In article ,
Alain Fournier wrote:
James Nicoll wrote:

In article ,
Dan Birchall wrote:

(Alain Fournier) wrote:

What surprises me is its density, about 1870 kg/m^3, which
is consistent with 75% water and 25% rock. I'm no expert
in this area, but so much water (or other low density
liquid or solid) seems improbable to me.

I'm not a planetary scientist either, but I googled, and this
density is roughly comparable to that of our outer planets -
Neptune's density is 1760 kg/m^3, Pluto's is 2030 kg/m^3. The
planet's orbital radius and temperature would probably help shed
light on what elements would most likely be found there in solid,
liquid or gaseous forms.


The six most common elements in the universe are hydrogen,
helium, oxygen, carbon and neon*. If a world isn't massive/cold enough
to hold onto H2 (and given that neither helium nor neon will form
chemical compounds) the most common chemical compounds should
be made up of combinations of H, O and C. In other words, water
should be pretty common.


* From memory, by mass it's something like 70% H, 24% He, 11% O, 4.5% C
1% Ne.

But this planet is near to its star and not very big. Close to the
star water not already on a planet there is likely to be a gas and
blown away. So I have a hard time imagining water accreting to form
a planet there. I would expect that you would need a big rock
core before it can hold on to water. Being so close to the star
the hypothesized 25% rock seems barely enough. Mars has a hard
time holding on to its water and it is much colder, Venus didn't
hold on to much water. I guess the planet could have been formed
in two steps.
1- The 25% rock forms a nucleus (that is a little more than Earth's mass).
2- The 75% water can now accrete to the nucleus.
But I have a hard time imagining the water staying in low stellar orbit
long enough for that to happen, wouldn't it be blown away?

My impression is that worlds like these form beyond the frost line and
then migrate. In our solar system, there's reason to think Uranus and
Neptune have migrated outwards but in other systems, large worlds can be
seen in orbits where it appears unlikely they could have formed. Presumably
they migrated in.

A popular account of this process can be found he

http://en.wikipedia.org/wiki/Planetary_migration
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