Dr. Bertram Bitsch gained his Diploma degree (Master equivalent) in Physics in 2008 from Tübingen University, Germany, with his thesis: “Radiation hydrodynamics in protoplanetary discs”. In 2011 he was awarded his PhD in Physics, also from Tübingen University, for his thesis: “Planet-disc interactions in radiative discs”.
Until 2013 he was a Postdoctoral Research Assistant at Observatoire de la Cote d’Azur in Nice, France, working on simulations of protoplanetary disc structures. When he moved to Lund Observatory, Sweden, in 2014 as a Senior Research Fellow, he combined his previous knowledge with planet formation simulations to figure out how planets form. This was also the research topic of his European Research Council (ERC)-funded research group at the Max-Planck-Institute for Astronomy in Heidelberg, Germany, where from 2018 to 2023 he and his team studied how planets can form in different disc environments through hydrodynamical and N-body simulations. Appointed in 2023 as a Full Professor of Astrophysics at University College, Cork (UCC), Ireland, Dr. Bitsch continues his research on planet formation simulations to attempt to answer the following detailed questions:
– How do giant planets get enriched with heavy elements?
– How do giant planets influence their inner systems?
– How did the Solar System form?
– How do stellar parameters influence planet formation and their compositions?
– How do planets form in different galactic environments?
(Sources: https://www.ucc.ie/en/physics/people/professorbertrambitsch/; https://www.ucc.ie/en/media/academic/physics/physicsmainwebsite/staffimages/academicstaff/BertramBitschCV.pdf)
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In this lecture to Cork Astronomy Club on 12 February 2024, Bertram outlines his research on computer simulations of accretion and planet formation in protoplanetary discs. Inside these accretion discs, small dust particles stick to each other to form pebbles, which in turn form planetesimals and planetary embryos, and grow to planets with orbital parameters that are subject to modification. These processes are influenced by the underlying structure of the protoplanetary disc, specifically the profiles of temperature, gas scale height, and density.
Right um so now I have uh great pleasure in on yeah uh in inviting uh Professor bur and to the stand um to give uh uh this after uh this evening’s lecture um beram is uh quite a new addition to the uh staff of the school of physics at
UCC and he comes and this is where I would do my cap if I had one uh he he he comes to UCC from the max PL Max plank Institute in Germany um and uh so we’re very privileged to have him his his uh his speciality is in
Exoplanets and that’s what he’ll be talking about tonight and specifically addressing the question of how planets actually form so B are you ready sure carry on sure okay thank you very [Applause] much yeah thank you very much for the for the introduction for the invitation
Um I’m very happy to talk to you a little bit about um our current understanding of how planets can form in the solar system and also around other stars so um let’s first take a look into our own uh into our own backyard so to
Say so this is a plot um of the solar system and uh what what you can see is here all of our eight major major planets Mercury Venus Earth Mars Jupiter Saturn Uranus and Neptune and this plot um oh thank you um so the planets
Actually um are to scale so you you see that the Earth compared to Jupiter is of course very very small what is not um uh in in a good way in this plot is the distance between the plants but we’ll come back to that uh in a little bit so
What you can already see is that there’s there seems to be uh some sort of division uh in the solar system between the inner terrestrial planets Mercury Venus Earth and Mars and the outer giant planets Jupiter Saturn Uranus and Neptune and we have to understand um of
Course not only how the planets in our own solar system can form but also around um stars that um around around other stars and the the interesting question of course that we have to that that we have to ask uh ourselves is how special is the solar system is the
Structure that we have in our own solar system something that is very common or is it something that is um very rare so we have to you know go a little bit into the teenage angst and worry are we special are we weird uh you know the
These kind of uh questions we have to to think a little bit um about this so um just so that we all on the on the on the same page um I want to quickly introduce the astronomical unit um because we we too lazy to measure in centimeters uh
The distances between the planets so we have here the Sun and then we say the distance from the Sun to the Earth that’s one astronomical unit so it’s just the distance between the Sun and the Earth then of course Mercury and Venus are closer in so they have4 and7
Astronomical units at distance Mars one and a half astronomical units and then you see the outer giant planets are much further away so Jupiter 5.2 astronomical units Saturn nearly 10 astronomical units and Neptune um 30 astronomical units so 30 times the distance between the Earth uh and um and the sun just to
Bring this a bit in perspective um just um you know I think it’s always good to put things in a perspective so if you if you if you play football uh then you know basically uh we are basically down here then Jupiter would be would be in
This circle and then you have to walk all the way to the penalty point that’s basically the outer outer distance to the uh to Neptune so it’s a very very large system and um unfortunately this also means that this um as you will see later on not all the systems that we can
Observe can be can observe to those distances but we’ll come to that in a second so it’s just really to put things into a perspective so how how big is the solar system actually now um the the planets U there there’s three different types of planets that I would like to to
Highlight here one of course is is the Earth so a terrestrial planet made out of rock um and of course with life life on it and a small um appraisable atmosphere um the the other um type of planet that is very common are Jupiter like planets so called gas giants um
These planets are very massive you see here Jupiter has a mass of 318 times the mass of the Earth so very very massive um it’s also much bigger than the earth so 11 times uh the 11 Earth’s radi so it’s it’s much it’s much bigger than
Than the earth the other the third type of planet um is Neptune like like planets they’re like um of course a bit more heavier than the earth like 17 times the mass of the Earth they also bigger four four times as big in uh in radius uh the difference the main
Difference here is that Jupiter is a gas giant so main composition is actually hydrogen and helium uh whereas for the terrestrial planets for the Earth it’s of course Rock uh and and iron uh for for Neptune um actually we not even sure exactly what Neptune is completely made
Out out of we sometimes call them ice giant because they’re in the outer regions of the solar system it’s very very cold and we suspect that it could har a lot of water ice or even CO2 ice or methane Ice uh so so we don’t really
Know what that world is uh what Neptune is actually made of uh because it’s very very very difficult to actually get there and there’s there’s the idea to send some Gres towards the outer solar system but the the last visit we actually had for Neptune was actually
The voyage appr so let’s see if something will happen in the next uh decades okay um the other thing I would like to to point out is um actually the distance um of the planets as I said Neptune is 30 astronomical units away and this means it takes around 165 years
To make one orbit for for Neptune around the Sun and this is very important because we have to keep keep in mind uh the so-called time scales that we have to care about so basically how how fast the material can do one orbit around the
Sun and the further out you are the much the longer it takes now we have these types of planets in our own solar system how can we actually detect planets around other stars there’s uh several methods how we can how we can do this and these methods are actually highly complementary to
Each other as we would see so one idea um or one way to measure is the so-call radial velocity method or it’s actually um also called the doler method because it works with the Doppler effect that you probably know when the ambulance is passing from one side uh to the other
One it’s exactly the same the same thing um except uh that of course now we use actually light because light um also um is a is a wave and then we can observe the blue and red shift um of the star so we have our star um and here we have a
