April 25, 2007

It has been 15 years since the discovery of the first planet outside our solar system. Now it seems astronomers may have accomplished what was, from the beginning, at the heart of their hunt for planets outside our solar system. They have discovered an Earth-like world that looks to be capable of supporting life.

Researchers working with Stéphane Udry and Michel Mayor from the Geneva Observatory discovered the planet in the orbit of the star Gliese 581. There are even indications that Gliese 581 -- one of the 100 stars closest to Earth, at a distance of only 20.5 light years -- has a system of at least three planets.

The team is not new to planet hunting. Mayor discovered the very first exoplanet -- as planets outside our solar system are called -- orbiting around a star similar to our sun in 1995. And two years ago, the same team discovered a planet the size of Neptune in the orbit of the red dwarf.

That astronomers have now demonstrated the existence of an Earth-like planet is quite astonishing: Boulders whose weight and size roughly resemble those of Earth are tiny in comparison to the more than 200 exoplanets already known -- and therefore hard to detect. "I have been telling people that the first Earth-like planet would probably not be found for another 3-5 years," Sean Raymond a planet expert at the University of Colorado, told SPIEGEL ONLINE in an e-mail. He wrote of an "exciting discovery."

A temperate climate?

The newly discovered planet, says the research team, is about 50 percent larger than the Earth and about five times as heavy. "We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius (32 and 104 degrees Fahrenheit), and water would thus be liquid," says Udry. "Models predict that the planet should be either rocky -- like our Earth -- or covered with oceans," he adds.

Such basic data electrify scientists: The existence of liquid water at moderate temperatures is considered the most important precondition for the development of life. "Of course you would also need other elements such as carbon and nitrogen, but they are probably present," Thierry Forveille, a member of the research team working from Grenoble, France, told SPIEGEL ONLINE. "Then you also need a trigger mechanism for the development of life -- which no one knows anything about."

Xavier Delfosse, another French member of the research team, is already dreaming about sending a mission to the planet due to its potential for harboring life and because it lies relatively close to Earth. "On the treasure map of the Universe, one would be tempted to mark this planet with an X," he says.

Taking a closer look

Udry and his colleagues used the HARPS (High Accuracy Radial Velocity for Planetary Search) spectrograph, developed specifically for hunting planets, will peering through the 3.6 meter telescope of the European Southern Observatory in La Silla, Chile. The Earth-like planet drew attention to itself by the slight wobbling motion it imposes on its host star -- an effect roughly comparable to the whirligig movement of a hammer thrower rotating around his own axis. The researchers will soon present their discovery in the professional journal Astronomy & Astrophysics.

But whether they have really found a life-friendly world is still not 100 percent certain. Only a planet's light spectrum can reveal the chemical composition of its atmosphere -- and provide information on whether that atmosphere allows for life. But that requires observing an Earth-like planet directly, which is hardly possible using today's technology. Only the next generation of instruments -- such as the US James Webb Space Telescope (JWST), for example, or the recently launched European CoRoT (Convection Rotation and planetary Transits) satellite -- are said to be capable of such observations.

In other words, one should approach the discovery with some caution. "Our estimates on its size and weight are based on calculations by other research teams," Udry's co-author Forveille said. Lisa Kaltenegger from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, believes, on the other hand, that her colleagues may have scored a direct hit. "We have already simulated the atmospheres of planets of this size," the German researcher told SPIEGEL ONLINE. The result: "The planet could be habitable." But, she adds, the atmosphere simulations would have to be adjusted more precisely to the environment of red dwarf stars like Gliese 851.

Comfortably warm or red hot?

A mass of five Earths, one-and-a-half times our planet's size -- "it all sounds very reasonable," Ralph Neuhäuser from the German Center for Exoplanet Research in Jena also said. But, he added, it is important not to forget that the "wobble method" used by the Geneva scientists provides only minimum values. Furthermore, of the roughly 200 exoplanets detected by this method so far, only 17 have been confirmed by means of other measurement methods, Neuhäuser said.

Speculation about comfortable temperatures on the Earth-like planets also needs to be handled with care. It orbits its host star in just 13 Earth-days; its average distance from Gliese 581 is one fourteenth of the distance between the Earth and the Sun. "The reason temperatures on the planet are not much hotter than on the Earth is that Gliese 581 is substantially smaller and colder than our Sun," Forveille explains.

But this intimate closeness could also have another consequence: Perhaps the planet and its host star are "tidally locked" -- meaning that, despite their rotating motion, each of them always only presents one hemisphere to the other -- just like the Moon and the Earth. That would largely take care of moderate surface temperatures: One side of the planet would then probably be glowing hot, the other freezing cold.

Forveille does not want to speculate on whether the new planet does in fact orbit around its host star in this manner -- just as he does not want to speculate on the question of what that would mean for the boulder's habitability. "Habitable conditions could then still exist, at least, in the border regions between the two hemispheres," Forveille said. "Or the planet could have an atmosphere that distributes heat very effectively -- like Venus, for example."

But those are only conjectures, he adds. "We still don't know with final certainty whether liquid water actually exists on the planet," Forveille says. "While H2O is a molecule that is found very frequently in space, final certainty can be achieved only through direct observation."