What is the giant planet migration?

When planets were first found around other stars, the planets that were found—at least around stars like the sun - were giant planets; they were more like Jupiter than Earth, so they were many hundreds of times more massive than the Earth.  instead of being rocky planets they were big balls of gas like Jupiter.  They were found very close to their stars, maybe 10% or even less of the Earth’s Sun distance, and because of this they were called “hot Jupiters” because they were gas-like planets.  When a few of these were found, people tried to study exactly how these planets would form. All of the models require them to form much further away from the star:  it’s thought that they can’t form that close to the star, they actually form much further out, and then somehow they move in.  The process of planetary migration is the process of these planets moving inward closer to the star.  How it happens is thought to be because of interactions with the gas discs.  These planets are very massive, they’re several hundred times the mass of the Earth, but the gas discs from which they’ve formed is many times more massive than the planets, so interactions between these planets that form and these gaseous discs can cause the orbits of the gas giant planets to move inward in time.   They may start much further out, maybe around where Jupiter is now, which is about 5 times the Earth-Sun distance and slowly move inward to their current orbits, which is very close to their stars.  That process is called planetary migration.

In your report [1] you simulate the territorial planet growth during and after the giant planet migration.  What’s happening?

As these planets move inward to their final orbits, very close to their stars, they pass right through the region where planets like the Earth and Venus and Mars are trying to form, which is what we would call the “terrestrial planet region.”  Many previous studies, have assumed that planets simply can’t form if this giant planet ploughs through this region.  What we did was to look in detail at this process of terrestrial planet formation - how these rocky planets like the Earth and Venus and Mars form - then throw in the extra factor of having a giant planet migrate through where these planets are trying to form.  What we found is that as the giant planet ploughs into all this rocky stuff, a couple of different things happen.  One thing that happens is, inside the orbit of the giant planet, some material is actually pushed inwards, so instead of colliding with the giant planet, this rocky stuff actually gets pushed in front of the giant planet as it moves inward.  By the time the giant planet ends up in its position as a “hot Jupiter”, very close to the star, what forms even closer to the star is terrestrial planets that are quite large, rocky planets, maybe 5 times as massive as Earth. We call these “hot Earth” planets.  This is because they are Earth-like, in terms of being mostly rocky, but they’re very close to their host stars, meaning that they’re very hot.  So, that’s one thing that happens, but not all material gets pushed inside; some material gets scattered outward as the giant planet moves in and from that material, planets like the Earth can form on orbits that are pretty similar to the Earth’s.

You state that more than one-third of these giant planets may harbor Earth-like planets.  How did you come to that conclusion?

What we did there was to combine our results with a previous paper I wrote a few months before.  In the context of how terrestrial planets form, giant planets are very important, and the reason for this is both giant and terrestrial planets form from discs of dust and gas around stars.  The trick is, even though the giant planets are bigger, much bigger—they’re ten, one hundred times as massive as Earth-like planets - they actually form faster.  Because they form faster, the final stages in the formation of rocky planets are very strongly affected by any gas from giant planets that have already formed.  By looking at the giant planets, you can get a handle on whether that kind of system could form planets like the Earth. 
What we’ve done is to combine these previous results, which are kind of a study of looking at how terrestrial planets form as a function of where the giant planet is.  All we do is have a disc of small rocky bodies and, in a computer, we simulate the orbits of these small rocky bodies and watch them grow.  They’re all on mutual gravity, as well as one giant planet; so we kind of play around with moving that giant planet’s orbit around and seeing which orbits allow planets like the Earth to form in what is called a “habitable zone”, which is the region around a star where the temperature is right for liquid water to exist on a planet’s surface. 
We mapped out which kind of giant planets’ orbits allow planets like the Earth to form.  These orbits of giant planets can either be kind of similar to Jupiter, which are much further away from the star, or they could be hot Jupiters, which are very close to the star; if you have a hot Jupiter that’s a little too far out, that’s a little to close to this habitable zone, then it’s unlikely to have a planet like Earth actually form in the habitable zone.  It’s kind of a typical spacing between giant and terrestrial planets that form.  We calculated these limits for giant planets’ orbits which allow these terrestrial planets to form in the habitable zone.  Then we took those limits and looked at the known samples of extrasolar giant planets, and based on that, we saw which of the known giant planets met our limits.  And the number is about a third—it’s a little more, about 35% of the giant planets look like they could form planets like the Earth in the habitable zone. 

So what are the next steps of your research?

We’re trying to put things in context.  We’re trying to make terrestrial planets, through computer simulations of planet formation, in a way as realistic as possible. We’re adding in more physical effects and we’re trying to start at an earlier stage of formation of planets instead of starting at these late stages when you already have these bodies as big as the moon.  The hope is to have a theory for how these planets form that is very robust and also starts at an earlier stage so we don’t miss any key parts.  That’s the hope, that’s the long term goal.

Thank you, Sean.

[1] Raymond et al., Science 313, 1413 (Septembre 2006)

Sean Raymond works at the laboratory for Atmospheric and Space Physics at the University of Colorado.

Interview by Gilles Prigent and Thanh Tam Candice Vu.

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