View Full Version : Epistellar Neptunes

Tom Mazanec
2014-Dec-04, 07:22 PM
How do they differ, atmosphere and core, from our system's ice giants? How are they alike?

2014-Dec-05, 03:21 AM
I don't think we have enough data to infer much about the details of the interiors of these exoplanets.

2014-Dec-05, 05:56 AM
It would be a very, very cold world with only heat from the masses of their cores.

2014-Dec-05, 11:53 AM
It would be a very, very cold world with only heat from the masses of their cores.

I believe that Tom is referring to 'hot neptunes', planets that are close to their star and are therefore hot throughout. Epistellar is a word sometimes used to describe planets near the star, as in epistellar terrestrials and epistellar terrestrials.

The first thing that would be different is that a hot Neptune would have less hydrogen than a planet with a similar original composition that exists out in the very cold depths of the solar system. The local star would heat the planet and cause hydrogen to escape, depleting the atmosphere of the world.

Although most depictions of the interior of Neptune simply make a distinction between the outer hydrogen/hellium atmosphere and the inner water/ice mantle, these layers will themselves be differentiated by pressure and depth; at some point the hydrogen/helium atmosphere will become liquid under pressure, and the water/ice mantle will have several phase changes due to depth; each of these phase changes will occur at deeper levels in a hot neptune than a cold neptune, because of the temperature/pressure relationship.

But many hot neptunes will probably be radically different inside to our own familiar Neptune; they could contain considerably less water, and/or considerably more rock. Some might even contain large quantities of other elements, such as carbon and neon. I would be prepared for almost anything in the planetary zoo, within reason.

See the Sudarsky classification for some idea of the external appearance of these planets, although there may be many factors that affect this.


Tom Mazanec
2014-Dec-06, 01:10 PM
Yes, I was referring to Hot Neptunes (which should be called Hot Uranuses, if you go by discovery of the first example. Although we never will, even in Futurama the planet was named Urectum :-) )