Can the WISE telescope detect black holes?

Question

Black holes are hot, aren't they? With its infrared scan, could the WISE telescope also detect a black hole? The hypothetical planet beyond the Kuiper belt could actually be a primordial black hole. The WISE telescope might already have imaged the object but could it have done it also if it is a black hole?

Answer 1

Black holes are black. They are only observed directly by telescopes if they are accreting matter. Any radiation observed comes from the matter surrounding the black hole. Generally speaking, the smaller the black hole, the hotter the accreted material becomes. For something of planetary mass, one might expect X-rays and Gamma rays from accreting material.

A non-accreting black hole would produce no signature at any wavelength and there is very little material to be accreted in the outer solar system, except perhaps a small possibility of seeing X-rays if something like an Oort cloud object interacts with the black hole.

It is possible that such a black hole could be detected via its lensing effect on light coming from distant stars, but since the black hole would be very small as seen from Earth, this is unlikely, and WISE would not be the instrument to do that.

EDIT: Just to prevent any confusion. "Naked" black holes are hypothesisd (but not yet observed) to produce Hawking radiation. Planet X, if it exists, is thought to be about ten times the mass of the Earth. Such a black hole would have an event horizon about 20 cm in diameter and would emit Hawking radiation with a luminosity of $10^{-20}$ Watts at a blackbody temperature of around 0.002 K. This is not going to emit any detectable infrared radiation.