Planet around it and what then happens is that the planet and the star they orbit around the Common Center of mass so the star is not fixed in this Frame it actually wobbles a little bit around because it feels the gravitational pull from the planet now if this happens if
The um star compared to us to us The Observers actually moves towards us or away from us we can actually um we actually observe that there’s a shift in the light so we had a blue shift and a red shift of the light and this is the
Same story like if you have the ambulance that you feel that you hear a different tone if the ambulance approaches you over moves away from and this is the same same effect uh here but uh again just for the line what we can determine with this method uh is of
Course the mass of the planet because you can easily imagine if you have the star and a very very big planet then it will have a larger influence compared to a small planet because it’s just a gravitational influence um this field in itself um is actually relatively young
So the first exoplanet that was detected around the main sequence star with this method was actually just in 1995 so nearly 30 years ago and um actually Michelle May andas were awarded the Nobel Prize for this detection a couple of years ago the other main method uh that we
That we use to detect them is the so-call transit method you see it Illustrated here so we have our star and then we have a planet around it and the planet moves in front of the star when that happens uh the planet actually just blocks out some light from the Star we
See a dip in the brightness this is what you see here so the small dots would correspond to the observations and in red we can make a fit to the to the data and then from the from the shape of the curve from the depths of it we can
Actually understand how big the planet is so what the planetary radius is because you can easily imagine a planet that is much larger would plock out more light from the sun and therefore we would see a deeper dip into the in in the brightness now um if we actually um
Are able to combine both techniques at the same time so if you can do the radial velocity measurement and at the same time the planet is transiting then we can actually determine the density of the planet because we have the mass we have the radius so we have the size and
If you have mass and size you can easily calculate the density of the planet and if you know the density of the planet you can actually learn something about the composition of the planet because for example if you look at the gas giant um of like like Jupiter 318 Earth masses
But has a density of roughly of basically of roughly of water so around 1 G per cubic cm the Earth on the other hand much smaller but we have an average density of like 5.5 G per cubic cm so we learn something of the composition of the planet by knowing something about
The density and this we can do with both of these methods can be applied to the same star um at the same time this is not necessarily the case actually most most exoplanets that we detect we can only detect with one of the two methods
Um we will come back to that in in in a little bit now um if you have this method you can actually detect um the exoplant so this is a very recent uh plot um you see from like a couple of days ago um for all the exoplanets that
Are detected that we have detected so in the plot what you have here on the uh on the x axis this the sem major axis or the distance in astronomical units and on the y axis you have the planetary Mass so i’ Illustrated here also so this
Is where the Earth sits and up here we have um our our Jupiter so I should say this is a logarithmic um scale um so and this is um just in order to highlight that we detect many many many more planets in the inner regions of the solar system in
The the inner regions of the exosystems compared to our own solar system right if you if you remember me our our innermost Planet had a distance of point4 astronomical units so it would be would be around uh around here and what we observe is there’s a lot of planets
And actually orbiting their host are much much much closer than what we have in our own solar system you also see um you can detect three or three different kind of groups um of planets so you have the group um up here so these are planets that are
Very similar in Mass to our own Jupiter so these are the cold Jupiter planets because they’re far away from the Star so it’s presumably cold um and then you have here this population mostly in in purple and these are the so-called hot Jupiter planets so planets that are
Jupiter Mass but very very close to the star so they Orit the stars in just a few hours or a few days so they get get receive a lot of um solar radiation so they are very very hot uh this temp temperatures of a of a few thousand
Degrees degrees Celsius so these are the hot jup pitures and then down here uh in this group you have Gs of a few Earth masses um like you see five five to 10 Earth masses that orbit very very close to the central stars and these planets are presumably Rocky in composition um
So they are a few times the mass of the Earth so what we we call them the super Earth so basically a super massive um um Earth Light cents and these two groups of planets the hot Jupiters and the super Earth we don’t have any analogous in our own solar
System and considering um actually um if you if you look at the um occurrence rates so basically how many of these planets actually exist then we find that these super Earth’s planets basically every second star should actually have one so in this way we could actually
Think maybe the solar system is a little bit weird but I let let’s see um we we will we will come back to that um just for for information the different colors here correspond to the different detection techniques so the Purple colors would correspond to the transit
Methods the green one is the radial velocity method and we have some other detection methods I unfortunately don’t have the time to go into detail in all of them but I’m happy to discuss um um afterwards now um we can also look um a little bit um at this planetary systems
In itself because it’s it’s I mean it’s nice to have um a statistic of all the different planets that we have but of course the planets that we observe they don’t come come alone actually in planetary systems and this is what you what you see here is like a um a small
Snipplet um of different planetary systems that we uh that we have um observed just to illustrate again the denseness of these planetary systems so here are what is marked in the soul so this solar system so we have our four terrestrial planets Mercury Venus Earth and Mars uh these are these are these
Planets and then you see here we can compare this to these different um other exoplanetary systems uh that uh that we have um observed so this is just a small fraction of the systems you have observed and what you see is that that basically nearly all of these systems
Have planets um that are interior to the orbit of mercury so they’re very densely packed into the inner system additionally we can um actually if you look at each system individually uh like for example this system if you look at all these planets you actually see that
These planets seem to have very similar sizes within itself so the planets are very similar within the same system of course you have a variations between different systems but in each system individually the planets are very very similar in their size additionally these planets are also even evenly spaced so
You could do a calculation to compare the period ratio so how fast does a one planet go around the uh the um the Sun or their their star and you see that also these planets are actually very evenly spaced and this basically gives us um a characteristic like like the
Peas in a PO um the the planets have all the same size and they’re all equally spaced so they’re very very orderly um arranged and this this is also something that we have to that we have to understand that there’s not a great variety within each system individually
Of course between the different systems we can have a large variety so it’s something we have to to understand from from our formation theories now um how do planets form so um the the standard theory is that we have a large molecular cloud so 20,000 astronomical unit um units or even a bit
Larger that consists mainly of hydrogen helium into a tiny bit um of what we call heavy metals in astronomy so this basically everything that is not hydrogen at Helium we in that sense we very simple simple people okay now what happens is that this Cloud um actually it has a
Small rotation attached to it and then the by by gravitational pole the cloud will concentrate towards in the Middle where we will start to form a star the rotation of that cloud um actually um allows us to give angular momentum and we will form a dis around uh around this
