JACKSON LAKE, WYOMING—The Milky Way abounds with low-mass planets, including small, rocky ones such as Earth. That's the main conclusion of a team of European astronomers, based on their latest haul of extrasolar planets. The new discoveries—55 new planets, including 19 "super-Earths"—were presented here today at the Extreme Solar Systems II conference by team leader Michel Mayor of the University of Geneva in Switzerland. "We find that 40% of all Sun-like stars are accompanied by at least one planet smaller than Saturn," he says. The number of Earth-like planets is expected to be even higher.
The new planets were found with HARPS (High Accuracy Radial velocity Planet Searcher), an extremely sensitive instrument used to analyze starlight, mounted on the 3.6-meter telescope of the European Southern Observatory (ESO) at Cerro La Silla in northern Chile. HARPS detects the minute periodic wobbles of stars, caused by the gravity of orbiting planets. So far, HARPS has discovered 155 exoplanets, including two-thirds of all planets less massive than Neptune.
Of the 19 newly found super-Earths (exoplanets between a few and 10 times the mass of Earth), the most intriguing is HD 85512b, which weighs in at only 3.6 Earth masses. Its orbit lies in the habitable zone of its parent star, which means temperatures are just right for liquid water to exist on its surface, says Lisa Kaltenegger of the Max Planck Institute for Astronomy in Heidelberg, Germany. "We're entering an incredibly exciting period in history."
Meanwhile, scientists disagree about which technique offers the best chances of finding the first true "Earth analog"—an Earth-like planet orbiting in the habitable zone of its Sun-like star. (H85512b is too massive, and it's star is too cool.) Mayor says HARPS might find this Holy Grail of exoplanet research within 5 years or so, after new upgrades to increase the instrument's sensitivity. But planet hunter Geoffrey Marcy of the University of California, Berkeley, disagrees. NASA's Kepler space telescope is "by far the best," he says. "We will find them if they're there, probably within the next 2 or 3 years."
At the meeting, Kepler co-investigator Natalie Batalha of NASA's Ames Research Center announced that the number of exoplanet candidates from the Kepler mission has increased by some 50% since last February, to 1781. Most are less than three times the size of the Earth. Kepler, launched in March 2009, finds planets by measuring the slight periodic dimming of their parent stars, when they happen to pass between the star and Earth.
No matter who finds the first Earth analog, the HARPS planets offer better prospects for detailed follow-up observations, Mayor says, because HARPS focuses on relatively nearby stars, while almost all Kepler stars are much farther away. For instance, ESO astronomer Markus Kissler-Patig predicts that the future 39.2-meter European Extremely Large Telescope (E-ELT) should be able to directly image HD 85512b. Analyzing the starlight it reflects will provide important information about the planet's atmospheric composition. "The E-ELT will be able to probe for biomarkers," Kissler-Patig says, referring to chemicals thought to indicate the presence of life.
While ESO is planning more-sensitive planet-hunting instruments for its existing Very Large Telescope and for the future E-ELT, Kepler is facing an uncertain future. "Kepler's goal of finding true Earth analogs can only be reached by extending the mission duration" past its planned operational lifetime of 3.5 years, Batalha says. In February 2012, NASA will decide on a possible mission extension. Marcy is optimistic. Kepler is so incredibly successful, he says, that it seems unlikely NASA will terminate the mission next year. "I'm sure NASA is wiser than that."