Most of us who read up on extrasolar planets know of the theory of epistellar planets that involves a gas giant forming further from its host star and being drawn into a closer orbit, likely via braking from the surrounding nebular material early in its solar system's history (did I describe that right?). However, I've read a lot about the possibility that epistellar planets like 51 Peg b might not be hot gas giants, but rather could be giant terrestrial planets. Such reputable sites as Extrasolar Visions, Sol Station, and Exoplaneten.de (Chris Kullmann's site) have mentioned this "giant terrestrial" theory of epistellar giants in their entries regarding 51 Peg b.

According to the radius-mass relation chart in the Cambrige Guide to the Solar System, a planet the size of 51 Peg b (~150 Earth masses) would have a radius of about 20,000 km (about 3 Earth radii) assuming a similar composition. Of course, given its distance, I'd think that such epistellar planets would likely have a much higher percentage of refractories and/or metallic material than what we find on Earth, so a giant terrestrial might have a smaller radius and higher density.

So what do you think is most likely the case for epistellar planets like 51 Peg b. Hot Jupiters or Giant Terrestrials?

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I think it would depend on the mass of the planet and the star type (OBAFGKM). Since we know Jupiter is a failed star, we have a basis for comparison.
It is quite possible that it is a terrestrial planet, but not enough information is available.

-Kevin

We do? I am not disputing the claim, i just never heard this. I'm gonna research it some.


/edit after some quick research
Not to nitpick, but the "Jupiter is a failed star" may be BA. In Fact to quote this article here on this very site.

and a bit further down

What I take from this is that Jupiter is a planet because it was created in the same manner as planets are created, and not created the way stars are created. therefore Jupiter cannot be a failed star Unless stars have been known to form from the protoplanetary disk around a new star.

I may be way off base so please correct me if I am missing something. I am not by any stretch of the imagination knowledgable about this, and I know better than to believe everything I read.

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This question is a big poser to current planetary science. First, we say that large planets are usually gas giants, but gas giants can't exist very close to a star. These planets are massive and extremely close to their respective stars. I voted undecided because we just don't have the evidence.

The question says "... most likely ... ", which I interpreted to mean, "according to present, prevailing theories on the formation of planets etc".

In the sense of "we don't yet know the mass, composition, radius, etc of such objects", then the answer would have to be "not yet determined".

Darned tootin'! Stars in binary systems tend to be in eccentric elliptical orbits. The orbits of Jupiter and the rest of the Sun's planets* are damn near circular.



*) Purists might argue that I should say "the rest of the Sun's planets except for Pluto," since Pluto's orbit has a pronounced eccentricity. Well, said purists can eat my shorts. Pluto is a KBO. :P

Brady says,

...which seems to suggest that these super-massive planets that wind up close to their primaries can expect to have their atmospheres stripped away, leaving terrestrial (or even super-terrestrial) cores.

Maybe that's what produced the terrestrial planets in this system?

That's a really good point. Are you suggesting that our Earth would have been a gas giant if it wasn't as close as it is? Then the Earth could be a gas giant core!

One piece of evidence that suggests that giant epistellar planets were once gas giants in the outer solar system is the discovery of an evaporating world, HD209458b. (sometimes called Osiris)

This world is losing its atmosphere at a fantastic rate, and has a massive cometary tail composed of hydrogen and traces of oxygen and carbon;
an interesting page about this planet can be found here.
http://vega.lpl.arizona.edu/~gilda/extrass.html

This planet has a thick atmosphere, which it is in the process of losing gradually; I don't think anyone knows how thick it is now, or how lt has been evaporating, but if the planet formed in the outer solar system it would have enough mass in its atmosphere for this evaporation to continue on a timescale of a gigayear or so...

if the planet formed in the inner system it would have contained less hydrogen, and the evaporation would have been more rapid.
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Large terrestrial planets may well form in the inner solar system, but they probably would end up as giant water worlds like my imaginary planet Panthalassa;
this planet is based on the simulations made by the same french team of planetary scientists as are working on HD209458b
http://arxiv.org/abs/astro-ph/0308159


the most interesting type of epistellar world to me would be a giant planet of whatever original type which ahd lost all its atmosphere and become a rocky, redhot, high gravity ball;
these have been called Chthonian worlds.

http://www.universetoday.com/am/publ...ng_planet.html
quote-

The whole evaporation mechanism is so distinctive that there is reason to propose the existence of a new class of extrasolar planets - the chthonian planets, a reference to the Greek God Khtôn, used for Greek deities from the hot infernal underworld (also used in the French word autochton). The chthonian planets are thought to be the solid remnant cores of ‘evaporated gas giants’, orbiting even closer to their parent star than Osiris. The detection of these planets should soon be within reach of current telescopes both on the ground and in space.

Perhaps 51 Peg b is one of these.

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I don't believe the earth is big enough to act as a core for a gas giant, though it is an interesting thought.

Note that Bellerophon (51 Pegasi b) is farther away from its parent star than Osiris is. Bellerophon may be losing atmosphere, too, but at a slower rate than Osiris so that we haven't been able to detect its much thinner comet-like tail yet.

If Osiris is going to take an entire gigayear to lose its atmosphere, it would certainly take longer than that for Bellerophon to lose its atmosphere. Maybe all epistellar giant planets are gas giants losing their mass!

Actually, this isn't too far-fetched. The cores of Uranus and Neptune are not that much more massive than Earth. If Earth had formed at a distance of, say, 15 or so AU, it could've ended up like them, assuming that the core accretion model is the correct explanation for the formation of the "ice giants." Of course, details about ice giant formation are uncertain, so maybe an Earth-mass terrestrial planet could exist by itself in the outer parts of another solar system.

Interesting thought. However, the cores of these jovians-turned-terrestrials would have to be very big. Jupiter's core is some 10 to 15 Earth masses. Assuming that any one of these epistellars have been completely stripped of their atmospheres, this would mean that the remaining terrestrial body would be in order of several times the mass of Jupiter's core. About 13 epistellars within 0.1 AU are less than Jupiter's mass (~317 Earths), and about 9 of those are less than 0.5 Jupiter masses. HD 49674 b, the lightest epistellar, is about 38 Earth masses, 51 Peg b is about 150 Earths, and upsilon Andromedae b is about 230 Earths. As I mentioned in my earlier post, this would put their radii in the order of 2.5 to 3 times that of Earth if they were composed of rock and metal (a ~20,000 km radius is about as big as a terrestrial can get; they just get denser the bigger they get, assuming that there are terrestrial hundreds of Earths in mass).

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And don't forget -- those "masses" we've determined for these epistellar giants are minimum masses. If their orbital planes are inclined somewhat from our perspective, so that we're not seeing them edge-on, their real masses will be higher.

Well, if 51 Peg b is 150 x Earth M and is not as close as Osiris, on balance it seems likely to be a gas giant which has migrated inwards and retains some or most of its atmosphere.

If it were a terrestrial planet it would have a ferocious gravity and it would retain a very thick atmosphere anyway, which would comprise a respectable fraction of the total mass;

there may be a class of large inner solar system warm Jupiters which we have not yet detected (unless that middle one at 55 Cancri is an example)

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