Uh these Stars these are the so-called Proto protoplanetary discs that we uh and inside of these discs we can then form our planetary systems and this is um actually the so this is my actually my area of research um to understand how we can grow from these inside of these
Protoplanetary discs inside of these discs we have small micrometer centimeter millimeter to centimet size dust trins and they have to grow all the way to form uh to form planets so these discs um of course we can also observe so we do not only have to rely on
Cartoons luckily um so this is actually this is the Orion Nebula that was actually introduced just before um and inside this Orion Nebula we can observe these um protoplanetary at this so the Orion Nebula is a star forming region where we can observe these young protoplanetary disc so so this is um an
Image that was done as the the h space telescope but I think it illustrates very nicely the kind of environment where this this where this happens so you see here for example you see a star with a small disc around it but we actually have much better pictures of protoplanetary discs uh in
In our day so this is from the um Alma um um telescope in Chile so the at a k large millimeter um array uh this is one of the first pict pictures that they actually did from 2015 so they built the telescope and then they were like okay
Let’s just test it and then they got an image like this and we were like whoa what’s what’s going on here um so before before before that that Machinery became operational nearly 10 years ago we had no idea what the protoplanetary disc would look like and then the first thing
That we would think like oh a protoplanetary disc maybe it’s smooth and so on and then the first image that you get is like well this is not smooth right this you clearly see sub you clearly see structures in this this so you have um darker darker darker regions
You have brighter regions and they actually alternate with each other and uh it’s at that stage it was still um a bit confusing what could actually cause this so so I should first mention what we see here um is actually the thermal emissions of millimeter sized dust grain
So basically um it is a distribution of millimeter sized grains in this protoplanetary dis and this means um if you have if you’re a region that is very bright there should be more grains than in regions that are very D additionally this this project planetary disc is actually very large so
You see the size bar down here so that’s 14 astronomical units so 14 times the distance between Earth uh and and the sun you Su this disc probably like 100 150 astronomical units inside so even much much larger than our own solar system inside of these uh discs we have
These micrometer to cenm size dust trins and this is um roughly only about 1% of the amount of the material that is um that is in this uh dis so if you think if you think about it well uh the dis um is um a few percent the mass of the star
But only 1% of these few percents can can actually use to make planets so it’s not that we have an infinite amount of material available to make these planets now how does a planet form so uh here um is um also a protoplanetary disc but here um it’s actually um a picture
From micrometer dust so basically we have used a different telescope that would operate at the different wavelengths and then we can um observe in this case the micrometer sized um particles so this is um also in this uh in this case you see Bride rings and uh
And dark dark gaps now we can actually observe the same this this is the same disc but at millimeter WX so we see the millimeter sized particles in this case what you immediately notice of course is the millimeter dis is much more smaller than the than the micrometer disc and
The reason for that is um at least that what we what we think from a seral perspective is if you have grains uh the in the dis with different sizes it means that you must be some mechanism to allow the grains to grow more efficiently in
The inner regions of the dis compared to the outer regions of the dis now uh we have these small micrometer and millimeter sized uh particles in the dis but uh we actually want to grow towards planets so what we actually um have to um think a little
Bit about is how can we grow from micrometer siiz St scin so particles that like 10 Theus 6 M to full grown planets so that’s 10 7 m so we have to go over 13 orders of magnitude and size and uh this of course requires a computer models computer simulations to
Actually do to actually understand uh this process additionally to that it actually requires knowledge about many different areas um of physics because you can easily imagine um the the physics of small dust grains how they would Collide and grow is certainly different than if you have um already
Large planets that would mainly um operate via gravitational interactions now how do we model this uh so this is a small cartoon of the of the interplay what is going on here so we have a star uh then the star has a protoplanetary dis um around it uh in
This dis we have our small micrometer siiz dust trins and they can then grow to millimeter centimeter size soal Pebbles um this is just just a cartoon is they’re not really like you know rocky rocky things but if you go to the beach you have the Pebbles it makes
Sense to have this uh comparison now these Pebbles can actually grow and form asteroids so um we call them those sometimes also planetesimal so these are objects of 10 to 100 kilometers uh in size now these pebbl uh and asteroids can actually start to form planets so
The idea is you have your your planetesimal and then you start to um to accrete other planet tmals or other Pebbles and then you start growing um once become big enough uh you you might actually be able to AC create gas directly from the protoplanetary uh dis
Uh itself and while you do that you actually gravitationally interact with the dis in itself so you see this here this is from from a from a simulation so you have um the collar basically gives you the gas density uh in the disc and you see you have our planet some spiral
Arm in the Pro uh in the disc and that allows actually an exchange of angle of momentum between the planet and the dis and effectively the planet actually moves through the disc so it does not sit still it moves through the dis once the planet becomes big enough um it can
Actually start to open a a gap in the disc because it gravitationally moves away uh the material from its orbit you already see uh that could be maybe some you’ll see you have gaps gaps gaps here we have gaps here maybe we can link this
Together we’ll come back to this uh in a little bit then from our simulations perspective you have to care about the the the dis Evolution so there’s some evolution in this um and then we have to compare this from our whole sets of simulations to the exoplanet population
So we have to care about our super Earth we have to care about our giant planets and then we have to think about what the planets are made of and uh yeah this is all a bit uh confusing uh and uh you can lose a lot of hair um about this as you
Can see um but I mean um the general picture the general picture for for Planet formation uh is this so the idea is we start with the small micrometer sized dust trains that can then grow through mle collisions uh and we can we can finally form form the planets in the
Very end and these planets then move around in the protoplanetary dis through the interactions between the gas dis and the planet and then uh we can uh compare um our the results of our computer simulations with the exoplanet population in terms of the occurrence rates of planets like do we make enough
Of the super Earth do we make enough C Jupiters and also very recently also to want the composition of these planets because to some level we know what the planets are actually what the exoplanets are actually made of and the idea uh here of this talk I would like to walk
You a little bit through all the different question marks that are on this um on this uh on this plot so uh the first thing that we have to to think about is how do grains um actually grow there we are okay so um so so the
Question is how do grains actually grow that’s one of the questions that we that we have to think um a little bit um about so what you see here is like a gallery of different outcomes of grain grain interaction so these are actually motivated by laboratory experiments uh
Where the people take small dust trins they put it in a cylinder in a drop tower to to look at it at zero gravity or they go to the parabola flight um and the group actually had also some experiments on the um ISS uh to to understand how the grains uh can
