Planet hunters have dramatically improved their techniques in the two decades since first discovering worlds beyond our solar system, most of which were gas-giant scorchers. Now they are searching for small, Earth-size exoplanets, such as the one said to circle Alpha Centauri B, which recently made headlines. Yet optimism about finding such planets may be premature. The problem is that stars swarm with surface activity that can mask or mimic the signs of tiny exoplanets. The putative planet orbiting Alpha Centauri B may, in fact, only be a mirage of stellar jitter.
Astronomers found the planet with a standard technique. Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA) and his colleagues monitored the star's light for periodic shifts in frequency, a sign that a planet's gravitational tug is causing the star to wobble.
When Artie Hatzes, an astronomer at the Thüringian State Observatory in Tautenburg, Germany, reanalyzed the data with two different methods, he found conflicting results: one showed a wobble; the other found none at all. He described his work in a June issue of the Astrophysical Journal. “If one analysis produces a planet and another doesn't, that's not robust,” he says. (To be fair, Dumusque and his team flagged significant uncertainties in their announcement last October.)
Alpha Centauri B's disputed world is not the first to come under close scrutiny. In 2010 an international team announced the discovery of a small planet around the star Gliese 581, smack-dab in the middle of the star's Goldilocks zone—the region where temperatures are just right for plentiful liquid water. Yet other researchers looking through their own data found no sign of the planet. Many other candidate detections are just as marginal, says David Latham, a planet-hunting veteran at the CfA, but remain unpublished.
The growing catalogue of hazy claims suggests that researchers must gather more data and resist the pressure to publish Earth-analogue discoveries too early, Hatzes says. He knows from experience: he now suspects that a planet his team announced in 2009, a gas-giant world thought to circle the star 42 Draconis, might also be a noise-induced illusion.
Going forward, Latham notes, planet hunters should focus on quieter stars and develop new models for sources of stellar jitter. Better spectrographs, such as the new HARPS-North on the island of La Palma in the Canary Islands, will help by reducing instrumental noise. Even so, Hatzes says, “at some point you're going to hit that wall, which is the noise level of the star.”