I’ve been reading a couple of articles about the discovery of water in the atmosphere of an extra-solar planet. Yes, exciting and all but the thing that startled me the most in the article is that this is apparently a gas giant in a 3 1/2 day orbit around its sun with an orbital radius of only 4 million miles. (That’s about what, a tenth of Mercury’s orbit?)
I admit to being surprised that it is so close; I would have guessed that tidal forces would have torn it apart at that distance (or prevented it from forming in the first place). Or, would it have to get a lot closer still before that became a problem? Is this something that would be expected from what we know of planetary formation, or is this planet interesting for reasons other than the fact that it has water?
In theory the planet will deform and break up inside the Roche Limit. What that distance is depends, among other things, on whether or not the planet is rigid.
There isn’t any one definite answer to the question. A satelite held together only by its own gravity gives one answer. One that has considerable tensile strength in addition to gravity holding it together gives another, etc.
In theory the planet will deform and break up inside the Roche Limit. What that distance is depends, among other things, on whether or not the planet is rigid.
There isn’t any one definite answer to the question. A satelite held together only by its own gravity gives one answer. One that has considerable tensile strength in addition to gravity holding it together gives another, etc.
However, any planet-sized body has very low tensile strength; it is held together by gravity.
But the Roche limit for a rocky planet around a star is usually inside the star’s radius (see the Wiki page cited above), since stars usually have low densities.
So 4 million miles, though close, is still well outside of the Roche limit for a star similar to the Sun. (If I’m doing my conversions right, from the linked Wiki page it looks like the Roche limit for the Sun is around 400,000 miles so this planet could get a lot closer.)
I’m sure it must be fairly distorted in shape though.
Which still leaves the second half of my question. My (admittedly vague) understanding of planetary astronomy was that the reason the inner planets are small and rocky and the outer planets are large gas giants is that, when the solar system formed, the Sun swept up most of the mass in the inner system and, when it ignited, the solar wind blew most of the lighter elements into the outer system. So, would we expect to see a gas giant that close in to its sun?
Well, we know there are lots of very close very massive planets, which was a pretty suprising finding. However this doesn’t mean that very close very massive planets are typical, just that these are the easiest extrasolar planets to detect. But current thought is that these are gas giants that formed farther out and then migrated towards the sun through some process, like coming close to and ejecting bodies in closer orbits. Or it might turn out that these close superjovian planets aren’t gas giants after all, but something else.
That was our understanding of the formation of planets until circa 1995, when the first “hot Jupiter” (51 Peg b) was found, and many similar planets were found soon after. The current theory is that hot Jupiters form at a distance from their star similar to Jupiter’s distance from the Sun, and migrate inward toward their star later.
51 Peg b is currently thought to be a gas giant planet, not a terrestrial planet. It is massive enough to resist the solar wind tearing its gas away. We don’t know if it’s distorted, or if so, how it is distorted- we don’t have any pictures of it. Any pictures you’ve seen are artist’s renditions that could turn out to be wrong.
We don’t know what limits there are on how close a planet can get to its star, other than the Roche limit. I would assume that, if they were too close in, they could be vaporized by the star. I don’t know of any calculations that have been done to figure out just how close to the star a planet could be, though. I considered doing my master’s thesis on this topic, but didn’t.
But we know that hot Jupiters do exist around a fair number of stars. Even if solar systems like ours turn out to be much more common, that means we know that hot Jupiters form in some not-too-improbable variation on the planet formation process. If we had just found one or two of them, we could consider really improbable causes, like near-collisions between stars. But we’ve found quite a few of them (there are over 200 planets outside our Solar System currently known, but I’m not sure how many of those are hot Jupiters), so an explanation like that won’t work.
Heat would gasify a planet before it got that close.
Solving the black body equation, Ts X (Rs/2D)1/2 = Te, here, for the distance from the sun at which iron will boil (T=3,000°K) gives a value of 1,370,000 km. That’s 674,000 km above the sun’s surface.
We found something that we didn’t know about before that our theories said could not be, so we modified our theories to be compatible with the new data. It’s a good example of the scientific method at work.