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goenitz
2012-Sep-12, 11:21 PM
I was wondering if astronomers have any idea what a maximum radius of a rocky planet could be and still remain earth-like (liquid water, continents, oxygen-nitrogen atmosphere, etc) Moreover, is there a formula or method that scientists use to figure out a relationship between mass/density/ radius and composition of a planet? For example, is there a dividing line where above a certain limit, a planet of a given mass would have to be a water world?

swampyankee
2012-Sep-14, 12:26 AM
For the first question, that's going to depend on the details of the model of planetary formation. For the second, there are some papers on arxiv.org

http://arxiv.org/abs/1206.3303 ("Characterization of exoplanets from their formation II: The planetary mass-radius relationship")

http://arxiv.org/abs/1102.4146 ("Theory of planet formation and comparison with observation: Formation of the planetary mass-radius relationship")

http://arxiv.org/abs/1001.4851 ("Mass-radius relationships for exoplanets")

http://arxiv.org/abs/0902.1640 ("A Study of the Accuracy of Mass-Radius Relationships for Silicate-Rich and Ice-Rich Planets up to 100 Earth Masses")

....

and more

At some mass -- probably somewhere over 500 Earth masses -- the planetary core will become degenerate.

PlutonianEmpire
2012-Sep-14, 03:18 AM
For the first question, that's going to depend on the details of the model of planetary formation. For the second, there are some papers on arxiv.org

http://arxiv.org/abs/1206.3303 ("Characterization of exoplanets from their formation II: The planetary mass-radius relationship")

http://arxiv.org/abs/1102.4146 ("Theory of planet formation and comparison with observation: Formation of the planetary mass-radius relationship")

http://arxiv.org/abs/1001.4851 ("Mass-radius relationships for exoplanets")

http://arxiv.org/abs/0902.1640 ("A Study of the Accuracy of Mass-Radius Relationships for Silicate-Rich and Ice-Rich Planets up to 100 Earth Masses")

....

and more

At some mass -- probably somewhere over 500 Earth masses -- the planetary core will become degenerate.
Terrestrials with 500 Earth mass, or gas worlds with that mass?

On a related note, I've always wondered what happens to the rocky cores of super-jovians. On another note related to THAT, do brown dwarves form like stars do (direct from nebulaic gas), or like jovians do (core-accretion), or does it depend on the situation? If a brown dwarf formed via core-accretion, back to my second question, what happens to the rocky core?

And how might one visualize all that in their head?

swampyankee
2012-Sep-14, 10:19 AM
Terrestrials with 500 Earth mass, or gas worlds with that mass?

On a related note, I've always wondered what happens to the rocky cores of super-jovians. On another note related to THAT, do brown dwarves form like stars do (direct from nebulaic gas), or like jovians do (core-accretion), or does it depend on the situation? If a brown dwarf formed via core-accretion, back to my second question, what happens to the rocky core?

And how might one visualize all that in their head?

First question: any planet.

Second: Depends on planetary mass. Jupiter is probably too small to have its rocky core become degenerate, but some of the super-jupiters may be large enough.

eburacum45
2012-Sep-14, 11:46 AM
You might like to explain what you mean by degenerate in this context. Even the Earth, the Moon or you yourself are ever so slightly degenerate, in the physical sense - that is to say, your atoms are compressed more than they would be if they were in free fall and subject to no pressure.