Actually grow in Ser gravitational environment so what you have here um is uh two columns so you have here on the um on the the very left this is a column where the grains have the same size the initial two grains have the same size
And here we you have a large grain and a small grain and um I mean and there are very clearly different outcomes what could happen for the for the grain grain Collision so you can for example have a hidden stick Collision so you have two grains that
Come come together and then they stick and you make a bigger grain um but on the other hand uh if your grains uh come in you can also get to fragmentation right so so if you just go Um um basically go too fast so the idea here
Is that the speed between the particles so the velocities of the particle determine the collisional outcomes right it’s like you know um if if you take your car and you drive towards your garage if you do very slow you just stick on the garage and it’s fine but if
You go too far you fragment right um so the point here is that the velocities of the grains determine the outcomes of the collisions uh and unfortunately um for planetary growth um it seems that the larger particles actually have larger velocities so this means the bigger you
Grow the faster you go and this also means that at some point you become too fast for your own good uh it’s too fast and Too Furious and then you um you know start to fragment again so at that point um we actually um have a limiting size
Of these small particles in the protoplanetary discs in the range of centimeter to decimeter sized um particles so um how how can we continue uh from this so the the first thing is that I should I mention is why do these particle actually move through the dis
And the reason for that is um that there’s um in this protoplanetary disc we actually have a pressure support in this disc so we have the star and then we have this protoplanetary disc around it that consists of um gas and and dust now close to the central star it is much
Much hotter compared to the outer regions of the dis so you have a hot inner dis and a cold outer dis and this naturally gives you actually a gradient in pressure now uh what does this um what does this uh how does this help us well if you have a pressure supported gas
That actually wants to go around uh on a typical karian velocity you actually don’t need to be on the karian velocity because you have the pressure support so the gas orbits the star on a on a velocity that’s lower than the karian velocity so the karian velocity is
Actually the velocity like for example like planets go around the star so that’s basically just the the classic velocity now the small dust graines on the other hand they don’t care about the pressure gradient so they don’t feel the pressure the pressure gradient and they want to
Orbit on a karian velocity now if you have uh two things that have different velocities so you have the gas that is slow and your particle wants to go fast what happens is you feel friction and the moment you feel friction you you slow down it’s like if you take your
Bicycle and you um basically drive against Against the Wind you always lose a bit of your velocity compared to the to the other direction so the point here is that the by the interaction between the dust particles and the gas um the dust particles lose angular momentum and
They move inwards and they move inwards on time scales that are much much shorter than the than the time scale of the gas so the small dust trins they want to go into the uh into the inner dis uh uh in very very short times compared to the lifetime of the
Protoplanetary disc so I I can illustrate this here so um I’ll show you show you a movie where we have here the um orbital distance again in astronomical units and what you have here on the on the right is the grain size so how big are the grains and the
Color coding would give you the density so basically a um a very bright color would mean you have a lot of those grains and a very dark color would mean you have basically no grains whatsoever of this size so you see this you see this here um let me start from the
Beginning so uh in the beginning you see the grains they start growing in the inner disc uh very very fast you your time is just a few 10,000 years so the inner and the inner disc grains go fast because the growth it happens on these orbital time scales that are much
Shorter in the inner regions of the disc so you can grow faster because it just takes uh less less time to find new neighbors to collide with in the outer dis it takes a little bit longer uh for you for you to to grow but also in the
Outer disk we can grow uh to to large grains uh in a few hundred thousand years and then you basically drain the whole discs of Pebbles uh on time scales of around like a million years sounds like a long time uh which certainly it is but uh we have to compare to the
Typical lifetime of these protoplanetary discs which are around an order of magnitude longer so the Dust We would drain in like a million years but this are supposedly living around 10 million years and now the question is well um does this actually matter well it actually matters because we observe
Protoplanetary discs that are a few million years old and they still have dust right so the theory would predict the dust just goes away so we should not see it but we we see it so what what is how can we explain this phenomena so the idea here
Is um actually that these protoplanetary discs are actually not smooth but they um actually have gaps gaps and rings in it and these gaps and Rings would actually correspond to points of highest pressure so you have a a high pressure point and the particles would actually
Go to to the point of highest pressure so you accumulate uh the um particles in your pressure maximum and in the minimum of your pressure your particles would move uh move away so the idea is uh if you see these bright Rings where we have a lot of dust this is actually a
Pressure maximum and uh there darker darker gaps we have a pressure minimum so if you just have some perturbations pressure perturbations in this protoplanetary disc we can actually explain the observations uh that we see now uh how does this work well so you can think a little bit about the um
If you make an analogy this like a beaver dam so the idea here for for the beaver As you have a dam the water can still go through but you filter out the larger the larger particles and this is the same for the gaps in the protoplanetary so you can basically stop
The motion of your um particles but your gas is still allowed fre to move uh to move through so for the beaver dam the water still still goes through but for example you you stop the leaves so this is like a a bit of an analogy to see uh
What actually happens uh in these um uh in these uh positions of the dis now um how do we make um planetesimal so um or these um asteroids of 10 to 100 kilom in size so the idea uh here is actually that you just take a lot of these particles of these small
Pebbles and and at one point if the concentration becomes high enough they will actually collapse under their own gravity so the idea one one idea here is we can have um solar you can have a Vortex and I think of something like if you look look at the maps if there’s
Hurricane season you always see these uh U um very beautiful but very destructive um objects and basically these objects carry material around it and you can actually build up a concentration of particles in the very uh in the very middle of them and these particles if
You put enough of them into a Vortex like this they could actually collapse under the influence of their own gravity so the gravity of all the small minium PES would then be enough to basically trigger gravitational collapse and then form a planetes um this um uh for example how
Can you reach these large um regions of um of Pebbles well this could actually be uh actually in this um pressure Maxima because here you accumulate automatically a large fraction of of these um particles so this could be a prime sides to make the first planetesimal now once you you become big
Enough you have to start wondering how do you continue to grow further if you’re small planet t mod and there’s one one idea that came up recently in the last couple of years this is so-called Pebble accretion so the idea here is that we do not only grow by the
Accretion of these large planet tesm the smack into each other and grow but we can actually agre the small the small millimeter centimeter size Pebbles and the reason for that is um if your planet sits here but then at the same time the small millimeter Pebbles they actually
Drift through the disc so so basically they come to you it’s like the food comes to you it’s like if you’re at the Rolling Sushi Buffet you know the food comes to you and you start uh start eating um as as much as you can um or or
