Meet our closest neighbour

The discovery of a possible extrasolar planet that has the same mass as Earth and orbits α Centauri B, a member of the closest star system to the Sun, is both a technical achievement and cause for excitement. See Article p.207

One big goal of astronomers studying exoplanets — planets that orbit stars other than the Sun — is the detection of an Earth-mass planet in the habitable zone of a Sun-like star. The habitable zone is usually defined as the range of distances from the parent star at which water, if present, would be liquid. On page 207 of this issue, Dumusque et al.1 report the discovery of a candidate exoplanet that brings this goal one step closerFootnote 1.

“The authors' spectral analysis of the system is a demonstration that weak planetary signals can be extracted from a star's spectrum.”

Finding exoplanets is nothing new — several hundred have already been discovered. What makes the planet identified by Dumusque and colleagues special and exciting is its mass and location: it has approximately the same mass as Earth, and it orbits α Centauri B, a member of the closest star system to the Sun. Because of its proximity, it would be a good target for further investigations. For example, reflected starlight or radiated light from the planet would enable us to study its atmosphere, if present, or possibly its surface composition. So far, such studies have been possible only for much larger planets2,3. In addition, the authors' spectral analysis of the system is a demonstration that weak planetary signals can be extracted from a star's spectrum.

If it is confirmed, the new candidate planet would qualify as the nearest exoplanet to our Solar System. The planet is too close to its host star, and therefore too hot, to be habitable — its orbital period, or 'year', is only 3.236 days. However, as previous research has shown, multi-planet systems are common4: where there is one planet there may be more. So it is conceivable that α Centauri B has more companions, maybe even in the habitable zone. But this is speculation, and detecting further planets would be even more difficult than finding this one.

To understand the significance of this finding, some context is needed. Since the discovery5 in 1995 of the giant exoplanet 51 Peg b, the first planet to be found orbiting a Sun-like star, the detectable mass of exoplanets has decreased from the mass of Jupiter to the mass of Earth. An Earth-mass exoplanet is 150 times smaller than 51 Peg b. Planet hunters have been able to find ever smaller planets owing to a combination of improved instruments and better analysis methods. Dumusque et al. detected the new candidate planet using the 'Doppler wobble', which is the effect caused by the planet's gravitational pull on the motion of its host star6. If confirmed, this would be the lowest-mass planet discovered using the Doppler-wobble method.

In their search for the exoplanet, Dumusque and colleagues faced two main challenges. The first was detecting such a small Doppler wobble, a mere 0.51 metres per second. In comparison, the Doppler wobble caused by 51 Peg b is 50 m s−1, or about 100-fold bigger. Doppler measurements this fine require very stable instruments. The authors used the HARPS spectrograph mounted on the European Southern Observatory's 3.6-m telescope located in La Silla, Chile — the most stable spectrograph in the world for this type of measurement.

The second and much more daunting challenge was the extraction of the planet's signal from the 'noise' caused by the variability of the star. Like our Sun, α Centauri B has spots (regions that are darker and cooler than the rest of the star's surface), which are caused by magnetic activity. These spots can create signals in the data that look similar to that caused by the planet, making it hard to distinguish between planetary and stellar signals. The data show that the stellar-activity signal was three times larger than that due to the planet. The researchers used 23 parameters related to the star's rotation period to model the variation in stellar activity, and then filtered it out from the data, unveiling the planet's signal. The fact that so many parameters had to be used emphasizes the complexity of the stellar signal.

So is this Earth-mass planet real? Only time will tell. As the American astronomer Carl Sagan once said, “Extraordinary claims require extraordinary evidence”. Although a planet-like signal is present in the data, the discovery does not quite provide the “extraordinary evidence”. It is a weak signal in the presence of a larger, more complicated signal. In my opinion, the matter is still open to debate. Other analytical tools, using alternative ways of filtering out the stellar variability, might arrive at different conclusions on the basis of the same data. However, if we want to find a real Earth twin around a Sun-like star, we have to devise robust methods for filtering out the star's variability. By providing researchers with a valuable data set for testing their analytical tools, the present study is a step in that direction.

In the coming months, astronomers will certainly be scrutinizing these measurements. Only if other analyses come to the same conclusion can we be sure that this planet exists. Better yet, independent measurements should be made with other facilities and instruments to confirm this candidate planet.


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Correspondence to Artie P. Hatzes.

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Hatzes, A. Meet our closest neighbour. Nature 491, 200–201 (2012).

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