Are there planets around Proxima Centauri? an update / ¿Existen planetas alrededor de Proxima Centauri?

Saturday, July 12, 2008

Imagen:Proxima Centari.jpg
Here's a synopsis of a paper that was just submitted yesterday with an update on the search for planets around Proxima Centauri, the red dwarf that is part of the Alpha Centauri trinary system, and the closest star to our sun. I think I'll go over the synopsis and try to provide a bit of explanation for some of the terms contained within (the ones I know).

The precision of radial velocity (RV) measurements to detect indirectly planetary companions of nearby stars has improved to enable the discovery of extrasolar planets in the Neptune and Super-Earth mass range.
Radial velocity = the wobbling of a star caused by a planet as it moves around it, which lets us find planets and determine their mass without having to directly image them. Works best with large Jupiter-type planets. Super-Earth to Neptune mass is anything from somewhat larger than our mass, to 15 times the Earth.
Discoveries of Earth-like planets by means of ground-based RV programs will help to determine the parameter Eta_Earth, the frequency of potentially habitable planets around other stars. In search of low-mass planetary companions we monitored Proxima Centauri (M5V) as part of our M dwarf program. In the absence of a significant detection, we use these data to demonstrate the general capability of the RV method in finding terrestrial planets. For late M dwarfs the classic liquid surface water habitable zone (HZ) is located close to the star, in which circumstances the RV method is most effective. We want to demonstrate that late M dwarfs are ideal targets for the search of terrestrial planets with the RV technique.
M dwarfs look like the star on the left here:

The Morgan-Keenan spectral classification

The sun is a G-type star.
We obtained differential RV measurements of Proxima Cen over a time span of 7 years with the UVES spectrograph at the ESO VLT. We determine upper limits to the masses of companions in circular orbits by means of numerical simulations. The RV data of Proxima Cen have a total rms scatter of 3.1 m/s and a period search does not reveal any significant signals. As a result of our companion limit calculations, we find that we successfully recover all test signals with RV amplitudes corresponding to planets with m sin i > 2 - 3 M_Earth residing inside the HZ of Proxima Cen with a statistical significance of >99%. Over the same period range, we can recover 50% of the test planets with masses of m sin i > 1.5 - 2.5 M_Earth. Based on our simulations, we exclude the presence of any planet in a circular orbit with m sin i > 1 M_Neptune at separations of less than 1 AU.

The last line there means that a planet in a circular orbit greater than the mass of Neptune at a distance less than that of Earth's orbit is impossible, as this would have caused the star to wobble about to an extent that hasn't been measured. I think. The longer one observes a star the longer you can tell what planets _aren't_ orbiting them, but what planets are orbiting them is another matter.

So now you know what isn't orbiting Proxima Centauri. Proxima Centauri is a flare star, however, which means that it probably wouldn't be that hospitable to life anyway even if an Earthlike planet is found within the habitability zone, but then again with so many ways for life to exist in the universe (see extremophiles for example) we really just don't know.


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