The hypothetical ninth planet of the Solar System could shine brightly.
Planet 9, if it exists, is thought to be an ice planet that is slightly smaller than Neptune, orbiting in the far outer Solar System. Esther Linder and Christoph Mordasini of the University of Bern in Switzerland modelled the evolution of the planet's probable internal structure. On the basis of its estimated mass and location, they conclude that the planet is still giving off residual heat that was generated when it was formed. This would cause the planet to emit light in the mid- and far-infrared range.
The authors say that future telescopes, such as the Large Synoptic Survey Telescope, or dedicated surveys should be able to detect Planet 9 — or rule out its existence.