Kepler-78b

[Dimitar Sasselov] "It couldn't have formed further out and migrated inward, because it would have migrated all the way into the star [by now]."

It is statistically unlikely, but obviously possible, that we got 'lucky' and caught it just before the final plunge. Even if it did not happen this time, sooner or later it will.

"... [it] couldn't have formed in place because you can't form a planet inside a star."

If we suppose that planets can form in a manner similar to that suggested in the Fission Hypothesis, then one could also say that this formation process is very much like a planet forming inside a star.

Or rather, inside a proto star - IOW before the star has finished collapsing to it's 'final' size. So this looks like it is not a result of over spin and fission. Unless we got lucky again (statistically unlikely) and caught it just after the final over spin event.

In this case we would be seeing a star that has just graduated from proto star to star. We would expect there to be two planets of roughly equal mass. So the orbital altitude and period might be other than suggested by assuming that only one body is present. But the two would have to be exactly opposite to keep us from detecting this for more than a few dozen orbits.

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Nice news catch, but it seems unlikely.

Thanks,
LB

Could it be an escaped moon of a supergiant planet?

Possible, but if so then where is the gas giant? An escapee in such circumstance is unlikely to move very far from the parent (in terms of radius from the star).

There are some theoretical reasons to be skeptical of of the large number of hot Jupiter discoveries. (Two smaller planets farther out can, under some conditions, cause the parent star to wobble in a way that mimics a single larger planet that is closer.)

Of course, there is a lot of statistical crud involved in such speculations. We really need to go there and watch for a while before we start knowing with any certainty.