American astronomers have discovered what they believe to be two rocky or icy planets outside our solar system, a finding that could open the door to the discovery of Earth-like planets in the near future.
Speaking at a NASA-sponsored press conference, the astronomers said that one of the newfound planets orbits the star 55 Cancri, 41 light-years away from the Earth, while the other orbits Gliese 436, a small star 30 light-years away. Both planets are about the size of Neptune, making them the smallest extrasolar planets, or exoplanets, ever detected, the astronomers said.
The findings are significant to planet hunters because all of the 130 or so exoplanets that were discovered previously are thought to be too large to be composed of rock and ice. Rather, they have had masses similar to that of Jupiter and Saturn, making them more likely to be gas giants. But recent advancements in Earth-based and space-based telescopes have allowed scientists to look for smaller and smaller planets that are closer to Earth in composition.
Finding an Earth-like exoplanet in a "sweet spot" that is not too close or too far from its sun is the holy grail of the search for extraterrestrial life.
"The news we are announcing today is going to bring us closer to the question: Are we alone in the universe?" said NASA Science Directorate spokeswoman Anne Kinney.
University of California at Berkeley astronomer Geoffrey Marcy, a member of the team that discovered the Gliese 436 exoplanet, said the scientific community should expect to see a number of breakthroughs in the field over the next 10 years.
"We can't quite see the Earth-like planets yet but we are seeing their big brothers, and hopefully we will be bearing down on the small-mass planets soon," he said.
The newfound exoplanets were detected using a technique that measures the "wobble" exhibited by a star whenever a planet passes by. That wobble is caused by the planet's gravitational pull and can be measured by examining the Doppler shift in the light emanating from the star.
A European team announced last week that it had used a similar technique to discover a Neptune-size planet circling a star known as mu Arae, 50 light-years from Earth. But the group has not yet had a paper on its findings accepted by a peer-reviewed astronomy journal. Until it does so, NASA's Kinney said it would not be appropriate to discuss the discovery.
The two American teams, meanwhile, have already had their papers accepted by the Astrophysical Journal. The papers are expected to be published in the December issue.
The proximity of the American and European announcements highlights the friendly but active competition that has driven planet hunters on both sides of the Atlantic in recent years.
During Tuesday's press conference, Carnegie Institution of Washington astronomer Alan Boss, who was not involved in the recent discoveries, prodded his European counterparts with references to the Olympics. "We are here to increase the number of U.S. gold medals," he said. "These (American) teams should get two more gold medals for these discoveries."
When asked about the unpublished work of the European team, Boss responded, "Maybe I would award them a bronze in this case."
Astronomers believe there may be as many as 20 billion rocky, or terrestrial, planets in the Milky Way Galaxy alone. NASA plans to launch at least three spacecraft over the next 15 years to photograph and further explore these planets. The first of the missions, dubbed Kepler, is expected to launch in 2007.
