Upsilon Andromedae 2
 

Upsilon Andromedae (υ Andromedae, υ And) is a binary star located approximately 44 light-years away from Earth in the constellation Andromeda. The primary star (Upsilon Andromedae A) is a yellow-white dwarf star that is somewhat younger than the Sun. The second star in the system (Upsilon Andromedae B) is a red dwarf located in a wide orbit.

As of 2010, four confirmed extrasolar planets are known in orbit around the primary star. All four are likely to be jovian planets that are comparable to Jupiter. 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 was ranked 21st in the list of top 100 target stars for the NASA Terrestrial Planet Finder mission, which has, however, been reported to have been cancelled as of 2011.

Upsilon Andromedae is located fairly close to the Solar System: the parallax of Upsilon Andromedae A was measured by the Hipparcos astrometry satellite as 74.12 milliarcseconds, corresponding to a distance of 13.49 parsecs (44 light years). Upsilon Andromedae A has an apparent magnitude of +4.09, making it visible to the naked eye even under moderately light-polluted skies, about 10 degrees east of the Andromeda Galaxy. The dimmer star Upsilon Andromedae B is only visible with a telescope.

Upsilon Andromedae A is a yellow-white dwarf of spectral type F8V, similar to the Sun, but younger, more massive, and more luminous. According to its entry in the Geneva–Copenhagen survey, the star is around 3.1 billion years old, and has a similar proportion of iron relative to hydrogen to the Sun. At around 1.3 solar masses, it will have a shorter lifetime than the Sun. The amount of ultraviolet radiation received by any planets in the star's habitable zone would be similar to the ultraviolet flux the Earth receives from the Sun.

Upsilon Andromedae B is a red dwarf of spectral type M4.5V located at a distance (in the plane of the sky) of 750 AU from the primary star. The true separation between the two stars is unknown because the displacement along the line of sight between us and the Upsilon Andromedae stars is unknown, so this value is a minimum separation. It was discovered in 2002 in data collected as part of the Two Micron All Sky Survey. The star is less massive and far less luminous than the Sun.

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.

The star rotates at an inclination of 58+9

−7 degrees relative to Earth.

The innermost planet of the Upsilon Andromedae system was discovered in 1996 and announced in January 1997, together with the planet of Tau Boötis and the innermost planet of 55 Cancri. The discovery was made by Geoffrey Marcy and R. Paul Butler, both astronomers at San Francisco State University. The planet, designated Upsilon Andromedae b, was discovered by measuring changes in the star's radial velocity induced by the planet's gravity. Because of its closeness to the parent star, it induced a large wobble which was detected relatively easily. The planet appears to be responsible for enhanced activity in the chromosphere of its star.

Artist's conception of the planets of Upsilon Andromedae.

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. 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 outer planets were designated Upsilon Andromedae c and Upsilon Andromedae d in order of increasing distance from the star. Both of these planets are in more eccentric orbits than any of the planets in the Solar System (including Pluto). Upsilon Andromedae d resides in the system's habitable zone.

The system is not coplanar, with each other or with the stellar rotation. The mutual inclination between c and d is 30 degrees. In 2001, preliminary astrometric measurements suggested the orbit of the outermost planet is inclined at 155.5° to the plane of the sky. However, subsequent investigation of the data reduction techniques used suggests that the Hipparcos measurements are not precise enough to adequately characterize the orbits of substellar companions. Astrometry of the innermost planet, meanwhile, constrained its inclination to 30-90 degrees. Full publication is expected in 2008. The orbit of Upsilon Andromedae c gradually oscillates between circular and eccentric states every 6,700 years. 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.

Some 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. If so, the rogue planet would have had to eject immediately; it is unclear how likely this situation might be. Other models are possible. However, a fourth planet (Upsilon Andromedae e) was discovered in 2010. This planet is in a 3:1 resonance with Upsilon Andromedae d.

Upsilon Andromedae does not appear to have a circumstellar dust disk similar to the Kuiper belt in the Solar System. This may be the result of perturbations from the companion star removing material from the outer regions of the Upsilon Andromedae A system.