The first star other than the Sun known to have a multiple-planet system. The existence of three planets around the star was confirmed in 1999. All the planets are much larger and heavier than the Earth, with masses up to about five times that of Jupiter. Upsilon Andromedae is somewhat hotter, bigger, and brighter than the Sun, and also younger, with an estimated age of 2·6 billion years. It lies 44 light years away and is faintly visible to the naked eye at 4th magnitude.
Upsilon Andromedae (υ Andromedae / Ups And) is a binary star, approximately 44 light-years away in the constellation Andromeda. The system contains a yellow-white dwarf star (Upsilon Andromedae A) similar to the Sun and a dim red dwarf (Upsilon Andromedae B).
As of 1999, three extrasolar planets are known in orbit around Upsilon Andromedae A. Upsilon Andromedae was both the first multiple-planet planetary system to be discovered around a main sequence star, and the first multiple-planet system known in a multiple star system.
Upsilon Andromedae A is ranked 12th in the list of top 100 target stars for the NASA Terrestrial Planet Finder mission.
Distance and visibility
Upsilon Andromedae is located fairly close to our solar system: the parallax of Upsilon Andromedae A was measured by the Hipparcos astrometry satellite as 74.25 milliarcseconds, corresponding to a distance of 13.5 parsecs.
System components
Upsilon Andromedae A is a yellow-white dwarf of spectral type F8V, similar to our Sun but rather younger, more massive and luminous.
Upsilon Andromedae B is a red dwarf of spectral type M4.5V located at a projected distance of 750 AU from the primary star.
The Washington Double Star Catalog lists two optical components, however these do not share the system's proper motion and only appear close to Upsilon Andromedae because they happen to lie near the same line of sight.
Planetary system
The innermost planet of the Upsilon Andromedae A system was discovered in 1996 and announced in January of 1997, together with the planet of Tau Boötis and the innermost planet of 55 Cancri A. The planet, designated Upsilon Andromedae b, was discovered by measuring changes in the star's radial velocity induced by the planet's gravity.
Even when this planet was taken into account, there still remained significant residuals in the radial velocity measurements, and it was suggested there might be a second planet in orbit. The two outer planets were designated Upsilon Andromedae c and d in order of increasing distance from the star. Both of the two outer planets are in more eccentric orbits than any of the planets in our solar system (including Pluto).
The existence of further planets too small or distant to detect has not been ruled out, though the presence of Jupiter-mass planets as close as 5 AU from Upsilon Andromedae A would make the system unstable. Simulations show that the eccentricity of the system's planets may have arisen from a close encounter between the outer planet and a fourth planet, with the result that the fourth planet was ejected from the system or destroyed. This may be the result of perturbations from the companion star removing material from the outer regions of the Upsilon Andromedae A system.
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