Hints of Earth-Size Planets around Alpha Centauri B

Alpha Centauri Bb is a possible Earth-size exoplanet in a 3.2 day orbit around Alpha Centauri B, a K-type star located 4.37 light-years away. The discovery of Alpha Centauri Bb was announced by Dumusque et al. (2012) from measurements made using the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument at the La Silla Observatory in Chile. The HARPS instrument measures the tiny gravitational wobbles an orbiting planet exerts on its host star. Since then, more attempts have been made to try and confirm the existence of Alpha Centauri Bb.

Figure 1: Artist’s impression of a hot Earth-size planet. Image credit: Invader Xan.

Because Alpha Centauri Bb orbits so close to its host star, the probability that it transits in front of its host star as observed from Earth is estimated to be between ~10 to 30 percent. Using the Hubble Space Telescope (HST), an attempt was made by Demory et al. (2015) to detect the transit of Alpha Centauri Bb in front of its host star Alpha Centauri B. The search involved observing Alpha Centauri B twice in 2013 and 2014, for a total of 40 hours. With a confidence level of 96.6 percent, the search ruled out any transits of Alpha Centauri Bb.

Nevertheless, a single transit-like event was found in the data that was collected. This transit-like event lasted for 3.8 hours and it is consistant with the presence of another Earth-size planet on a longer period orbit around Alpha Centauri B. The Alpha Centauri system, consisting of Alpha Centauri A, Alpha Centauri B and the red dwarf Proxima Centauri, are the closest stars to the Sun. The search for nearby alien worlds is a fascinating prospect and additional observations will be necessary to verify the presence of multiple Earth-size planets around Alpha Centauri B.

Figure 2: Light curve of the single transit-like event indicating the possible existence of an Earth-size planet on a longer period orbit around Alpha Centauri B. Demory et al. (2015).

References:
- Dumusque et al., “An Earth-mass planet orbiting α Centauri B”, Nature 491, 207-211 (08 November 2012)
- Demory et al. (2015), “Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri Bb”, arXiv:1503.07528 [astro-ph.EP]