Are Small Rocky Worlds Naked Gas Giants?

astroengine writes "The 'core accretion' model for planetary creation has been challenged (or, at least, modified) by a new theory from University of Leicester astrophysicists Seung-Hoon Cha and Sergei Nayakshin (abstract). Rather than small rocky worlds being built 'bottom-up' (i.e. the size of a planet depends on the amount of material available), perhaps they were once the cores of massive gas giant planets that had their thick atmospheres stripped after drifting too close to their parent stars? This 'top-down' mechanism may also help explain how smaller worlds were formed far from their stars only to drift inward toward the habitable zone."

Comments

Re:Hot Jupiters?

[Anonymous Coward]

I think the general idea is that these would in fact be proto-planets. Essentially any of these gas giants rotating close to their suns could evolve into an Earth like planet. The question would be how old are the systems that we are finding these gas giants close to their stars? If they are all old systems then I doubt the theory will hold up. If they are mainly younger systems then it's possible. There has to be more than one way for planets and moons to form if this theory is true because the theory wouldn't explain the Earth's moon as well as most other moons. It would explain iron cores and could potentially explain the differences between the Earth and Mars if say the Earth started out as a gas giant and Mars was made up of asteroid like debris. Earth having an iron core and Mars having a less defined core. Mars has a lot of surface iron but lacks a strong magnetic field so it lacks a large iron core. If the near star gas giants are proto Earths then it raises the likelihood of large numbers of Earth like planets since near star gas giants are so common. Just look at older systems for Earth like planets.

Re:Trivial?

[Teancum]

How does this square with the idea that gas giants have materials like metallic hydrogen at their cores? I can see how accretion can occur without the necessity for a rocky core, and I can see how it would occur with one.

There isn't necessarily a dilemma here as the theory still can be consistent for both results, in terms of rocky "terrestrial" bodies being naked gas giants and "traditional" gas giants still having a metallic hydrogen core.

Planets like Jupiter and Saturn, while certainly the bulk of their current mass is Hydrogen and Helium, they do have other elements that comprise their structure and more than likely you would find at the core of these planets a "rocky" core that would include Iron, Nickle, and other elements that would be more identifiable with what we have here on the Earth. The question would be how large would that tiny "rocky" core would be if you stripped off the outer gaseous layers.

The problem with this theory is mainly how you go about stripping off that outer layer. The presumption here is that objects close in to a star like the Earth, Mars, and Venus are at the moment would have had this outer "shell" being stripped off at some point in the distant past as the Solar System was being formed.

Highly unlikely theory

[gstrickler]

As this calculation for CoRoT-2b indicates, at 6M tons per second, a hot super-Jupiter would need more than 39B years (~3x the age of the universes) to be "blown/boiled away". Jupiter is ~1/3 the mass of CoRoT-2b, so at 6MT/s, it would last 13B years. The rate of loss of atmosphere would have to be at least a factor of 10 greater than on CoRoT-2b, or greater than 60MT/s just for a Jupiter mass planet to to reach an Earth mass core in 1.3B years. Our solar system is estimated to be ~5B years old and that Earth and Mars both appear to have been rocky for more than 2B years, so 1.3B years to blow off an atmosphere seems to be a generous estimate of quickly it must have happened.

Given that our sun is only converting ~600M tons/sec of hydrogen into ~594M tons of helium, a net loss of 6MT/s, therefore a Jupiter mass planet would need to be receiving a enough of the solar radiation to blow off 60MT/s. Yes, E=mc^2, and c^2 is large, but you're still talking about a lot of mass to move out of notable gravity well (first out of the Sun's gravity well, then move more mass out of Jupiter's gravity well). If jupiter were in earth's orbit, would it receive enough solar radiation to lose 60MT/s? Not from solar wind, the total solar wind mass is ~1.85MT/s. even if all 1.8MT were directed at Jupiter and Jupiter had no magnetopause to protect it from the solar wind, 1.8MT/s would not strip 60MT/s of atmosphere. So you have to come up with a theory where the EM radiation causes the the planet to eject it's own atmosphere, which is still going to be virtually impossible.

Re:Highly unlikely theory

[Teancum]

The Sun (Sol in Latin and derivative languages) is more like a 3rd or 4th generation star in terms of material recycled from previous stars that has gone through supernovas and reformed to become new stars. At least that is where exotic elements like Uranium, Gold, Silver, and just about everything heavier than Oxygen have come from.

I presume the objection here is that 1st generation stars (which at this point are very old stars which likely have had any planets around them ripped off simply by passing near other stars on any journey they have made going around the galactic core or even a small globular cluster) would behave differently than something you would see around the Sun. Certainly compounds more exotic than water would be quite rare and even water would be minor.

Still, of the planets that are being considered with this model, I think the GP post is fairly on spot in terms of skepticism on this theory. EM radiation alone is unlikely to be able to provide the energy needed to strip gas giants of their atmosphere, where I think you would need some kind of gravitational actor as well. The problem with that theory is it introduces a 3-body problem and requires an explanation for where that object went, whatever it was. The 3-body problem is a big deal because at the very least any planet would likely be in a highly elliptical orbit where the presence of that gravitational anomaly would leave some evidence behind. I don't think that is necessarily a good idea either.

The environment in a stellar nursery would be rather complex, where perhaps a "nearby" neutron star emitting x-rays and other complex aspects of the environment might also be a factor. The ignition sequence of what happens with a star finally starts the fusion process could also be a factor here, where there might be some added complexity in the protostar cloud before the star finally settles down into a stable main sequence pattern. A brief (on the scale of a typical star's lifetime) period of intense radiation and/or stellar wind when this ignition starts might be something to consider. Current theory suggest this is a rather benign event where gravity merely starts compressing the gasses that gradually start producing more fusion before it becomes stable, but that might be mistaken. For small stars (stellar class M objects, for instance) that may be the case, but larger stars certainly do have their own peculiar life cycles anyway so the "birth" of a large star might be nearly as dramatic as its death, just as the death of class M stars is rather wimpy too.