[LINK] "An Earth mass planet orbiting Alpha Centauri B"

[rfmcdonald]

The search for planets at Alpha Centauri has just begun with the confirmed existence of Alpha Centauri Bb, an Earth-mass planet orbiting orange dwarf B at a distance of 0.04 AU. The discovery paper is here.

One of the major challenges in the search for exoplanets is the detection of an Earth twin, i.e. an Earth-mass planet orbiting in the star’s habitable zone. Towards this goal, Alpha Centauri B is one of the most interesting targets. At a distance of 1.3 parsecs, it is a member of the closest stellar system to the Sun, composed of itself, Alpha Centauri A and Proxima Centauri. It also exhibits low stellar activity, similar to the solar activity level, usually associated with a small perturbing contribution of stellar intrinsic activity to measured radial velocities. Alpha Centauri B is cooler than the Sun (effective temperature3,4, 5, 6 of Teff = 5214 ± 33 K, spectral type K1V), and have a smaller mass that our parent star7 (M􀄑 = 0.934 ± 0.006 Msun). These two conditions ease the detection of a potentially habitable planet with radial velocities, the first one implying a habitable zone closer to the star, and the second one, a stronger radial-velocity variation for a similar mass planet. In addition, theoretical studies show that the formation of an Earth twin is possible around Alpha Centauri B8, 9. Finally, the brightness of the star (visual magnitude V=1.33) would allow for an efficient characterization of the atmosphere of potential orbiting planets.

An Earth twin induces a typical radial-velocity variation of a few tenths of a meter-per-second on a star like Alpha Centauri B. Such detections, technically possible with the most stable highresolution spectrographs, are however challenging due to the presence of stellar intrinsic signals inducing a radial-velocity “jitter” at the level of a few meters-per-second, even for quiet stars.

We report here the discovery of a planetary companion around Alpha Centauri B, unveiled by a radial-velocity signal with a semi-amplitude K of 0.51 meters-per-second, a period P of 3.236 days, and a semi-major axis a of 0.04 AU. This planet, with a minimum mass similar to Earth, is the lightest orbiting a solar-type star and the closest to the solar system found to date. Being much closer to its parent star than the Earth is to the Sun, it is not yet an Earth twin. However, the small amplitude of the signal shows that the radialvelocity technique is capable of reaching the precision needed to detect habitable super-Earth planets around stars similar to our Sun, or even habitable Earths around cooler stars (i.e. Mdwarfs).

In addition, statistical studies of exoplanets suggest that small-mass planets are preferentially formed in multi-planetary systems10, 11, 12. There is therefore a high probability that other planets orbit Alpha Centauri B, maybe in its habitable zone.