EXOPLANETS—those that orbit stars other than the sun—are one of the hottest topics in astronomy. The first was found in 1992, going round a neutron star (a strange beast made of matter as dense as that in an atomic nucleus) 1,000 light-years from Earth with the rather pedestrian name of PSR 1257 +12. Planets going round more conventional stars turned up three years later, and since then the initial trickle of discoveries has become a flood. The present tally is 841 confirmed (through a mix of ground-based searches and space-based telescopes) and another couple of thousand probables.

So it takes a lot, these days, for news of a freshly discovered planet to raise eyebrows. But it still happens from time to time. And October 16th was one of those times. On that day a paper was published by Nature, in which a team from the Geneva Observatory claimed to have found evidence of an Earth-sized planet orbiting a star called Alpha Centauri B.

If the name sounds familiar, that is because, at just over four light-years away, the Alpha Centauri system—a group of three gravitationally bound stars—is the sun’s nearest neighbour. The planet, therefore, is not only the closest to Earth ever discovered, it is just about as close to Earth as any exoplanet can be. Only if it were orbiting Proxima Centauri, the third member of the system after Alpha Centauri A and B, could it be closer. It is small, too, with a minimum estimated mass just 1.13 times that of Earth. And the star it circles is very like the sun: 93% as massive, and with a surface temperature just a few hundred degrees cooler.

There, though, the similarities with Earth end. The new planet is very close to its parent star, completing an orbit every 3.2 Earth days. (By way of comparison Mercury, the scorched innermost planet in the Solar System, has a year that is 88 days long.) It is also almost certainly tidally locked to its parent, meaning that one side is constantly baked and blasted by stellar radiation, while the other faces the freezing blackness of space. Life on such a body is, presumably, out of the question.

Some needle; some haystack

That Xavier Dumusque (the paper's lead author) and his colleagues were able to find the thing at all, however, is newsworthy of itself. They ran their quarry to ground using what is known as the radial-velocity method, which examines stars for the tiny wobble caused by the gravitational pull exerted by the planets that circle them. The method itself is well established. But the smaller a planet relative to its star, the smaller the wobble it induces, which makes looking for Earth-sized tiddlers with this technique particularly difficult. To be exact, the team managed to discover a wobble of 0.5 metres a second in a star that is roughly 41,300,000,000,000km away, the most precise such measurement ever taken.

Indeed, the tininess of the measurement is cause for caution among some planet-hunters, made wary by past experiences of planets being announced and then evaporating on closer inspection. In this case, a great deal of statistical gymnastics was necessary to extract the wobble caused by the planet from the bigger wobbles that arise from the star’s internal churnings, noise in the instruments, the motion of Earth (and therefore of the scientists and their telescope) through space, and contamination of the signal with light from Alpha Centauri A, the biggest and brightest star in the Alpha Centauri system. “My reaction to this paper is...maybe,” says Hugh Jones, a planet-hunter at the University of Hertfordshire, in Britain. “A very small signal is being extracted from a not particularly quiet star, with a very nearby companion that is three times brighter.”

Others are more sanguine. Gregory Laughlin, of the University of California, Santa Cruz—a pioneer of planet-hunting—argues that were these results from any star less famous than Alpha Centauri, they would be accepted without a murmur. Debra Fischer, of Yale, who has also been searching for Earth-sized planets around nearby stars, has already analysed some of Dr Dumusque's data with help from Rebekah Dawson, a graduate student. “We do not rule out the possibility that this signal is real,” she says, with guarded optimism.

Dr Dumusque’s team, for their part, reckon that the probability of a false alarm is less than one in five hundred. Their observations were painstaking, taking place over three and a half years. One condition of their paper’s publication by Nature was that their data be made immediately available to other researchers, who will be able to double-check the team’s sums.

Assuming the planet is real, other astronomers will be keen to study it further. As Dr Dumusque's team point out in their paper, enough exoplanets have been found for people to begin detecting statistical patterns. One pattern seems to be that if a star has one smallish planet, it is quite likely to have several more. So confirmation of the team’s discovery would boost the odds that more worlds are circling Alpha Centauri B, perhaps even in the so-called habitable zone where the temperature would allow liquid water to exist on the surface. The team's technique is sensitive enough to detect a planet about four times the mass of Earth within the habitable zone, although it would require around a decade of observations to confirm.

The radial-velocity method used Dr Dumusque and his colleagues will reveal the presence of a planet and its mass. But in order to characterise it better, astronomers will be looking for a transit, in which the planet crosses in front of its sun, as seen from Earth, slightly dimming the star's light. Whether they will get one is a matter of luck—it depends on the orientation of the planet's orbit relative to Earth. But if they do, the dimming will allow them to estimate the planet's size. And because solar systems tend to be flat, if one planet can be seen transiting, then others might, too.

And since Alpha Centauri is so close, it may even be possible to isolate sunlight that has travelled through the atmospheres of its planets—if they have them. That would give astronomers a list of the chemicals present in the atmosphere, which would in turn provide hints about the planets’ geology. And, to top off the chain of speculation, Alpha Centauri's relative proximity means it is just about conceivable that, in the far future, a sufficiently committed and well-financed group of scientists could send a probe to take a closer look—although the trip would take several decades at the very least.

Something new under the suns

Exciting as Dr Dumusque's news was, not all eyes have been turned to Alpha Centauri. On October 15th, at a planetary-science conference in Reno, Nevada, hosted by the American Astronomical Society, another unusual planet was announced. Named PH1, it is notable both for the fact that it boasts four suns and for the method by which it was found. As is described in a paper to be published in the Astrophysical Journal by Meg Schwamb, another astronomer at Yale, and her team, it was discovered by two members of Planet Hunters (whence its name), a web-based citizen-science project in which ordinary people are given access to data from NASA’s planet-hunting Kepler space telescope and encouraged to look for planets that the computer programs which crunch Kepler’s data might have missed.

The planet hunters in question are Kian Jek, a web developer, and Robert Gagliano, a doctor. The discovery was a textbook example of the usefulness of having people check the data alongside computers, says Chris Lintott, an astronomer at Oxford who helps to manage the Planet Hunters project. Kepler detects planets by noting the drop in light caused when one of them passes in front of its parent star. That works well for single-star systems, and can even be made to work with binaries. In PH1’s case, however, the variations in light generated by a planet interacting with four separate stars were enough to baffle the computers—but not the eagle-eyed humans.

Besides being fascinating in its own right, such exotic finds are a good test of astronomers’ theories about how planets form. In PH1’s case, its four stars are actually a pair of binaries. Conventional planetary-formation theory holds that worlds condense out of a disc of dust and rubble early in a star’s life. But in this case, “the second binary would sit right at the edge of the protoplanetary disc,” notes Dr Lintott. Computer models suggest that the gravitational influence of the second pair of stars ought to disrupt the disc and prevent the formation of planets. Reality, in this case, disagrees with the models—and that is how science advances.

(Picture credit: ESO/L. Calçada)

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