I have heard that some planets "migrate" in towards their star. How does this work?

Hello Troy,
I am going to guess at the answer to your first question, then pass it on
to my officemate, Geoff Bryden, who models protoplanetary disks and
early planet migration. He may be one of the few people in the world
who has a good idea of what is happening at this time.

I believe the migration of a giant planet would occur on a timescale of
tens of millions of years. Certainly, less than a billion years. Since
we are talking about an early stage in planet formation, the remnant disk
will be full of junk (gas, dust, asteroid-type bodies, planets). That
means that collisions will be common (as evidenced by geologically old
cratering on solar system bodies like the moon), and I am guessing that
it is collisions that destabilize the orbit. The planet around 51 Peg
is almost undoubtably the last of a series of planets that did not
survive - that is, there were likely planets that migrated ahead of this
planet and plumeted onto the star. The reason that 51 Peg survived was
that the number and mass of the colliding objects had finally tailed off.
We are trying to think of ways to look for signatures of these lost
planets right now!

The reason that I believe the planet has a massive atmosphere is
1) any planet with a lot of mass also has a lot of gravity - that
means that it can hold onto it's atmosphere more efficiently.
2) The planets condense from the disk around a star. That disk is made
up of the same stuff the star is made out of - i.e. there is plenty of
hydrogen and other light gases as raw material in the disk.
3) If you assume the planet is completely rocky - with only a thin or
shallow atmosphere, then the rocky material would account for almost
*all* the rocky material in the protoplanetary disk. Physical processes
are just not that efficient. There is bound to be a lot of rocky material
that "got away" - likely in the form of other planets that fell into
the star.

Glad to discuss your interesting questions about a topic that is near and
dear to me!

Good luck, I will forward your question and this email to Geoff, and
see if he has anything to add!

Cheers,
Debra Fischer

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