Hydrodynamical models of Extrasolar Planets: Formation and Evolution

Lead Research Organisation: University of Cambridge
Department Name: Applied Maths and Theoretical Physics

Abstract

Planets form in discs of gas and dust that are commonly found around young stars. Within these discs, small dust grains collide with each other to create bigger dust particles. In a few million years, this process can lead to the formation of a full-sized planet like the Earth. If there is enough material available, more massive 'Superearths' can be formed. When they are massive enough, typically a few times the mass of the Earth, planets may start to attract gas from the surrounding disc, which eventually results in the formation of a gas giant planet like Jupiter, which is more than 300 times the mass of the Earth. Because more material is available far away from the star, we would expect gas giant planets to form far from the star, while Earth-like planets form closer in. This is indeed what we see in the Solar system. The first extrasolar planet, however, was a gas giant planet that was orbiting more than 20 times closer to its star than the Earth is to the Sun. These planets are now called 'Hot Jupiters'. They were a real surprise, but fortunately there was a relatively easy explanation: this planet was not formed where it is now; it has migrated inward from its place of birth under influence of the surrounding disc. Eventually the disc evaporates, and migration will stop. However, theoretical migration rates are much to fast: they predict that all planets should quickly plunge into the central star before the disc has time to evaporate. This is a serious problem, and it is important to understand the details of planetary migration in order to understand the formation of planets. Much progress has been made over the past few years in refining numerical models to come up with more realistic migration rates. Very recently, it became clear that modeling the detailed temperature structure of the disc is imminent. Many of the implications of these temperature effects remain to be resolved, and this is part of the proposed research. A few years ago, a new, very fast type of planetary migration was discovered. This runaway migration may take place in very massive discs, for intermediate mass planets (like Neptune), and because it is so fast it can easily come to dominate the overall outcome of migration. Because the disc is very massive, it should feel its own gravity in numerical models. However, this is very difficult to model because it is computationally very expensive. As part of this proposal, I would like to use the newest computational facilities to resolve this issue and study this very interesting type of migration in discs that feel their own gravity. Another interesting surprise regarding extrasolar planets was their occurence in binary stars. The planet orbits one of the stars, with the companion star orbiting around them. This poses a real challenge to planet formation theory, because the gravitational influence of the companion strongly disturbs the dust particles that are supposed to form planets. But not only are the dust particles disturbed, the gas is as well. Recent studies on planet formation in these interesting systems did not take into account the evolution of the gas. As part of this proposal, I would like to study the influence of real gas dynamics on planet formation in binary systems, to get a better picture of the possibility of forming planets in these hostile environments. Extrasolar planets are mostly found on very eccentric orbits, for which the disctance to the central star varies by more than a factor of 2 during one orbit. The Hot Jupiters, however, are all on circular orbits, and we think that this is due to tidal effects. The strong tides that are raised on a planet so close to the star slowly forces the planet into a circular orbit. During this process, a huge amount of energy is dumped into the atmosphere of the planet, and some may not even survive. I would like to study the effect of these tides on the structue and appearance of close-in extrasolar planets.

Publications

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Paardekooper S (2010) VORTEX MIGRATION IN PROTOPLANETARY DISKS in The Astrophysical Journal

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Paardekooper S (2009) Disc-planet interactions in subkeplerian discs in Astronomy & Astrophysics

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Paardekooper S (2010) Planetesimal collisions in binary systems in Monthly Notices of the Royal Astronomical Society: Letters

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Paardekooper S (2010) A torque formula for non-isothermal type I planetary migration - I. Unsaturated horseshoe drag in Monthly Notices of the Royal Astronomical Society

 
Description Young planets, embedded in circumstellar discs, were thought to migrate inward at alarming rates. New models including a proper energy budget show that these rates depend very sensitively on the thermal structure of the disc, and that migration can be stalled or even reversed.
Exploitation Route The migration recipe that we came up with is now widely used in models of planet formation.
Sectors Education

 
Description Planetesimal collisions 
Organisation Observatory of Paris
Country France 
Sector Academic/University 
PI Contribution Putting all ingredients into a global model of planetesimal evolution in binary systems.
Collaborator Contribution Helped putting in collisional evolution in our models.
Impact 1 peer-reviewed paper.
Start Year 2010
 
Description Planetesimal collisions 
Organisation University of Bristol
Department School of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Putting all ingredients into a global model of planetesimal evolution in binary systems.
Collaborator Contribution Helped putting in collisional evolution in our models.
Impact 1 peer-reviewed paper.
Start Year 2010
 
Description Saving the Earth 
Organisation American Museum of Natural History
Department Department of Astrophysics
Country United States 
Sector Academic/University 
PI Contribution Helped putting in detailed migration laws into disc evolution models
Collaborator Contribution This collaboration has helped putting my models into the perspective of disc evolution.
Impact 1 peer-reviewed paper, one article in a popular science magazine.
Start Year 2009
 
Description Torque formula 
Organisation Eberhard Karls University of Tübingen
Department Department of Computational Physics
Country Germany 
Sector Academic/University 
PI Contribution Deriving a formula that predicts the direction and magnitude of planet migration.
Collaborator Contribution Provided results using an independent numerical method.
Impact 2 peer-reviewed papers.
Start Year 2009
 
Description Torque formula 
Organisation University of California, Santa Cruz
Department Department of Astronomy and Astrophysics
Country United States 
Sector Academic/University 
PI Contribution Deriving a formula that predicts the direction and magnitude of planet migration.
Collaborator Contribution Provided results using an independent numerical method.
Impact 2 peer-reviewed papers.
Start Year 2009
 
Description Torque formula 
Organisation University of Nice Sophia-Antipolis
Country France 
Sector Academic/University 
PI Contribution Deriving a formula that predicts the direction and magnitude of planet migration.
Collaborator Contribution Provided results using an independent numerical method.
Impact 2 peer-reviewed papers.
Start Year 2009
 
Description Popular article 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact An interview with a journalist led to a popular article in a Brazilian science magazine.

None.
Year(s) Of Engagement Activity 2010
 
Description Science et Vie 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact An interview with a French journalist led to an article in "Science et Vie".

None
Year(s) Of Engagement Activity 2010