3 small planets orbiting stars

American astronomers say they have discovered the two smallest planets yet orbiting nearby stars, trumping a small planet discovery by European scientists five days ago and capping the latest round in a frenzied hunt for other worlds like Earth.

All three of these smaller planets belong to a new class of "exoplanets" — those that orbit stars other than our sun, the scientists said in a briefing Tuesday. They define this new class by the planets' smaller mass — roughly 14 to 18 times the size of Earth and equivalent to Neptune in our solar system.

Evidence of two planets announced Tuesday was spotted by two separate teams of U.S. researchers using telescopes in Hawaii and Texas. Scientists not involved in the projects lauded both, saying their planets should be recognized as the first discoveries of planets in this class — rather than the Europeans who announced their planet last week.

The dueling announcements reflect the intensity of the race to discover exoplanets. The big prize, of course, would be to find an Earth-size planet capable of supporting life, but today's instruments cannot detect bodies that small.

"We can't quite see the Earthlike planets yet, but we are seeing their big brothers," said planet hunter Geoffrey Marcy of the University of California-Berkeley, a leader of one of the teams.

Using an Olympics analogy, other scientists said the American groups have tied for first place in the planet hunt and should share a gold medal.

They noted the Americans' findings have been accepted for publication by international science journals, while the discovery led by a pair of prominent Swiss astronomers still is being reviewed for publication.

"These two were submitted in July and August, while the Swiss discovery is still in consideration," said planetary theorist Alan Boss of the Carnegie Institution in Washington, D.C., who did not participate in any of the discoveries.

"On that basis," he said of the Europeans, "I would award them the bronze" medal.

Over the past decade, astronomers have found as many as 135 planets orbiting various stars, but all of them are giant gas planets similar to Jupiter and Saturn.

Researchers don't know the composition of these new, smaller planets — or even what they look like, since they can't actually see them. To detect such bodies, astronomers measure such things as how much a star wobbles from the unseen planet's gravitational tug.

In our solar system, Neptune and Uranus are of similar size and they are composed of an icy, rocky core enveloped in a thick atmosphere of hydrogen and helium. But they sit in the farthest coldest reaches of our solar system.

By contrast, both new planets are very close to their stars, making them difficult to spot.