Lalande 21185, located just 8.3 light years away, is the fourth-nearest star to the Solar System after the Alpha Centauri triple, Barnard's Star, and Wolf 359. At magnitude 7.47, it about three times too faint to be seen with the naked eye, but is visible in binoculars. The earliest known recording of this star was by Joseph-Jerome Lefrancais de Lalande at the Paris Observatory some time before 1801.
Motion
Lalande 21185 has the 8th largest annual proper motion known. It is moving at a relatively high velocity (47 km/sec) in an orbit perpendicular to the plane of the galaxy. It is thought to belong to a "thick disk" of old stars moving rapidly in eccentric orbits within 5,000 light years of the galactic plane.
Properties
Lalande 21185 is a cool and dim red dwarf star of spectral type M2.1Vne. It has an absolute magnitude of +10.5, corresponding to a luminosity just 0.0048 of the Sun's. The star is though to have only 46% of the Sun's mass, and 46% of its radius. The star is less enriched in elements heavier than hydrogen ("metals") compared to our Sun, with around 63% of the solar abundance of iron. Lalande 21185 is a flare star, probably much older than the Sun, but less than 10 billion years old.
Planets
Lalande 21185 is suspected to have as many as three Jupiter-class planets. In 1996, a team led by George G. Gatewood of the Allegheny Observatory reported detection of possible planetary companions from astrometric perturbation analysis of photographic plate data collected from 1930 to 1984. However, the planetary detection has not yet been confirmed. The planetary candidate closest to Lalande 21185 ("b") lies within 2.2 AU of the star. It has about nine tenths of Jupiter's mass, a highly circular orbit, and a period of 5.8 years. The second planet ("c") may lie about 11 AU away, in the first planet's orbital plane, with perhaps 1.6 Jupiter masses, a circular orbit, and a period of 30 years. If the second planet's mass has not been significantly underestimated, then there may be a third planet ("d") lying beyond 11 AU with a mass around one Jupiter, and a longer orbital period.
In September 2002, a team of astronomers announced the detection of water "maser" emissions from three of 17 star systems suspected of hosting planets, including Lalande 21185. These microwave emissions could be generated from water molecules in a possible planet's atmosphere, excited by the infrared light of its host star. However, the signals could have come from the parent star and not its planets; water maser emissions from the planets would not be strong enough to be detected from the Earth. Additional telescopic observation should be able to pinpoint the exact source of the signal.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]