What Is Lurking in the Outer Reaches of Our Solar System?

Artist’s conception of Planet Nine. Credit: Caltech/R. Hurt (IPAC)

The Solar System has eight planets. In 2006, astronomers reclassified

The discovery photograph of Pluto found by Clyde W. Tombaugh of Lowell Observatory in 1930. Astronomers today suspect that there might be a previously unknown Planet 9 in the distant solar system, but a new search at millimeter wavelengths has failed to find any convincing candidate. Credit: Lowell Observatory; Tombaugh

This speculative “Planet 9,” according to estimates, would be about 5-10 Earth-masses in size and orbit about 400-800 au from the Sun. A planet at this distance would be extremely difficult to spot in normal optical sky searches because of its faintness, even to telescopes like PanSTARRS and LSST. Most solar system objects were discovered at optical wavelengths via their reflected sunlight, but the sunlight they receive drops as one-over-their-distance-from-the-Sun squared; moreover, the reflected portion then travels back to telescopes on Earth and so declines again by a similar factor.

In the outer reaches of the solar system these objects, although cold, might emit more infrared radiation than the optical light they reflect, and astronomers in the past have used infrared surveys like the Wide-field Infrared Explorer (WISE) to search, but without success.

DOI: 10.3847/1538-4357/ac2307