Space Forum / Astronomy / September 2006

Wasn't it  who wrote:

The first extrasolar planet atmosphere detection was in 2001.

http://www.space.com/scienceastronomy/astronomy/extrasolar_atmosphere_01
1127-1.html

http://tinyurl.com/5bfog

Well, to zeroth-order you could start with the cosmic abundances of
these things as a guide. For every 100 atoms of silicon (the way these
things are usually quoted), you have...
340 atoms of neon
310 atoms of nitrogen
10.1 atoms of argon
and 270,000 atoms of helium
I toss in that last to show just how *bad* an approximation this can
be. First, some of these gases willescape (so it will depend on the
size of the planet,and the temperatre). Second, and more difficult to
estimate, is how they get incorporated into the planet in the first
place. If it's not a gas giant (accreting gas directly from the
nebula), then you are only going to get an "atmosphere" from outgassing
the the accreted materials... and what gets accreted depends on both
the temperature at which you're doing this at, as well as the chemistry
(which can be tough to get a handle on). Neon & argon are going to be
hard to build up, because they don't form solids (elementally or as
compounds) at reasonable temperatures. So as to likely mixes, no, I
can't think of any.

As to possible, well, if you figure out a way to dump a
solar-composition atmosphere on a planet, and allow the H/He to escape,
it would seem you could certainly have an atmosphere that was around
half neon.

As an aside, I once heard somebody propose SF6 as a "safe" option (for
a very very VERY dense atmosphere)... if you can manage to get an
atmosphere dominated by almost nothing other than sulfur and florine
(good luck with that, abundance of 0.08 atoms for every 100 of
silicon).