Extrasolar Gas Giants?

[Dano32839]

So far the only extrasolar planets we have found are all gas giants.

My question is WHY do they think they are all gas giants? Most of them are very close to their star. Wouldn't the solar winds blow the atmostphere of the gas giant away?

Is there any reason that giant TERRESTRIAL planets cannot exist? We do not have them in out solar system, but does that mean they cannot form?

Just curious, thanks.

Dan,

 

[MeteorWayne]

That's not correct. A variety of planet types have been discovered.

Giant planets close to the star are the easiest to detect using the earliest method, the doppler shift of the star's light by the tug of the planet.

Other methods have found big objects further away, some of which are not gas giants.
Here's a good site to look through for the types of planets and methodology.

http://exoplanet.eu/catalog.php

Wayne

 

[Dano32839]

You are missing my point.
Why do they think they are gas giants? Is there a REASON that Terrestrial giants cannot form?
Or do they just ASSUME that they are gas giants because they are massive?

 

[BoJangles2]

I'm no expert on this however, no one really knows if you can make a rocky giant planet as we have no analogue for this in our solar system. As far as we know, the maximum size of a terrestrial planet is somewhere between one and ten Earth masses, or between the mass of the Earth and the mass of Neptune.

Though In my opinion the only limiting factor for terrestrial planet size is the ratio of material available at formation and environment/gravity within the system it forms, i.e. that’s to say when a star is formed the elements seem to be sorted like a big centrifuge. The elements which make gas giants are more common, therefore there is more probability for large gas giants to form opposed to super-earths.

Check out the video below I just added to the Lectures and Documentaries thread, it talks about planet formation and has simulations which touch on your question.

Stellar Simulations: searching for habitable planets

I hope this helps.

 

[vogon13]

Even an undense gas giant close to a star would have a Hill Sphere higher than the top of it's atmosphere. (and even if some 'leakage' occurred, recall we have a large volume of gas to dissipate) If there was a circumstance of an "evaporated"
gas giant,(and it probably happens) we would not be detecting that one, just the ones that are still around. (think about it)

 

[doubletruncation]

There are some ~58 known transiting planets (some of these may be false positives, but the majority are probably real planets). For these planets we can measure both the mass and the radius. All but two of them have periods less than 10 days (and would be considered "Hot Planets"). All but four of these planets appear to be gas giants (based on their mean densities). The four that might not be gas giants are the four least massive planets known to transit their stars. This includes: Corot-Exo-7b which (if it's a real planet and not a blend of some sort) is a super-Earth, and could either be a terrestrial planet or an ocean planet. Then there are GJ436b and HAT-P-11b which are Super-Neptunes, the models that best-fit these planets have very large cores taking up ~90% of the mass of the planets with a thick gas envelope on top. Finally there is HD149026b which is somewhat more massive than Saturn, but has a radius that is only ~80% that of Saturn. The high density for this planet suggests that it has a huge core with something like 70 Earth masses of material. Above this there are a few other fairly dense planets (like HAT-P-3b), but most of them have radii similar to Jupiter, or slightly larger than Jupiter. One fun fact to note is that models for planet/brown dwarf/stellar structure predict that objects ranging from 1 jupiter mass up to ~100 jupiter masses (the smallest stars, which are roughly 10% the mass of the Sun) all have roughly the same radii (1 R_J). This is observationally confirmed for planets and for the smallest stars, and for a few "brown dwarfs" with masses of 10-20 jupiter masses. Anyway, so you might wonder if there is an observational bias that prevents us from finding jupiter mass solid cores orbiting very close to stars. Well, it's true that they would be quite a bit tougher to detect in transit than the gas giants (though Corot would have found them fairly easily). However, people have done careful statistics on the results of a few of the transit surveys to determine the fraction of stars like the Sun that have Jupiter-size planets with periods less than ~5 days. It's roughly ~0.5%. The radial velocity surveys (which would not be biased against finding terrestrial Jupiter-mass planets since they are not sensitive to the radius) find a comparable Hot Jupiter frequency. So I think this shows that if there are terrestrial Hot-Jupiters, they are not the norm, most Hot Jupiters are in fact gas giants.

 

[AceSuperVillain]

The truth is probably that most of the planets we've detected are probably things we've never dreamed of. Hot Jupiters and Pulsar Planets were never expected even in Science Fiction, and I wouldn't be surprised if the rest of it is equally unimaginable.

As far as whether or not a 'Rock Giant' might exist, it would have to be really really really really massive to cram an entire Jupiter sized ball full of rock, probably to the point where it would have to be a brown dwarf instead of a planet. Bear in mind that all of the solid planets and all of the asteroids wouldn't even half fill Jupiter (you could probably cram the Kuiper Belt in there too), and Jupiter with its current mass is heavy enough to displace Sol's barycenter outside of the solar radius on its own. In order to even get that much rock, you'd have to steal from the debris disk of another star.

So while that logic may or may not be true, that is the why we assume that very massive planets are gaseous. It is not just because all the massive planets in our system are gaseous.