Upsilon Andromedae d
|Extrasolar planet||List of extrasolar planets|
|Star||Upsilon Andromedae A|
|Right ascension||(α)||01h 36m 47.8s|
|Declination||(δ)||+41° 24′ 20″|
|Distance||44.0 ± 0.1 ly
(13.49 ± 0.03 pc)
|Radius||(r)||1.480 ± 0.087 R☉|
|Temperature||(T)||6074 ± 13.1 K|
|Semimajor axis||(a)||2.54 ± 0.15 AU
|Periastron||(q)||1.88 ± 0.18 AU
|Apastron||(Q)||3.19 ± 0.28 AU
|Orbital period||(P)||1276.46±0.57 d
|Inclination||(i)||23.8 ± 1°|
|(ω)||279 ± 10°|
|Time of periastron||(T0)||2,448,827 ± 30 JD|
|Semi-amplitude||(K)||63.4 ± 1.5 m/s|
|Mass||(m)||3.75±0.54[dubious ] MJ|
|Discovery date||April 15, 1999|
|Discoverer(s)||Butler, Marcy et al.|
|Discovery method||Radial velocity|
|Discovery site||California and Carnegie
50 Andromedae d, Upsilon Andromedae Ad
Upsilon Andromedae d is an extrasolar planet orbiting the Sun-like star Upsilon Andromedae A. Its discovery in April 1999 by Geoffrey Marcy and R. Paul Butler made Upsilon Andromedae the first star (other than the pulsar PSR 1257+12 and the Sun) to be known 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.
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
Upsilon Andromedae d orbits its star in an eccentric orbit, more eccentric than that of any of the major planets in the 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 be ejected 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.
Given the planet's high mass, it is likely that it is a gas giant with no solid surface and surface gravity of over 25 times that of Earth. Since the planet has only been detected indirectly through observations of its star, properties such as its radius, composition, and temperature are unknown.
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.
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