Exoplanets: Small, furry creatures from Alpha Centauri? 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) 1,500 light-years from Earth. Planets orbiting more conventional stars turned up three years later, and since then the initial trickle of discoveries has become a flood. The present tally is 843 confirmed 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. October 16th was one of those times. On that day Nature published a paper in which a team from Geneva University claimed to have found 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 is the sun's nearest stellar 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 much like the sun: 93% as massive, and with a surface temperature just a few hundred degrees cooler. There, though, the similarities end. The new planet is extremely 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 and his colleagues were able to find the thing at all is, however, newsworthy in itself. They ran their quarry to ground using what is known as the radial-velocity method, which examines stars for tiny wobbles caused by the gravity of planets that circle them. This method is well established. But the smaller a planet is, the smaller the wobble it induces, which makes looking for Earth-sized tiddlers 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. Indeed, the tininess of the measurement is cause for caution. In the past, several apparent planets have evaporated on closer inspection. In this case, a great deal of statistical gymnastics was necessary to extract the planet-induced wobble from bigger ones caused by the star's internal churnings, noise in the instruments, the motion of Earth through space, and contamination of the signal with light from Alpha Centauri A, the biggest member of the Alpha Centauri system. Mr Dumusque and his colleagues reckon, however, that the probability of a false alarm is less than one in 500. Their observations were painstaking, and spread over 3½ 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 the paper points out, enough exoplanets have now been found for statistical patterns to emerge. One such is 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 a decade of observations to confirm such a discovery. Better telescopes than exist now might then study the atmospheres of such planets, looking for evidence about their surface chemistry and (inevitably) for signs of life. Most intriguingly, Alpha Centauri's proximity means that a sufficiently committed and luxuriantly financed group of scientists could send a probe there to take a closer look. Such a trip would take decades - possibly more than a century - but there are several designs for craft that could do it using existing technology. Exciting as Mr Dumusque's news was, though, 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. This object, named PH1, 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, an 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 things 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. Their 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 even for binaries (two stars revolving around a common centre of gravity). In PH1"s case, however, the variations generated by a planet's interactions with four separate stars baffled the computers - but not the eagle-eyed humans. Besides being fascinating in its own right, such an exotic find is a test of astronomers" theories about how planets form. PH1"s four parents are actually a pair of binaries: each binary revolves around a common centre, like an ordinary binary, and the two binary systems then revolve around each other. Planetary-formation theory holds that worlds condense out of a disc of dust early in a star's life. But in this case, according to Dr Lintott, computer models suggest that the stars" gravitational dance would have disrupted the disc and stopped the formation of planets. Reality thus disagrees with the models - and that is how science advances.