An artist’s impression of Gliese 581 d may oversell the reality (Image: Trent Schindler and the National Science Foundation)

Type the name “Gliese 581 d” into a search engine, and you’ll find hundreds of tantalising images of an Earth-like world. The exoplanet has been a top contender for the most life-friendly world beyond our solar system since it was discovered in 2007. There’s just one problem – it probably doesn’t exist.

New analysis of the Gliese 581 star system suggests that signals previously attributed to two of its six suspected planets actually come from sunspot-like regions on the star itself. The same analysis builds support that three of the other four planets are real, but none of these worlds would be able to support life as we know it.

Several promising exoplanets have been cast into doubt in the past. The gassy giant Fomalhaut b, for instance, was hailed as one of the first exoplanets to have its picture taken – but it may be nothing more than a blob of dust. And the closest Earth-sized world to us, Alpha Centauri Bb, could just be noise in the data.


The probable fall of Gliese 581 d only serves to emphasise the need for caution amid the recent explosion in exoplanet discoveries, says Paul Robertson at Pennsylvania State University in Philadelphia.

“The initial discovery of Gliese 581 d dates back really quite a few years now, and there has been a whole bunch of different teams looking at this data set,” says Robin Wordsworth at the University of Chicago, who in 2011 led research naming Gliese 581 d the first known rocky world capable of supporting life. “If the new paper is correct, it’s a very disappointing result.”

Magnetic illusion

Gliese 581 is a red dwarf star about 20 light years from Earth. Stéphane Udry at the University of Geneva, Switzerland, and his colleagues found the first four proposed planets in the system, including Gliese 581 d, using a technique called radial velocity. The tiny gravitational tug of a planet on a star shifts the spectrum of the star’s light. The time between shifts reveals the length of a planet’s orbit, while the shift’s size gives information about the planet’s mass.

The data suggested that Gliese 581 d had the right mass to be rocky like Earth, and it orbited at the right distance from its star to potentially host liquid water, and therefore maybe life. In 2010, another team claimed two more planets in the system, including Gliese 581 g, another possibly life-friendly planet. The two additions were less confident finds, but many planet-hunters thought the first four were pretty safe bets.

Robertson’s team looked at a different part of Gliese 581’s spectrum and measured the rotation period of the star. They found evidence for magnetic regions similar to sunspots rotating with the star, which they say give the illusion of planets d and g. “The signals of these ‘habitable zone’ planets were no longer there,” he says, though the signals for b, c and e improved.

“I see that the authors have a point,” says Udry, who is now planning further observations of the system. “This is just the way science works, and it is perfectly fine.”

Lost worlds

So should astronomers – and the media, New Scientist included – be more cautious next time they trumpet an exoplanet haul?

For instance, many announcements come with eye-catching artists’ impressions, but only a handful of worlds have been directly photographed, and they show up as tiny pin-pricks of light. Most exoplanets are revealed only as subtle variations in the light from their star.

“I spend my days looking at squiggles on a graph,” says Robertson. “But a lot of science is publically funded, and the taxpayers who contribute to that deserve a return on their investment. I wouldn’t say we should shy away from artist impressions or anything that helps us communicate the results of our work to the public.”

Wordsworth agrees, but he adds that scientists could do more as a community to highlight any uncertainty to people when new worlds are announced. Changes in the way exoplanet research is conducted could also help, as upcoming telescopes and space missions move from bulk data collection to intensive study of candidate planetary systems.

“Now we do not just need to detect them, but we want to know more about them,” says Udry, and our understanding is continually improving. “That’s very positive, even if the price to pay would be to lose a good candidate from time to time.”

Journal reference: Science, DOI: 10.1126/science1253253