Gliese 876

2008/9 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

Gliese 876, also cataloged as IL Aquarii, is a red dwarf star approximately 15 light-years away in the constellation of Aquarius (the the Water-bearer). As of 2006, it has been confirmed that three extrasolar planets orbit the star. Two of the planets are similar to Jupiter, while the closest planet is thought to be similar to a small Neptune or a large Earth.

Distance and visibility

Gliese 876 is located fairly close to our solar system. According to astrometric measurements made by the Hipparcos satellite, the star shows a parallax of 212.59 milliarcseconds, which corresponds to a distance of 4.70 parsecs (15.3 light years). Despite being located so close to us, the star is so faint that it is invisible to the naked eye and can only be seen using a telescope.

Stellar characteristics

As a red dwarf star, Gliese 876 is much less massive than our Sun: estimates suggest it has only 32% of the mass of our local star. The surface temperature of Gliese 876 is cooler than our Sun and the star has a smaller radius. These factors combine to make the star only 1.24% as luminous as the Sun, though most of this is at infrared wavelengths.

Estimating the age and metallicity of cool stars is difficult due to the formation of diatomic molecules in their atmospheres, which makes the spectrum extremely complex. By fitting the observed spectrum to model spectra, it is estimated that Gliese 876 has a slightly lower abundance of heavy elements compared to the Sun (around 75% the solar abundance of iron). Based on chromospheric activity the star is likely to be around 6,520 or 9,900 million years old, depending on the theoretical model used.

Like many low-mass stars, Gliese 876 is a variable star. It is classified as a BY Draconis variable and its brightness fluctuates by around 0.04 magnitudes. This type of variability is thought to be caused by large starspots moving in and out of view as the star rotates.

Planetary system

In 1998 an extrasolar planet was announced in orbit around Gliese 876 by two independent teams led by Geoffrey Marcy and Xavier Delfosse. The planet was designated Gliese 876b and was detected by making measurements of the star's radial velocity as the planet's gravity pulled it around. The planet, around twice the mass of Jupiter, revolves around its star in an orbit taking approximately 61 days to complete, at a distance of only 0.208 AU, less than the distance from the Sun to Mercury.

An artist's impression of the outer most planet, Gliese 876b.

In 2001 a second planet was detected in the system, inside the orbit of the previously-discovered planet. The 0.62 Jupiter-mass planet, designated Gliese 876c is in a 1:2 orbital resonance with the outer planet, taking 30.340 days to orbit the star. This relationship between the orbital periods initially disguised the planet's radial velocity signature as an increased orbital eccentricity of the outer planet. The two planets undergo strong gravitational interactions as they orbit the star, causing the orbital elements to change rapidly.

An artist's impression of the inner most planet, Gliese 876d.

In 2005, further observations by a team led by Eugenio Rivera revealed a third planet in the system, inside the orbits of the two Jupiter-size planets. The planet, designated Gliese 876d, has a minimum mass only 5.88 times that of the Earth and may be a terrestrial planet. Based on the radial velocity measurements and modelling the interactions between the two giant planets, the system's inclination is estimated to be around 50° to the plane of the sky. If this is the case and the system is assumed to be coplanar, the planetary masses are around 30% greater than the lower limits established by the radial velocity method. This would make the inner planet have a true mass around 7.5 times that of Earth. On the other hand, astrometric methods suggest an inclination around 84° for the outermost planet, which would mean the true masses are only slightly greater than the lower limit.. Another investigation led by Paul Shankland (which included Rivera and others), reveals a lack of any astronomical transits of the planets across the face of the star -- along with a radial velocity 'tilt' away from 90° (caused by the Rossiter-McLaughlin effect) -- and so indicates the notional ~90° inclination is further unlikely.

Both of the system's Jupiter-mass planets are located in the 'traditional' habitable zone (HZ) of Gliese 876, which extends between 0.116 to 0.227 AU from the star. This leaves little room for an additional habitable Earth-size planet in that part of the system. On the other hand, large moons of the gas giants, if they exist, may be able to support life. Furthermore, the habitable zone for planets whose rotation is synchronous with their orbital motion may be wider than the traditional view, which may enable the existence of habitable planets elsewhere in the system.