Gliese 86 b – an Extrasolar Planet

Gliese 86 b, also known as Gliese 86 A b or Gl 86 b, is an extrasolar planet located approximately 35 light-years away in the constellation Eridanus. It is an exoplanet similar to Neptune that orbits an M-type star. It has a mass of 9 Earths, takes 24.2 days to complete one orbit around its star, and is 0.1344 AU away from it. In November 1998, French scientists discovered the planet orbiting a K-type main-sequence star (Gliese 86 AB). The planet orbits the star very close, completing an orbit in 15.78 days.

Gliese 86 is approximately 35.6 light-years from Sol. It is located in the southwestern quadrant of the constellation Eridanus, slightly northwest of Phi Eridani, northeast of Chi Eridani, and northeast of Achernar (Alpha Eridani). Many astronomers now refer to this star by its designation in Wilhelm Gliese’s (1915-93) famous Gliese Catalogue of Nearby Stars. Gliese was a longtime astronomer at Heidelberg’s Astronomiches Rechen-Institut.

According to preliminary astrometric measurements made with the Hipparcos space probe, the planet has an orbital inclination of 164.0° and a mass 15 times that of Jupiter, indicating that it is a brown dwarf. However, further analysis indicates that the Hipparcos measurements are not precise enough to reliably determine the astrometric orbits of substellar companions, leaving the orbital inclination and true mass of the candidate planet unknown.

On November 24, 1998, a team of astronomers (including Michel Mayor, Didier Queloz, and Stephane Udry) announced the discovery of a substellar companion “b,” whose latest minimum mass estimate is 4.02 times that of Jupiter with a similar diameter. This object orbits Gliese 86 A at an average separation of 0.11 AUs, which is close to Mercury’s orbital distance in the Solar System. Its highly circular orbit (e= 0.04) takes less than 15.8 days to complete.

Astrometric analysis suggests that the object has a mass 15 times that of Jupiter and an inclination of 164.0° from the Earth’s line of sight. As a result, the object could be an extremely dim brown dwarf, a substellar companion of Gliese 86 A. The authors consider their analysis to be preliminary, requiring confirmation with additional astrometric and other observations.

Once the motion caused by this planet is removed, the radial velocity measurements of Gliese 86 show a linear trend. This could be due to the white dwarf companion star’s orbital motion. The second star in the binary system, Gliese 86 B, is a DQ6, with a mass of 0.590 that of the Sun and a temperature of 8180K.