While investigating planet formation (to justify support complement a PhD proposal on ocean planets), I found this discussion over at physicsforums following a blog entry by Dr Steinn Sigursson. (He uses Planet X in his posts but that is too frequently linked with the dwarf planet Xena Eris, and besides, Pluto has been demoted, so its Planet 9 we're looking for ).

The question is, how was our solar system created ?

"Looks like the outer solar system, with late heavy bombardment, would have come together nicely if there was another Neptune out there to begin with."

Basically, it appears as though there is a planet or substellar companion star out in the Oort cloud. This is not likely to be the Nemesis object of Hut's/Muller's theory, but would have been responsible for the Kuiper belt. Most of the discussion is about brown dwarfs, but following links leads to a paper by Gomes et al., simulating the possibility of outer Neptunes or Jupiters at 4000 AU or 5000 AU. The thread is all about the dynamics of such bodies, but what would they look like?

• How bright would they be? Not very. Grabbing some used envelopes and the usual definition, we get magnitude 27 for a Jupiter at 5000 AU, or 29 for Neptune at 4000 AU. The limiting magnitude for Hubble is about 30; finding one would be hard at those brightnesses. This is from reflected (visible) light from the Sun (solar irradiance at 5000 AU is down to 5 uW / m2 !); perhaps it might radiate a bit more from internal heat in the infrared.
• Whats the temperature? At the surface, this depends on how much sunlight it absorbs and how much it reflects: its albedo. Using albedos of 0.3 ( typical for an asteroid, "dirty snowball; see eg. Delbo et al.) , 0.39 for Earth, or 0.8 for clean ice, we get effective temperatures of 2.5 to 3.5 Kelvin ( -270 ° C)

Now, Hydrogen freezes solid at 14K, Helium is liquid at 4K. So we're talking about a liquid helium ocean on top, with no atmosphere worth speaking of.

But Could it sustain life?

Not as impossible as it seems. Ehrenreich and Cassan point out that an Ice Giant could host a liquid layer underneath an ice shell surface, heated by radiogenic energy, that could last for billions of years before freezing.

So where would it be? This is the time to submit a grant proposal for more envelopes ...