Fifty new alien worlds, including 16 "super Earths," have been found—the largest extrasolar planet haul announced at one time, astronomers say.
The discoveries bring the total number of known extrasolar planets, or exoplanets, to 645.
"The harvest of discoveries ... has exceeded all expectations, and includes an exceptionally rich population of super-Earths and Neptune-type planets hosted by stars very similar to our sun," study leader Michel Mayor, an astronomer at the University of Geneva in Switzerland, said in a statement.
One of the newly discovered worlds, dubbed HD 85512b, lies at the edge of its star's habitable zone—the region around a star where liquid water, and thus life as we know it, can exist. (See "New Planet May Be Among Most Earthlike—Weather Permitting.")
In Search of Smaller Planets
Radial velocity, also known as the Doppler wobble technique, involves searching for wobbles in a star's light that indicate the gravitational pull of an orbiting planet.
The method most easily finds very massive planets that orbit close to their host stars. But in recent years astronomers have used radial velocity to find or confirm a number of planets down to a few times Earth's mass.
In fact, HARPS alone has detected about two-thirds of all known exoplanets with masses less than that of Neptune. In 2007, for instance, HARPS discovered another super-Earth, called Gliese 581d, that may lie within the habitable zone of its star.
For the new study, astronomers used HARPS to observe 376 sunlike stars in our home galaxy, the Milky Way.
Based on the number and variety of planets found around these stars, the team estimates that about 40 percent of all sunlike stars in the galaxy host at least one planet that is less massive than the gas giant Saturn.
Life-Supporting Planets Within Reach?
Scientists next plan to upgrade HARPS hardware and software and use the instrument to conduct a more refined search of nearby sunlike stars for rocky Earthlike planets that could support life.
"In the coming 10 to 20 years we should have the first list of potentially habitable planets in the sun's neighborhood," Mayor said.
Future planets discovered by HARPS could also be among the best candidates for space telescopes to look for chemical signs of life—such as oxygen—in the planets' atmospheres, the team said.
MIT astronomer Sara Seager said that the new batch of planets orbits relatively bright stars, which could allow for followup observations using other planet-seeking methods that might glean more information than the radial velocity technique.
"It's great to have entered the era that discovering super-Earths is routine," added Seager, who was not involved in the HARPS research.
"Golden Age" of Exoplanet Discovery
Alan Gould, an astronomy educator at the University of California, Berkeley's Lawrence Hall of Science, called the discoveries "pretty exciting."
"This big batch of planets contributes to our overall knowledge of what extrasolar planet systems are like—how common Jupiter-type giant planets are relative to Neptune-types and super Earths, and whether they're close in or far out from their host stars," said Gould, also a member of NASA's Kepler exoplanet-finding mission.
"These kinds of statistics can help us figure out what star-planet systems are like in general, and that has implications for whether there are Earthlike planets or not.
"This is looking more and more like a golden age of exoplanet discovery."
The 50 new planets were announced at the Extreme Solar Systems II meeting, held this week in Moran, Wyoming.