Want and this is basically the same the same same um process that works here so a planet that can grow very efficiently by pip accretion because uh you basically have access to the whole Mass reservoir of the disc so you sit happy so your planet happily sits at some
Point in the disc but all the small material comes actually to you because they drift inwards very very fast so you have have access to the full Mass Reservoir and you will can grow very very quickly now um at some point you actually stop growing um just because
You you you you ate too much and this is uh in this case um actually when the planet starts to influence the um the structure of the protoplanetary disc so the planet actually would open a small Gap in the protoplanetary disc so you’ll see this here Illustrated from from the
Simulation so again the color coding gives you U the U the density we have our planet the dark color means we have um a low density so we open a gap and the greenish reddish color would mean we have an over density so why does this
Happen so this is um just an exchange of angular momentum in the protoplanetary disc so the inner dis wants to basically wants to give angular momentum to the planet but if you give angular momentum away you lose angular momentum right so the inner disc gives angular momentum to
The planet but loses the angular momentum at the same time and if you lose angular of momentum you move inwards at the same time the planet gives angular momentum to the outer regions of the disc and if the planet gives angular momentum to the outer regions of the dis
The outer dis um actually ex extend its or so your planet basically pushes away the material from its own uh own orbit now uh if the planet is doing this you actually generate um a pressure bump exterior to the planet you see this uh Illustrated uh Illustrated here so this
Is um our planetary position we have this here A basically a function of um um velocity um so here in the outer regions we have our low um basically the normal pressure support so our small Pebbles would would move inwards but here um ex uh just exterior to the
Planet we actually hit uh this pressure bump where the the gas um actually from a pebble’s perspective is going the opposite direction so basically if you go back to the analogy with riding a bicycle suddenly the wind changes so you don’t lose the the um angular momentum
You don’t go in and here you would would actually B basically stay stay out so the material accumulates exterior exterior to the planet then um uh if when that happens then uh the planet um actually cannot agree the Pebbles anymore because they’re trapped exterior to the planet so the planet can
Start to cool because you can imagine accretion is a process that releases a lot of energy and then the moment you start to cool you can actually contract uh and you can uh can start to accre gas and become a gas giant now this process in itself um can actually explain some
Of the observations that we have so and this is just just a cartoon to to illustrate it is so we have our planet uh that sits here at um r equal one so here and then we generate a pressure bump exterior to the planet so we trap
Uh the the particles uh in it and now we can actually um take a look back at the pictures of the protoplanetary discs that we had before so now we know from from our modeling well if a planet sits here uh you make a Deep Gap in the disc
Where you have no um pebbles no grains and we have a bump exterior to it well then uh to explain the observations it would mean that in uh the gaps we could actually have planets and um because these planets can then explain the gaps so the void of the Pebbles but at the
Same time we can explain uh the um the pride Rings exterior to it so basically we could say well maybe this just implies that in each Gap that we see in these Proto is there could be a planet of course this is very very highly debated because um uh as you can imagine
It’s very very hard to detect planets in these uh in these discs uh in itself so it’s still an open question if um all these gaps uh and rings are actually caused by uh by growing planets but it is one um uh um one of the leading ideas
How we can explain these features of these Proto of these discs that we observe so what we see um actually when we observe these discs is basically Planet formation in the making now um I I would like to talk a little bit um about the formation of um
Um of super Earth so how can we uh can we make uh these uh these Earth these um planets of a few Earth masses very very close to the to the central star so um there’s um a few a few um um ideas um
That we that we follow so this is from a set of simulation where we have um orbital distance against the planetary Mass so we have our plan that start here and then they start growing uh in these Within These trajectories we have here two sets um of simulations where that
Are just distinguished by this Factor um f nor so which basically corresponds to the amount of material you have available so what what you see in the end if you have um in this set of stimulation around 100 Earth’s masses in Pebbles available you make planets that
Are roughly the size of Mars but if you have more material available you can actually make much bigger planets and this of course makes sense because the more food you have available the bigger you can at the same time uh we actually um uh wait we come back back to that in
In a little bit so now we can actually um take a look and look at the planetary systems that would actually come come out of this as a function of how much material we have um available so you see here on the plot these are the planets
In our simulation after 3 million years so this is the end of the lifetime of the protoplanetary is so the end of the lifetime of the gas phase of the disc and then once the gas phase disappears we only have gravitational interactions between the um objects and they can
Actually the objects can actually Collide and start growing and this is what you see on the right after 100 million years um of evolution in these simulations and what we find is that if you have a low relatively low flux um of Pebbles you can make uh the terrestrial
Um like like planets they start growing and colliding over time scales of 10 to 100 million years similar to what we believe the terrestrial planets in our own solar system have formed on the other hand if you have a high Pebble flux uh we already start making these planets of a few Earth’s
Masses during the gas phase of the protoplanetary discs and then we have some uh rearrangement um after the gas disc is gone that would give us the final systems that we observe so the so the idea that we can have here is if you want to explain the formation of the
Solar system in this context um it actually uh implies that we had maybe a very low Pebble flux uh in the inner regions of the solar system because we want to make a terrestial planet uh and um not a not a high flux because if you
Had to have a to high flux of Pebbles you would actually have super Earth in our own solar system and one idea that that can actually explain this is actually the presence of Jupiter because when as Jupiter grows and becomes big it actually opens this uh this Gap in the
Protoplanetary dis and then when you open open the Gap you drop the Pebbles exterior to the orbit and then the inner system cannot reach uh cannot be reached by these Pebbles anymore so Jupiter grows it blocks the Pebbles and basically the inner system is starved of Pebbles and we basically stop growing in
The inner system and we can form the terrestrial plants now um I would like uh to to show you also a bit of a movie to get a bit of a better feeling of how that um actually this process can actually um happen so what you will see in this
Movie um is uh the um major axis and the planetary mass and then you will have a lot of these small dots that would correspond uh to the to the planets that that can grow we have here in this protoplanetary disc we also have the ice
Line so the Water Ice Line so the inner regions of the disc is very hot so we have no water ice in the outer regions it’s cold so water is actually frozen out and we have water ice now we can take a look um at the um
At the movie you see here the planets start growing in the protoplanetary dist they start to move through the disc they start to migrate through the disc and then uh they they slowly make their way uh inwards so this gray region uh is a region where the migration Direction um
Is actually slightly slightly different and then we see we have a planetary system uh in the inner regions um of this um of the system so let’s start again so you see the planets start growing by creating Pebbles and then you um they they form um a chain of
Of planets and then they start um actually Collide colliding with which with each other um you see this probably here yeah boom and that the planets start um um colliding and uh and growing and what you have in the very end is actually um a system of close in super Earth
