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GREAT NEIGHBORSThis diagram shows the position of the four planets in the Gliese 581 system. The newly found, innermost body is Gliese 581 e, the smallest exoplanet yet discovered. The habitable zone (blue region), where water could exist as a liquid, clearly includes the outermost planet, Gliese 581 d.IMAGE: ESO, based on a diagram by F. Selsis/Univ. of Bordeaux

Inching ever closer to the goal of discovering a planet just like home, Swiss astronomers have announced finding the smallest extrasolar planet ever detected. The object, a mere 20.5 light-years away, could be as tiny as 1.9 Earths and isn’t likely to exceed twice that amount.

The feat of detecting a planet not much heavier than Earth, says Stephane Udry of the University of Geneva’s observatory in Sauverny, Switzerland, shows that astronomers “are on the right track” for the ultimate discovery: finding an Earthlike planet that orbits another star in the habitable zone, the region around a star in which water could exist as a liquid.

Veteran planet hunter Michel Mayor, also of the Geneva Observatory, described his team’s findings on April 21 at the European Joint National Astronomy Meeting at the University of Hertfordshire in Hatfield, England. Researchers have now found more than 340 extrasolar planets.

Another planet in the same system, although not as close in size to Earth, is now the only known low-mass planet that does in fact orbit in the habitable zone, the team also announced at the meeting. “Gliese 581 is a truly fascinating exoplanet system,” comments theorist Sara Seager of MIT. “It is like a gift that keeps on giving.”

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VIEW FROM GLIESE EAn artist's rendering of one of the planets orbiting the nearby star Gliese 581, now known to host the smallest known exoplanet (illustrated in foreground) and also a relatively small planet that orbits at just the right distance from the star to have liquid water (illustrated as the blue planet).IMAGE: European Southern Observatory

According to the leading model of planet formation, an orb as small as the newly found planet, 581 e, would almost certainly be rocky like Earth, rather than icy or gaseous. But because the planet lies so close to its parent star, a red dwarf called Gliese 581, it’s hot enough to boil away any surface water and could not support life similar to that on Earth.

Mayor and his colleagues, including Udry, detected the tiny planet indirectly by the gentle pull the planet exerts on its parent star — not much different than the minuscule tug a mouse might exert on an elephant. Just barely detectable by a sensitive spectrograph on the European Southern Observatory’s 3.6-meter telescope at La Silla in Chile, that wobble reveals that the close-in planet takes just 3.15 days to whip around Gliese 581 in a circular orbit.

Previous observations with the same spectrograph had already shown that Gliese 581 harbors three other relatively small planets: a Neptune-sized planet and two others dubbed superEarths, each with a minimum mass several times that of Earth’s (SN: 4/28/07, p. 259).

The newly discovered Gliese 581 e lies closest to the star. Next in line is Gliese 581 b, with a minimum mass of 16 Earths (similar to Neptune), Gliese 581 c, with a minimum mass of five Earths, and Gliese 581 d, which orbits in the habitable zone and has a minimum mass of seven Earths.

Earlier observations had indicated that Gliese 581 d orbited its parent star in about 80 days and resided right at the outer edge of the habitable zone. New measurements by Mayor's team place Gliese 581 d closer to the star, with a 68.6-day orbit, and unequivocally in the habitable zone, Udry says. It's even possible, he says, that Gliese 581 d could have a deep ocean.

Although the wobble method reveals only the minimum mass of Gliese 581 e, it’s highly unlikely that the planet could be more than four Earth masses, notes Udry. That’s because the four planets are “packed so tightly,” says Seager, that were Gliese 581 e much heavier, gravitational interactions between it and the other planets would force the body into a more elongated orbit.

The Swiss team is now trying to determine whether the new-found planet transits, or periodically passes in front of its star as seen from Earth. If it does, the amount of starlight the planet blots out would reveal its radius and precise mass. That in turn would indicate whether the planet is truly rocky or its density is too low for rockiness. In addition, starlight filtering through the planet’s atmosphere during transits would reveal the composition of the atmosphere. The team will start looking for transits using ground-based telescopes about a week from the announcement, followed up by observations with NASA’s orbiting Spitzer Space Telescope.

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