Upsilon Andromedae

Upsilon Andromedae (υ Andromedae / υ And) is a star system of about three stars approximately 44 light-years away in the constellation Andromeda. The primary star (Upsilon Andromedae A) is a solar twin that is somewhat younger than the Sun. The second star in the system (Upsilon Andromedae B) is a red dwarf that orbits around 750 astronomical unit from the primary star. Not much is known about third star (Upsilon Andromedae C) except that it is also a solar twin.

As of 1999, three confirmed extrasolar planets are known in orbit around the primary star. All three are likely to be jovial 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.

History

Upsilon Andromedae A is ranked 21st in the list of top 100 target stars for the NASA Terrestrial Planet Finder mission. Note NASA has cancelled this mission indefinently, due to funding limitations (See wikipedia article Terrestrial Planet Finder)

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. 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.

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. According to its entry in the Geneva-Copenhagen survey, the star is around 3.3 thousand million years old, and has a similar proportion of iron relative to hydrogen as the Sun. At around 1.3 solar masses, it will have a shorter lifetime than our 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 projected distance of 750 AU from the primary star. 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 our 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.

Planetary system

The innermost planet of the Upsilon Andromedae 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. 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.

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 Centre 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 the two outer planets are in more eccentric orbits than any of the planets in our solar system (including Pluto). The outermost planet resides in the system's habitable zone. In 2001, preliminary astrometric measurements suggested the orbit of the outermost planet is inclined at 155.5° to the plane of the sky, implying its true mass may lie between 9 and 10 Jupiter masses.

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. As a result, the orbit of Upsilon Andromedae c gradually oscillates between circular and eccentric states every 6,700 years.

Upsilon Andromedae does not appear to have a circumstellar dust disk similar to the Kuiper belt in our 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. In 2005, the eccentric orbits of the outer most planets have lead to the speculation of a fourth planet. The idea states that all the planets began in circular orbits, but chaotic evolution caused Upsilon Andromedae d (the outer planet) to be perturbed suddenly into a higher-eccentricity orbit. The coupled evolution of the system then causes slow periodic variations in the eccentricity of Upsilon Andromedae c (the middle planet). Whether this was caused from a hypothetical "Upsilon Andromedae e" is not yet confirmed and is debatable.