Saturday 7 July 2012

Two new more planets discovered in the Gliese 676 system.

The Gliese 676 system is made up of a binary pair of M Class Red Dwarf stars 53 light years from Earth in the constellation of Ara. The larger of these stars, Gliese 676A has 71% of our Sun's mass. This is orbited by the fainter, and less well understood, Gliese 676B at a distance of at least 800 AU (i.e at least 800 times the distance at which the Earth orbits the Sun), at which distance an orbit would take over 20 000 years to complete.

In February 2011 a team of scientists led by Thierry Forveille of the Laboratoire d’Astrophysique de Grenoble at Observatoire de Grenoble at the Université Joseph Fourier announced the discovery of a planet orbiting Gliese 676A in a paper in the journal Astronomy and Astrophysics. This planet, Gliese 676Ab (in naming objects in stellar systems stars are given an upper case letter, and planets  a lower case letter, proceeded by the star or stars they orbit), orbits the star at a distance of 1.8 AU (for comparison Mars orbits our Sun at 1.5 AU) every 1050 days, and has a mass 4.95 times that of Jupiter.

An artist's impression of Gliese 676Ab. Extrasolar Visions II.

Forveille et al. also found evidence for a second planet, Gliese 676Ac, though it was not possible to establish much about this at the time.

In a paper published on the online arXiv database at Cornell University Library on 29 June 2012, and in the journal Astronomy and Astrophysics on 2 July 2012, Guillem Anglada-Escudé of the Institut für Astrophysik at Universität Göttingen and Mikko Tuomi of the Centre for Astrophysics Research at the University of Hertfordshire and the Tuorla Observatory at the Department of Physics and Astronomy at the University of Turku describe the results of a new study of the Gliese 676 system, in which they produce additional information on Gliese 676Ac, as well as descriptions of two further planets in the system.

While not able to give a full description of Gliese 676Ac, Anglada-Escudé and Toumi conclude the planet has a mass of at least three times that of Jupiter, and orbits at a distance of at least 5.2 AU, taking over 4400 days (12 Earth years) to complete a single orbit. It is this distance and long orbital cycle that makes it hard to study Gliese 676Ac; the planets in the system have all been discovered by the wobble their gravity causes to the primary star as they orbit about it. Ideals several complete orbits are useful to understand the mass and orbital cycle of a planet, but with an orbit taking 12 years to complete, this will take decades to amass. 

The first of the new planets to be described is Gliese 676Ad, it has 4.4 times the mass of the Earth, and orbits at 0.413 AU (too close to support liquid water), completing one orbit every 3.6 days (86.4 hours). 

The second new planet is Gliese 676Ae, which has a mass 11.5 times that of the Earth (comparable to 15 times the Earth's mass for Uranus or 17 times the Earth's mass for Neptune), and orbits Gliese 676A every 35.4 days at a distance of 0.187 AU.

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