A Swiss-based team searching for planets outside our solar system, called exoplanets, say they have detected a planet the size of Earth orbiting the star Alpha Centauri B.
Named Alpha Centauri Bb, the planet is 10 times closer to its star than Mercury is to our Sun. This means the planet should be extraordinarily hot, ruling out any chance of life existing there.
In a paper published in Nature, Xavier Dumusque from the University of Geneva and colleagues say that exoplanets down to the size of Earth have been found, but not in the habitable zone – that is, at a distance from the parent star at which water, if present, would be liquid.
“There are planets in the habitable zone of stars cooler than our Sun, but for reasons such as tidal locking and strong stellar activity, they are unlikely to harbour water-carbon life as we know it. The detection of a habitable Earth-mass planet orbiting a star similar to our Sun is extremely difficult, because such a signal is overwhelmed by stellar perturbations.”
The researchers describe how, despite the stellar perturbations, they detected an Earth-mass planet orbiting Alpha Centauri B, a member of the closest stellar system to the Sun. The planet has an orbital period of 3.236 days and is about 0.04 astronomical units from the star (one astronomical unit is the distance of the Earth from the Sun).
The ‘perturbations’ are not the only observational problems specific to the Alpha Centauri binary system: another bright star is very close to Alpha Centauri B – Alpha Centauri A. The two stars orbit each other with a binary period of almost 80 years so the velocity signal due to the binary orbit must be removed when analysing the velocities of one component.
Two other teams of astronomers are also observing the Alpha Centauri system in search of low-mass planets: one at the Cerro Tololo Inter-American Observatory in Chile and another at the Mount John University Observatory in New Zealand.
Both teams have obtained high-precision radial-velocity observations of Alpha Centauri B, which are observations of the velocity of the star straight towards and away from Earth. With the measurement, the groups are in a position to confirm or refute the claim made by the Swiss researchers.
Astronomers say that, if confirmed, the presence of an Earth-sized planet around Alpha Centauri B would not be too surprising. Results from the exoplanet-hunting Kepler spacecraft indicate small planets are extremely common – since 1995, more than 800 have been located – and astronomers believe that eventually they will find that most stars are circled by one or more rocky planets.
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