Peering at dim specks of stars light-years from Earth, U.S. and European astronomers have made the first discovery of rocky planets beyond our solar system, a scientific leap that bolsters the possibility of Earth-like planets in the cosmos.
Until now, the only known extrasolar planets were gas giants, akin to Jupiter and Saturn, that are at least 300 times the size of Earth.
The three newly discovered planets are 15 to 20 times the mass of Earth but small enough that scientists believe they are composed primarily of rock and ice.
One of the new extrasolar planets, moreover, is the fourth planet discovered circling its star, making it part of the most complex known planetary system outside our own.
"We are beginning to see smaller and smaller planets. Earth-like planets are the next destination," said astronomer Geoff Marcy of UC Berkeley, co-leader of one of the discovery teams.
"Up until now, the technology has limited planet detection to those in the Jupiter- and Saturn-mass range," added his coauthor, Paul Butler of the Carnegie Institution of Washington.
"We've entered a new era for planet hunting."
Two of the new discoveries were revealed at a NASA-sponsored news conference Tuesday. A third was announced Monday by Swiss researchers.
Reflecting the intense, though friendly, competition in the race to discover planets, the Americans dismissed the Europeans' priority claim because their paper had not yet been accepted for publication, whereas the Americans' had.
"Using an Olympics analogy, we would have to award the Swiss only a bronze medal," said astronomer Alan Boss of the Carnegie Institution, who was not involved in the research.
All the teams deserve medals for a very important reason, he added. "This is evidence for a new class of planets about which we have not had any information," he said.
"The fact that we are discovering a new class is very reassuring for the possibility that we would find new Earth-like planets."
Anne Kinney, NASA's director of astronomy and physics missions, said the discoveries may help answer a simple question: Are we alone in the universe?
Until nine years ago, astronomers had discovered no planets outside our solar system, and many researchers did not think such observations were feasible. Now astronomers know of 139.
Marcy and Butler pioneered the field using modestly sized telescopes to study the wobble of stars caused by the gravitational influence of one or more planets.
