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The planet HD 106906 b was found at 650 AU from its home star, which is not a star much different from the Sun. Fomalhaut b is also a young planet that is orbiting very far away from its sun.

These two planets were discovered mainly because they are still hot enough to be seen in infrared images, and this happens because they are young. I think that old planets in far away orbits would be entirely black with a temperature below 10K, and thus invisible.

Back to our Solar System, some people support the hypothesis that beyond Neptune, some Sun-orbiting non-dwarf planets might exist. The existence of HD 106906 b and Fomalhaut b suggests that this is possible. What are the odds that we find at least one of these planets in the Solar System? If they do exists, how could we find it?

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up vote 6 down vote accepted

If such a planet exists, WISE should observe it. WISE is an infrared satellite that imaged the entire sky. In particular:

  • it was able to detect anything with a temperature above 70-100 K, whereas the coolest known exoplanets are in the 100 K range (see histogram below, taken from the Exoplanets Catalogue);
  • it was able to detect objects larger than 1km up to 3 AU from the Sun, or objects of 2-3 Jupiter masses in a distance up to 7-10 light-years (so roughly up to 440,000 to 630,000 AU, see here and here);

so I would say that its existence is quite ruled out now.

enter image description here

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What are the odds that it exists but have a temperature far colder than that, lets say 10K to 15K? Or if it is a rocky Earth-sized or super-Earth planet? –  Victor Stafusa Dec 16 '13 at 16:13
@VictorStafusa I haven't done the calculations but I don't think such a large object could be so cold. Rocky sized Earth-sized planet possibly could. But you might want to ask that as a separate question. –  gerrit Jul 26 at 14:20

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