Orbit and Mass
Like the majority of long-period extrasolar planets, Upsilon Andromedae d revolves around its star in an eccentric orbit, more eccentric than that of any of the major planets in our solar system (including Pluto). The orbit's semimajor axis puts the planet in the habitable zone of Upsilon Andromedae A.
To explain the planet's orbital eccentricity, some have proposed a close encounter with a (now lost) outer planet of Upsilon Andromedae A. The encounter would have moved Upsilon Andromedae d into an eccentric orbit closer to the star and ejected the outer planet from the system. Subsequently gravitational perturbations from Upsilon Andromedae d moved the inner planet Upsilon Andromedae c into its present eccentric orbit. 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 d 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 d, which is about 10.25 times the mass of Jupiter.
Preliminary astrometric measurements suggest the orbit of Upsilon Andromedae d may be inclined at 155.5° to the plane of the sky. However, these measurements were later proved useful only for upper limits; worthless for HD 192263 b and probably 55 Cancri c, and contradict even the inner planet u And b's inclination of >30°. The mutual inclination between c and d meanwhile is 29.9 degrees.
Read more about this topic: Upsilon Andromedae D
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