Exoplanet neighbour

Astronomers have discovered our nearest neighbouring exoplanet around our nearest star – α Centauri B, which is just 1.3 pc away. Using the HARPS instrument on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile, a team lead by Xavier Dumusque  have been monitoring the Doppler motion of the star for 4 years. In this week’s edition of Nature, they have annouced their detection of a tiny signal from a planet orbiting  α Cen B every 3.2 days at a distance of just 0.04 AU from the star. The team estimate the planet’s mass to be a little more than that of the Earth, making it the first exoplanet found with a mass similar to Earth around a Sun-like star.

Phase-folded radial-velocity curve of α Cen Bb with a period of 3.2357 days. The red points are the radial velocities binned in phase, and the red curve represents the best fit for the planet with a semi-amplitude of 0.51 m/ s. (Credit: Dumusque et al., 2012, Nature.)

An “Earth twin” will cause a radial-velocity variation of only a few tenths of a m/s on a Sun-like star, making such detections extremely challenging.  α Cen Bb (the planet) induces a radial-velocity of just 0.5 m/s (or  1.8 km/hr, compared to the average human walking speed of 5 km/hr) on the star, making these observations the highest precision ever achieved using the Doppler method.  At this precision, there are challenges removing the intrinsic stellar “jitter” that results from stellar oscillations, stellar rotation, and starspots.  Furthermore, α Cen B is part of a triple system, comprising two Sun-like stars – itself and α Centauri A – in a binary system with a period of 79.91 years, and a dwarf star Proxima Centauri that orbits the binary at a distance of 12,300 AU.

Artist’s impression of the planet orbiting the star Alpha Centauri B, a member of the triple star system that is the closest to Earth. (Credit: ESO/L. Calçada/Nick Risinger/skysurvey.org)

The Swiss team are not the only group monitoring α Cen B, and two other teams using the Cerro Tololo Inter-American Observatory (CTIO) in Chile and the Mount John University Observatory in New Zealand, are now rushing to confirm – or not – these exciting results.

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