Upsilon Andromedae C - Orbit and Mass

Orbit and Mass

Like the majority of long-period extrasolar planets, the orbit of Upsilon Andromedae c is eccentric, more so than any of the major planets in our solar system (including Pluto). If placed in our solar system, Upsilon Andromedae c would lie between the orbits of Earth and Venus.

The high orbital eccentricity may be the result of gravitational perturbations from the planet Upsilon Andromedae d. Simulations suggest that the orbit of Upsilon Andromedae c returns to its original circular state roughly once every 6,700 years.

One proposal is that interactions between Upsilon Andromedae d and a (now lost) outer planet moved Upsilon Andromedae d into an orbit closer to the star, where it gradually caused the orbit of Upsilon Andromedae c to become eccentric. If so, the rogue planet would have had to eject immediately; it is unclear how likely this situation might be. Other models are possible.

A limitation of the radial velocity method used to detect Upsilon Andromedae c is that the orbital inclination is unknown, and only a lower limit on the planet's mass can be obtained. However, by combining radial velocity measurements from ground-based telescopes with astrometric data from the Hubble Space Telescope, astronomers have determined the orbital inclination as well as the actual mass of Upsilon Andromedae c, which is about 13.98 times the mass of Jupiter. The mutual inclination between c and d is 29.9 degrees. That, however, means that Upsilon Andromedae c narrowly exceeds 13 times the mass of Jupiter, which is the dividing line between planets and (brown dwarf) stars.