How is density determined of extrasolar planets?

I would like to know how the density of the three new extrasolar
planets around (HD 69830) was determined. The press release mentioned
that they thought the outermost planet had H2O.

There is no real evidence for the density of these planets.

The authors know the approximate masses and orbital radii
of the planets. They use a model of planetary formation to
simulate the birth of the planets from the circumstellar disk
around this star long ago. Their models suggest that two
of the planets formed close enough to the star that they
probably contain little icy material, and so might be dense and
rocky; while the third planet formed far enough away that it
may have a substantial icy component, with low density.

I wouldn't trust their model very far at all. I would treat their
statements about the density of the planets as reasonable
speculation, no more.

Michael Richmond

asks:

I would like to know how the density of the three new extrasolar
planets around (HD 69830) was determined. The press release mentioned
that they thought the outermost planet had H2O.

We don't know the density of these planets. In fact we know very little
about them other than their calculated masses, which are Neptune-class,
and their orbital parameters. From these two pieces of information we
can calculate their likely surface temperatures. But even with this
little bit of information we can make some reasonably educated guesses
about the nature of these planets.

As for how the planets are configured, they could be (and very likely
are) gas giants, with no firm surface. But they could also be
larger-than-normal stony iron planets, with a relatively thin
atmospheres. We simply don't know.

I'm not sure that anyone would have said that the outermost planet would
have had liquid water on its surface. If it is a smaller gas giant,
there won't be a surface per se. However, what people are saying is that
based on the temperature of the planet's central star and its distance
from it, the planet lies in the "habitable zone," a theoretical region
that surrounds every star that offers the possibility of liquid water
forming on an Earth-like planet, if it were there.

But I believe that is all that anyone is saying. The third planet lies
in this theoretical habitable zone, but it is not Earth-like enough to
actually support liquid water on any sort of surface.

Wirt Atmar

In article , Wirt Atmar
wrote:
We don't know the density of these planets. In fact we know very little
about them other than their calculated masses, which are Neptune-class,
and their orbital parameters. From these two pieces of information we
can calculate their likely surface temperatures.

We also have some limits on the radius and/ or orientation of the
planets and their orbits. For low values of orbital inclination (compared
to the line of sight) and lowest inferred masses, the likelihood of
transiting is increased. With orbital periods of 8 2/3 and 31.5 days, then
the absence of reports of transiting puts lower limits on the inclination
and therefore the mass.
(I *think* I've got that argument the right way round.)
--
Aidan Karley, FGS
Aberdeen, Scotland
Written at Thu, 25 May 2006 13:30 +0100, but posted later.