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Upsilon Andromedae d an extrasolar planet orbiting the Sun-like star Upsilon Andromedae A. Its discovery in 1999 by Geoffrey Marcy and R. Paul Butler made Upsilon Andromedae the first known star (other than the pulsar PSR 1257+12) to host a multiple-planet planetary system. Upsilon Andromedae d is the third planet from its star in order of distance and the outermost known planet in its planetary system.
Discovery
Like the majority of known extrasolar planets, Upsilon Andromedae d was detected by measuring variations in its star's radial velocity as a result of the planet's gravity. This was done by making precise measurements of the Doppler shift of the spectrum of Upsilon Andromedae A. At the time of discovery, Upsilon Andromedae A was already known to host one extrasolar planet, the hot Jupiter Upsilon Andromedae b, however by 1999 it was clear that the inner planet could not explain the velocity curve.
In 1999, astronomers at both San Francisco State University and the Harvard-Smithsonian Center for Astrophysics independently concluded that a three-planet model best fit the data. The two new planets were designated Upsilon Andromedae c and Upsilon Andromedae d.
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.
The planet's orbital eccentricity may be the result of 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.
A limitation of the radial velocity technique used to discover Upsilon Andromedae d is that only a lower limit on the planet's mass can be obtained. In the case of Upsilon Andromedae d, this lower limit is 3.75 times the mass of Jupiter, though depending on the inclination of the orbit, the true mass may be much greater than this value. Preliminary astrometric measurements suggest the orbit of Upsilon Andromedae d may be inclined at 155.5° to the plane of the sky. If these measurements are confirmed, this implies that the true mass may be around 9-10 times that of Jupiter.
Characteristics
Given the planet's high mass, it is likely that it is a gas giant with no solid surface. Since the planet has only been detected indirectly through observations of its star, properties such as its radius, composition and temperature are unknown. Assuming a composition similar to Jupiter and an environment close to chemical equilibrium, it is predicted that its upper atmosphere will contain clouds of water.
Upsilon Andromedae d lies in the habitable zone of Upsilon Andromedae A as defined both by the ability for an Earthlike world to retain liquid water at its surface and based on the amount of ultraviolet radiation received from the star. Simulations suggest that even on eccentric orbits, terrestrial planets may be able to support liquid water throughout the year. This suggests that large moons of Upsilon Andromedae d may be able to support extraterrestrial life. On the other hand, it is not clear whether such moons would actually form, and the detection of satellites orbiting extrasolar planets is currently beyond our observational capabilities.
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External links
"Upsilon Andromedae d"