Planets now um um this this plot maybe U contains a lot of information but let me quickly walk you walk you through it so what what we have here is the time evolution of our protoplanetary system over um a few um to 50 million million years uh and what you have all these
Lines correspond to the evolution of individual planets so we have here the semi major axis in astronomical units and we have here the planetary masses in uh in Earth’s masses so what happens is the planets slowly start accreting by by Pebble accretion then they stop growing
Because they reach uh the um this small pebble isolation mass and then they can further grow by by having collisions now at this um at this stage here this gray shaded area uh line that you see here this is when the dis um dissipates so we have a gas dis on the
Left we can um still have the planetary migration and PEB accretion here on the right uh we have no gas dis anymore we just have the gravitational interactions and what happens is that you have gravit that you have interactions between the planets where we can have Collision so
You see here so you have these black lines and they suddenly stop so when these black lines stop uh it happens that basic our planet disappeared and there are several ways how our planet can disappear in these simulations you either get kicked out of the system so
You get ejected or you actually agreed with another planet and you make a bigger planet and this is what you see here in the in the mass diagram so you have your a planet that has a certain mass and then boom you get a step function here where you suddenly
Increase your mass and it’s because you collided with another planet and started uh started grow and this uh process of dynamical instabilities has another um effect on the structure of the system and this is the inclination of the system so you see this here so this is an evolution time evolution of the
Inclination of the system so what happens is that the planets that we have at the end of these simulations all have some small mutual inclinations and this has very important consequences for the um for how we can observe these planets so um this is um you can imagine so if
You have a flat system where all all the planets are in one um basically on one line and then you would observe them via the transit method you can see A system that has multiple transiting planets but um if at the same time you would observe A system
That has small Mutual inclinations uh between the planets you might not observe all of the planets um because the the transiting area so if if you a star and your planet goes um in close close by just in front of it you you might see it but if it’s further away
And you have a small inclination you will actually you will actually um not see that planet and this is actually also true from the solar system so like if you if you if you were an alien uh somewhere in the galaxy and you would try to observe the solar system with
Transiting the transiting techniques you would only a maximum see three planets at the same time you will never see more than three planets transiting because of the small neutral inclinations uh between uh between the planets that prevents uh to to to see all planets at the same
Time now uh this is this is very important because with this um we can actually try to explain uh the observations of um um of of our EXO Planet a distribution so what you see you see here um is a plot where we have the number of detected planets by
Transiting um um um by the transit method and the gray gray curve that you see here that is the are the detections from the Kepler um capla space mission so was a a very very cool mission that lasted several years and detected several thousand um transiting plets so
What you see uh is that basically most of the systems so around 70 75% of the cap of the systems that were detected by the capit satellite um actually only have one single transiting Planet then um systems that have two two or more planets is around um like 20 20%
Uh 15 15% of the of the planets of the exoplanetary systems that we observe maybe have two transiting planets and if you go to larger and larger numbers of exoplanets you see that the um number of systems that have many transiting planets becomes much much becomes small
Relatively fast and this is just an effect of the small Mutual inclinations between uh the plants so when we observe a planetary system with the transiting method and we see one planet um we don’t know if there are any hidden planets in this uh in
This uh in the system now from from our the simulations uh that we did um a couple of years ago we can actually reproduce this um observation pretty pretty nicely so this is the green the green line that you that you have to um
Look at and what what we see uh is that um we can match it well because uh we we have different kinds of systems there that um that that match uh this very well we don’t want to go into into too much detail till here but I’m of course
Happy to discuss this afterwards so the the essential point that I would like uh to make here um is related to this so this is the same kind of plot we again have the um distribution the number of detected planets but now these colorful um inlays actually describe how many of
The planets we actually have in each simulation so so even so we do we do our simulations we have small Mutual inclinations between uh the planets and we do synthetic observations to understand how many planets of these systems would we actually see if you would um observe them and uh for our for
Our best fit of simulations this is this is the result so the number of uh systems where we only have one detected Planet per Transit um in actually in our simulations the large majority of these systems have actually two or more planets so you see here for example the
The this light blue color would would basically correspond that around 37.8% um of our um planets of our systems that have only one single transiting planets actually contain two planets and uh the the the yellow color would indicate that 26.8% of all these systems where we have one detected Planet actually contains
Three planets so our prediction is actually that there is hidden planets in these systems that we observed with the transiting method now um how can we test this well um the idea here is actually if you can combine multiple observational techniques at the same time we can actually see uh we can
Actually um check if the system contains hidden planets so again so this is the the stat the um statistic that comes out from the capler satellite so it’s a transiting method but if you would observe these same systems with the the radial velocity so with the douer method
We can actually see the planets that are not transiting and we can actually check if our simulations would be uh are actually correct or not so um we trying to convince observers uh to do this uh but uh uh let’s see uh if you find find
The results now um this this plot is very confusing but don’t don’t don’t worry um uh I I will there one one very important Point um about um about this uh plot so this plot shows you um the planetary Mass against the planetary radius so basically the the how how
Massive is the planet and how big is the um is the planet because for some of the planets as I said you can actually have measurements of the um of the size and of the mass and so we can actually put them in this diagram and these are the
All the circles that you have here and you also see here the arrow bars of these um measurements some some of these planets have very uh very small Arrow bars which I personally think is like very amazing that we have planets um orbiting around stars that are several
Hundred light years away and we can have these these very small measurements in mass and radius now the point that I would like to make here is that there’s a huge diversity in the composition of these planets so because you can you can actually run models where you say you
Have a you have a given material and you have a given mass and then you can calculate how big would actually the be the radius of this um object so for example in this red line it would say like let’s assume we have a planet that
Is to one 100% iron it’s of course unrealistic but just let’s make the Assumption you have a planet that is 100% um Iron and then if that planet would uh be 10 10 Earth masses of iron then uh we would have a radius of around 1.5 um Earth’s
Radi now we can do the same the same game uh and do take a line where we have 33% iron and the rest would be a rocky composition so it’s actually an earthlike composition you see the Earth um actually falls very nicely on this uh
