A team of astronomers including Christophe Lovis, member of PlanetS, confirmed in a press release distributed by ESO that Proxima is gravitationally bound to its neighbors Alpha Centauri A and B.
Since the discovery of Proxima in 1915 its proximity in the sky with Alpha Centauri and the similarity of their distance to the Sun have led astronomers to suspect that they are gravitationally bound. But to demonstrate this link, it is necessary to measure the relative velocity of Proxima with respect to Alpha Centauri with high precision.
The ESO’s planet-hunting HARPS instrument developed by the Geneva Observatory is the only existing instrument to provide an extremely precise measurement of Proxima Centauri’s radial velocity.
The measures done with Harps indicate that the velocity of Proxima with respect to Alpha Centauri is 309 +/- 55 m/s, when the escape velocity of Alpha Centauri at the distance of Proxima is 545 +/- 11 m/s, is significantly larger than the measured velocity. Proxima and Alpha Cen are therefore bound gravitationally.
Apparent orbit of Proxima around Alpha Centauri
Proxima and Alpha Centauri being bound implies that they very likely formed together and have the same age (except if Proxima was captured by Alpha Centauri). The age of Alpha Centauri, estimated between 5 and 7 billion years, then gives us the age of Proxima and its planet Proxima b. This planet, potentially habitable, is therefore older than the Earth (4.6 Gyr) by one or two billion years.
The Letter reporting these results will appear in the journal Astronomy & Astrophysics, and is available on the arXiv.org web site https://arxiv.org/abs/1611.03495
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