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Upsilon Andromedae c is an extrasolar planet orbiting the Sun-like star Upsilon Andromedae A every 241.5 days. Its discovery in 1999 by Geoffrey Marcy and R. Paul Butler made Upsilon Andromedae the first known star (excluding the pulsar PSR 1257+12) to host a multiple-planet planetary system. Upsilon Andromedae c is the second known planet in order of distance from its star.
Discovery
Like the majority of known extrasolar planets, Upsilon Andromedae c 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, 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. It is thought 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. Simulations suggest that the orbit of Upsilon Andromedae c returns to its original circular state roughly once every 6,700 years.
A limitation of the radial velocity method used to detect Upsilon Andromedae c is that only a lower limit on the planet's mass can be obtained. For Upsilon Andromedae c, this lower limit is 1.89 times the mass of Jupiter, though depending on the inclination of the planet's orbit, the true mass may be much greater.
Characteristics
Given the planet's high mass, it is likely that Upsilon Andromedae c 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 that of Jupiter and that its environment is close to chemical equilibrium, it is predicted that the upper atmosphere of Upsilon Andromedae c is cloudless.
References
See also
External links
"Upsilon Andromedae c"