On this line and then if you do this we see oh we have actually a very nice group of planets that seem to have a similar composition uh to the earth now uh we can of course um also take a look uh into um into a different composition
So we can follow the blue line so that would be if a planet is made to 100% all of water of course it does doesn’t exist but it gives you an illustration of what that actually imply now if you do the the same game for water we would take a
Planet that has 10 Earth’s masses and would consist to 100% of of water we would arrive at like 2.7 2 uh 2.8 times the radius of the um of the earth now uh we also see that there’s a large group um of um or a large number of
Planets that basically plot in this region of parameter space and this implies that actually uh these planets can have a large diversity of composition so there will be planets that are very Rocky and there will be planets that could be water worlds and we have to explain this so statistically
Speaking we want to calculate the super Earth or we want to calculate if it’s Rocky or water r i i i have some opinions which movie is better but okay anyway so so the point here is right um I don’t know if you if you if you paid attention for the
Movie but the problem in this movie that I showed you from our simulation is that in the end all the planets were blue and the blue actually means they have a large water ice fraction right so basically all the planets that we made in our simulations as nice as it is that
We can match the capit observations of how many planets are transiting we have the problem that all these planets are water rich but we have just seen that there has to be rocky planets so how do we make them Rocky um so there’s many many ideas uh
That we that we actually following up so this is one of the the active areas that I’m that I’m working on to understand the compositional concept um of these um of these planets so one one idea is could be related to the then maybe we calculate the movement of the water ice
In the dis too fast maybe we have different different sizes of um of Pebbles um but then if the pebbles in the inner dis are too large then there there’s a conflict with the um sizes of the particles that we know we have in our Sol solar system the so-called
Condules maybe there’s some idea that we have some radioactive energy um heating of the planets or maybe as the planet actually grows by by the accretion of of Pebbles maybe we don’t accrete all the water because you can imagine that as the plan as the planet grows it becomes
Very hot and then you have water ice that comes in it might actually uh evaporate in the upper regions of the dis and then just uh disappear and we might not ACR the water so there’s many different avenues that we have to uh that we have to follow uh at this stage
To understand the uh the composition of these um of these planets uh so this is the active active area of uh of research and this is actually very uh promising this actually brings me already to my summary uh because the um the future in that sense is very very
Bright because you have a lot of amazing telescopes that are that are online right now like the um the James web telescope that uh allows us to um probe actually the atmosphere um of exoplanets we can learn something about their composition uh we will soon um towards
The end of this decad have the uh elt so the extremely large telescope that is built in uh in in Chile that will ALS allow us to to understand planets better and at the end of this decade there the Arial Miss Mission so it’s an Isa mission that is just dedicated to
Observe um the atmospheres of of exoplanets so this Mission will characterize the atmosphere of up to a thousand exoplanets and this will give us an amazing set of data to understand um our simulations so to summarize um everything in the this moves dust Pebbles gas planets everything is moveth
It’s a very dynamical uh system uh to understand Planet formation we have to uh require knowledge about different stage areas of physics has span over many magnitudes of size so we have to understand how the small dust grains grow how they move through the disc how
They can make Planet tmal uh and planets we um at the moment uh we think that this accretion of these small millimeter to centimeter size cbles can actually help us to build the planet so as I said it’s the the Rolling Sushi Buffet that comes to you uh but of course there’s
Still many many open questions that we have to to understand um especially regarding the composition of planets so are they planets Rocky do they contain a lot of water what is the atmosphere made of and this is um actually one of the big um ideas that we have for our future
Understanding of planets we characterize these planetary anoses and then it will maybe tell us something about the formation mechanism um of these uh planets now I thank you for your attention I’m happy for any questions thank you B now first of all my head is reeling sorry and what
Um what what what strives me most is that you can talk with such confidence yes about things which are ridiculously far away which you’ve never seen and you give us graphs and diagrams and simulations and where do you get your confidence from it always is right ah I
Mean it’s a very interesting question I’ve actually never been asked that so so um um well I mean it’s it’s just it’s just the physics I mean I mean for for the detection of planets right I mean like for example the transiting planets um actually a
Couple of years ago there was also um you can observe the Venus Transit you can observe the macro transit in our own solar system and then of course it’s the question Do We Trust uh the instruments uh that were that were designed to do this on on distances of a few few
Hundred light years and and I think the data is is there so I’m I’m very very confident about this right well here’s a confident man so um any questions but I have a few but there’s one up there near the back I’ll just turn these lights on so you
Can get a better view of it yeah thank you that was pretty interesting as much as I could understand very interesting the um in your in your investigation so far you know if there’s a maximum size um so so so this is this is actually very very interesting because
The um so from an observational perspective at some point if the planet becomes bigger and bigger you would move towards the brown dwarf regime and then you would actually at some point if you become more and more massive you would actually start to make a small Star
Right so so essentially there’s um um so so Jupiter um as as we discussed it’s like 300 Earth masses but if Jupiter would be around 10 times more massive then it would already be fall into the category of a brown dwarf and if you would be um around a factor of 100 more
Massive you already be like a very small star so at some point there’s a trans transition between the between the planets towards the Stars however uh in these portal planetary discs they are just a small fraction of the mass of the star itself right so even if you would
Have cre the whole disc onto one object you cannot make a star out of it because it’s just not enough uh it’s not have enough M Mass you could you could probably make BR dwarves out of this in these discs but this is already a
Pushing a push in the limit but this is just a question for the mass but regarding the size this is actually um quite uh quite funny because Saturn which is um around a third of the mass of Jupiter is basically the same size as Jupiter and if you go to planets that
Are even bigger than Jupiter like let’s say 10 times the mass of Jupiter they would still be very similar in size to our own Jupiter and the reason for that is because of the gravitational compactification um of the um of the gas right so when you’re more massive you
Have a larger gra gravitational P so basically concentrate your mass more towards the towards the middle compared to if you’re lighter and it seems that from that around Jupiter the the size of Jupiter seems to be kind of the maximum in this kind of um function next question question there other systems no
You said there’s LS LS detect uh system is rotation is normal on presume Transit doesn’t work and doesn’t work is there way those ah right um yes so I didn’t go into the into the detail of this of course the um alignment of the system to whats of
Makes makes sense right so if you have the start start here and your planetary system would go like this of course you don’t see the transit because the planet never goes in front of the star at the same time uh you would actually also um also not see
The the doler shift because the doler shift would go up and down but you’re sitting here so in these systems we cannot see with the transiting or doler um adopter technique there’s other uh means what we what we can do in order to detect these planets uh like there’s um
