While none of these new bodies would be mistaken as Earth's twin, some appear to be noticeably smaller and more solid — more like Earth and Mars — than the gargantuan, gaseous giants identified before.
Planet-hunting is the hottest field in astronomy, with hundreds of researchers joining a race that just a decade ago was reserved for a few dreamers. This past week has been a dizzying one with three teams in the United States and Europe rushing to announce their discoveries of new exoplanets — those orbiting stars other than our sun.
On Tuesday, NASA was expected to cap the excitement with details on what the space agency describes as a "new class" of exoplanets found by one of the American teams, led by University of California-Berkeley astronomer Geoffrey Marcy.
At least two of the newly discovered bodies — including one NASA is expected to describe — probably are comparable in scale to intermediate-sized planets in our solar system like Neptune and Uranus, which are about 14 times the mass of Earth. That sounds huge, but many of the previous exoplanets have been closer to the size of Jupiter, about 318 times the mass of Earth.
"It's been a great week," said David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, where scientists announced a competing discovery last week. "They have finally broken through to a new level."
Now many experts say it won't be long before astronomers detect planets that are similar to Earth's dimensions and characteristics — perhaps even suitable for sustaining life with an oxygen-rich atmosphere and oceans.
NASA's announcement comes on the heels of the first discovery ever of a multiple planet system beyond our own solar system by a European team led by Michel Mayor and Didier Queloz of the University of Geneva. The pair discovered the first exoplanet in 1995, and has found dozens of others in what observers describe as a "good-natured," but serious race with the Americans.
NASA officials wouldn't discuss details of the latest findings Monday.
Even the largest planet cannot be directly seen by the best telescopes because it is hidden in the halo of its star's bright glare. But astronomers have come up with methods for detecting these bodies by measuring how much a star wobbles from the gravitational tug of an orbiting planet.
The European team describes its new object as a "super-Earth" that is the smallest planet to be found outside of our solar system.
The planet was spotted in June orbiting a southern hemisphere star called mu Arae located 50 light-years away in the constellation Alter. It orbits mu Arae every 9.5 days and has a temperature of more than 1,160 degrees. Its dimensions are more like Neptune or Uranus, and it represents the upper limit of the size of solid planets.
This "super-Earth" appears to be orbiting between the star and a larger, previously known exoplanet, making it the first multiple planet system to be spotted beyond our own solar system.
"We are getting closer to finding a solar system," more like our own — one that has an Earth-sized planet in the inner region and a Jupiter-sized planet in the outer region, said Alan Boss, a planet-formation theorist at the Carnegie Institution in Washington. Boss did not directly participate in the new planet searches.
The third and fourth planets are both Jupiter-sized, less Earthlike gas giants. One was spotted by the Europeans and is so close to its parent star that it completes an orbit in just four days. The other was discovered by the Harvard-Smithsonian center and orbits a star in the constellation Lyra 500 light-years away.
What makes the American discovery noteworthy is it was found through a network of small telescopes.
In the next 20 years, NASA hopes to launch new space observatories to get a sharper view of exoplanets and perhaps find some that are more Earthlike. The first mission, known as the Kepler observatory, is scheduled to launch in 2007.
Meanwhile, astronomers caution against jumping to grand conclusions about these strange new worlds.
"Very few solar systems seem to be built along the same lines as ours," said Timothy Brown of the National Center for Atmospheric Research in Boulder, Colorado, who was a leader of the Harvard-Smithsonian team.
Brown compares planet-hunting today to the efforts of early biologists who were confounded by strange new specimens.
"You tend to think that all fish have fins, and then you pull up an octopus," Brown said. "There's just a vast amount that we don't know."