Saturday, 8 June 2013

Questioning the possible planet of Alpha Centauri.

The Alpha Centauri system is our Solar System's closest visible neighbour, at a distance of 4.37 light years; it is the third brightest 'star' in the sky (it is a binary system, but appears as a single object with the naked eye), though it is not visible from the northern hemisphere. It comprises two stars, Alpha Centauri A, a G-type Yellow Dwarf star similar to, but slightly larger than, the Sun, with 110% of the Sun's mass, and Alpha Centauri B, a K-type Orange Dwarf Star with 90% of the Sun's mass, which orbits Alpha Centauri A at an average of 17.57 AU (17.57 times the distance at which the Earth orbits the Sun) every 80 years. A third star, Proxima Centauri (which is our closest neighbour at 4.24 light years) is 0.237 light years (13 000 AU) from this pair, and may be associated with them, making a triple, rather than binary system. Proxima Centauri is a dim Red Dwarf with 12.3% of the Sun's mass; it is not visible with the naked eye.

In 2012 a team of scientists led by Xavier Dumusque of the Observatoire de Genève published a paper in the journal Nature suggesting that a planet with 1.13 times the Earth's mass was orbiting Alpha Centauri B every 3.24 days. The study was based upon data from the High Accuracy Radial velocity Planet Searcher (HARPS) telescope at Observatoire de Genève which detected a wobble in the movement of Alpha Centauri B, which was attributed to the gravity of the planet, dubbed Alpha Centauri Bb. This was a significant discovery, since Alpha Centuari B is a variable star and the wobble was detected by means of Doppler Shift (i.e. variations in the apparent frequency of light produced due to the star moving slightly to-and-fro relative to us), and was achieved by using a mathematical test called a Fourier Analysis to filter out the random fluctuations in the stars output, leaving the regular signal caused by the planet.

An artist's impression of Alpha Centauri Bb. Luís Calçada/European Space Agency.

In  a paper published on the online arXiv database at Cornell University Library on 21 May 2013, Artie Hatzes of the Thuringer Landessternwarte presents a re-analysis of the HARPS data on Alpha Centuari Bb. Hatzes was able to reproduce the results of  Dumusque et al. using Fourier Analysis, apparently confirming the existence of the planet, but when he used a second technique often used to separate Doppler Shift data from stellar variability, Local Trend Filtering, he found that the planet disappeared, suggesting that the Fourier Analysis result is an artifact of the technique, rather than the presence of a planet. Based upon this Hatzes contends that the existence of Alpha Centauri Bb cannot be asserted with confidence, and cautions against the diagnosis of planetary finds based only upon a single statistical analysis technique.

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