This astrometry we can basically follow the motion um basically you can basically still see the movement um of the um of the star in this um basically as a function of time so the Gaia satellite um from from from Isa is actually um supposed to release data um
To for these type of planetary systems uh in the next I don’t think two or two or three years so we gathered a lot of data so we can actually do this so it would depend on the different uh you would just use a different detection uh
Method um so yes um uh uh it the alignment matters uh of course but from a statistical um perspective it is not um it is not interesting in that sense because you already have a lot of planetary systems so we can actually already do the St statistics regarding
The occurrence rates of these planetary systems of course if you miss some it’s of course it’s a it’s a Pity but there’s nothing not too much what we can do about it because the orientations are just just random right so there could be lot more systems out there yeah so yes
So so and in the in the in the plot that I that I showed you in the uh in the beginning uh here um so basically here uh in this plot we we have around ,000 detected exoplanets but from a statistical point we can you can you can just compare
Saying like look what is the probability that you actually have a planet transiting right because of the of the mutual inclinations about the orientation and you can use this to make a statistical analysis to calculate how what is the frequency um of these planets so for the super Earth planets
Basically the idea is that every second star should actually have super Earth but we just don’t see them because um of the of the of the transit uh Transit alignment so we can calculate the statistics of how how how frequent the different types of planets are oh over the
5,000 uh planets that we’ve discovered are we assuming that there are just one stair in the system and the plants Orit that one stair do we know any other stair systems with multiple stairs that planet go yes so the these actually exist um so um so the capler mission um
That I mentioned is basically basically all these purple dots from the transit Mission basically like 90% come from the Capa Mission um the Kaa also detected planets um orbiting around binary Stars there’s two types of binary so the ones that are very close close in and then
You have a planet that basically orbits both of those Stars at the same time like like in Star Wars tentu um or the other option is like you have a wide binaries that a bit wider separation you need have planetary systems around each stars um separately so these system
Exist there’s not not not that many um of um of them uh in the uh in the literature um I’m not entirely sure why that is uh to to be honest um I think I think one one of the reason is uh that if you have binary STS you get um like
This tortion in your light curve so it’s becomes a bit more dodgy to actually identify if there’s a planet um around it but there’s um a few 10 uh systems where you have planets around binary star so so they exist and from a theoretic perspective I don’t see why
Not there’s no nothing that speaks against it I mean it’s more complicated to understand but there’s nothing that speaks against it for us to find well yes thanks uh what will we be talking about in terms of telescope technology to establish that an nearo exop Planet definitely has an ocean and continents I
Mean it could that be anything that’s coming in the pipeline or is that sadly long after uh well um I don’t know what what progress medicine will make um so sorry no okay um just uh no okay more seriously right so so this um so we can
Observe the um the the composition of planetary atmosphere so we can we can do this with the um cor um with the James web Space Telescope can do it we can also do it or with the H based telescope we can also do it with groundbased um observatories we can determine the
Compos position of the atmosphere but we cannot see onto the planet right so so so essentially um what what happens is um so if if you do it by by the transiting method so you have the planet that goes into in front of the star but
Then you have a very small you have the atmosphere around it right and the light passes through the atmosphere and uh of that of that planet and then we can basically see from um the spectroscop spectroscopic line so absorption emission band you can basically determine what the atmosphere of that
Can of this but we can only observe the light that actually makes it through the atmosphere right if the light does not make it through the atmosphere we cannot see it right so so so we can determine the composition of the atmosphere but we cannot say anything about the surface of
The um of the planet I thought I might be the answer but I was whole 10 years no I’m I’m I’m sorry but but I mean there there’s an interesting uh interesting thing because not all planets have atmospheres actually right so so there’s actually some some planets
Uh that are very close to the star so we believe that the atmosphere has been blown away by the by the winds from from the Star and then we can actually determine the composition of the um of the um uh of the surface of those planets but of course if you don’t have
An atmosphere it’s very difficult to have uh liquid water on it so yeah from from your analysis and the data collected th far have you seen any correlation between the rotational plane of the Galaxy and the rotational planes of stars and Proto Proto spheres such um
Not not to my knowledge um it’s it’s all it’s all a bit it’s all a bit random because I mean even if you if your I mean your stars move around uh in the uh in the Galaxy but then what what matters um is of course the for gravity itation
Interaction the distance matters right so the stars that are very close close close to each other can have gravitational pulls uh on uh on each other and then the orientation uh can become randomized even if you go around in the same same Circle you know you can
Have random random random inter um um interactions to basically tilt your planetary systems and the orientation in itself of a planetary system basically comes from the from the collapse of the molecular cloud so basically if the cloud has just a different rotation you basically just get a different rotation
For your planetary system out of it and that doesn’t really is not really influenced by the motion in in the Galaxy now can I say that uh there there’s only time for a couple more questions I’m hoping that no one is going to wave their hands because I got one of my
Own that’s why you didn’t see me uh but when I close the meeting which will be a few minutes after we’ve finished the last question with birth um then I’ll invite you all down for tea and I hope but you can join us for Te and that then you’ll have the
Opportunity to uh uh hold ver one to one so is anyone going to contest with me the right of asking question no good but I’m not going sure to formulate this question but it’s all to do with the rotation of dist and so on now I mean planets only exist
Because they’re orbiting Their Stars if they didn’t orbit they would fall in but what what makes an orbit in the first place you start your very first slide you showed us a cloud and you said the cloud is rotating yep what makes the cloud taste this is ah okay um so this basically
Just an initial leftover of angular momentum right so so you have you have your your you keep using that word I don’t know what it means right sorry okay I’m I’m very sorry so basically um so so basically it’s a measurement of the rotation so how you how how you um
How how you rotate uh in the um um basically around your Center Center of mass so you have a molecular cloud but it would not sit still right so you so and then there will always be a little bit of of rotation and then that a little bit of rotation already gives you
The um basically the collapse uh that you make that you make a disc uh around it so so the cloud is is always has there’s some motion in the cloud and then as you basically concentrate your material towards the middle it will not just like fall like this but it will
Start to rotate and flatten and that’s what gives you the the dis in the uh in the very end I still feel I don’t understand maybe I was made to spend seven years studying physics um but I’m delighted to uh uh have made your acquaintance tonight um I
Hope this won’t be the last time we see you uh and on behalf of cor astronomy club I would like to thank you for uh coming to talk to us and all the uh preparation that you’ve um put into given this lecture um even though I have to confess that not every
Chart that you showed on the board did I understand but I did enjoy the simulation on behal of cus Club I hope you’ll accept this than you right I I promised I would close the meeting soon I will just a few closing remarks I