An Etymological Dictionary of Astronomy and Astrophysics

A hypothetical large planet in the far outer → solar system the gravitational effects of which would explain the unexpected orbital configuration of a group of → trans-Neptunian objects (TNOs). Trujillo & Sheppard (2014) noticed a clustering of the → argument of perihelion of bodies lying beyond ~150 → astronomical unit (AU), and attributed this to a hypothetical super-Earth body lying at several hundred AUs. Batygin & Brown (2016) showed numerically and analytically how the apsidal and nodal clustering of the distant TNOs arises as a result of resonant and secular dynamical effects from a distant perturber. They identified a range of semimajor axes (400-1500 AU) and eccentricities (0.5-0.8) for which a distant planet can explain the → orbital elements of the distant TNOs. The predicted planet would have a mass of 10 Earths (approximately 5,000 times the mass of → Pluto), a diameter of four times Earth and a highly elliptical orbit with an → orbital period of approximately 15,000 years.

See also: → planet; → nine.