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Many of the extrasolar planets to date are Jovian sized planets that orbit very very close to their parent star. Traditional planetary formation models say that it is extremely unlikely (if not impossible) for them to form there as there is not enough material to make planets of that size.

What is the mechanism or mechanisms responsible for getting a Jovian style planet this close to the star?

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New research is suggesting that solar systems may be inherently unstable. You may have read recently that wandering inter-stellar planets may be more common than stars. The prediction is that planets frequently become ejected from their solar systems early in their formations, and wind up wandering deep space forever.

There is a complicated interaction within (and in some cases outside of) a solar system, will all the planets tugging on each other, affecting each other's orbits in subtle ways--and nearby stars may also influence the stability of a solar system. Since ejected planets seem to be so common, it would stand to reason that these planets are crossing each other's orbits at high speeds--migrating inwards toward the parent star, and throwing others out of the solar system.

It's my understanding that planets migrating inwards is the reason for the large number of hot jupiters and wandering planets.

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If we take the notion that there is not enough matter to form a planet in such a close proximity then the only method that planet reaching the close circumstellar orbit would be the migration. The migration of the Jupiter sized planet could be caused by orbit evolution or some other factors.

On the other hand if one would take the model of young stellar object with its circumstellar envelope and rapidly accreting disk there is a possibility to create Jupiter sized planet so close to the star. The current understanding of this is when the circumstellar envelope starts to collapse it could create instabilities in the accreting disk creating condensations. These condensations usually would accrete into the star causing some seizable brightening effects. But they could be captured in the stable orbit where they might grow and merge with similar formations creating Jupiter sized planets as a result.

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This is by no means a settled question; rather it is at the forefront of exoplanet research. There are several ideas out there, and I'll list a few.

  • Migration via drag: In young stellar disks, there may be a bunch of leftover gas that can produce a drag force on planets. By the way, there is a subtle reason this works at all. Because gas will have some pressure support, it will rotate at sub-Keplerian velocities, and so any Keplerian planet will be slowed by it. However, the problem here is the rate of migration. Gas drag is very good for migrating centimeter- to kilometer- sized objects (too good in fact - another open question is how planets build up before all the protoplanets are dragged into the star), but not necessarily for Jupiter-mass things.
  • Scattering: You can imagine some multi-body interactions where a soon-to-be hot Jupiter gets close to another object, they interact strongly (compared to the effect of the star), and the other object is pushed away. Conservation of energy tells you the hot Jupiter must fall deeper into the potential well. The problem with this is your potential energy needs to decrease a lot to go from past the ice line (where certain gasses condense into ices thus aiding planet formation - we believe Jupiter-mass planets cannot form interior to this) to right next to the star. A single scattering event seems unlikely to transfer this much energy.
  • Tidal damping: Now introduce ellipticity. Perhaps the semi-major axis of the orbit is large, but something affects its angular momentum, putting it into a highly elliptical orbit. With every periapsis passage, the planet could loose orbital energy to tidally dissipated heat as it and the star are stretched and compressed in various directions. Under the right conditions, this can liberate a large amount of energy (see Io, for instance, whose great degree of volcanism is due entirely to tidal interactions with Jupiter). Over time, the orbit circularizes into something small.

There are other ideas out there, ranging from "perhaps they were just captured like that" to "perhaps two stars collided and this is what happened" (not unlike the formation of the Moon in the collision scenario). Likely some combination of these ideas will be needed. For example, scattering could be the mechanism that introduces the ellipticity needed for tidal damping.

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There is mounting evidence from the Kepler mission that these hot Jupiters migrated in by scattering other planets out. Of the 400-odd systems with multiple planets, almost none of them have a hot Jupiter. Statistically quite significant.

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