Cosmologists in Search of Planet Nine: the Case for CMB Experiments
Cowan et al
Cosmology experiments at mm-wavelengths may be able to detect Planet Nine if it is the size of Neptune, has an effective temperature of 40 K, and is 700 AU from the Sun. It would appear as a ~30 mJy source at 1 mm (or ~8 mJy at 150 GHz) with a parallax of ~5 arcmin. The challenge will be to distinguish it from the ~4000 foreground asteroids brighter than 30 mJy. Fortunately, asteroids can by identified by looking for sources that move across a resolution element in a matter of hours, orders of magnitude faster than Planet Nine. If Planet Nine is smaller, colder, and/or more distant than expected, then it could be as faint as 1 mJy at 1 mm. There are approximately 1E6 asteroids this bright, making many cosmology experiments confusion limited for moving sources. Nonetheless, it may still be possible to find the proverbial needle in the haystack using a matched filter. This would require mm telescopes with high angular resolution and high sensitivity in order to alleviate confusion and to enable the identification of moving sources with relatively short time baselines. Regardless of its mm flux density, searching for Planet Nine would entail obtaining frequent radio measurements for large swaths of the sky, including the ecliptic and Galactic plane. Even if Planet Nine had already been detected by other means, measuring its mm-flux would constrain its internal energy budget, and therefore help resolve the mystery of Uranus and Neptune, which have vastly different internal heat.