Astronomers report smallest planets yet discovered

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-sized planet capable of supporting life, but today's instruments cannot detect bodies that small.

'Big brothers'

"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.

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

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 of the new planets are very close to their stars, making them difficult to spot.

One of them orbits very close to the star named 55 Cancri, which is about the same size as our sun and located 41 light-years away in the constellation Cancer. The new planet was located by University of Texas-Austin astronomers using the Hobby-Eberly telescope in the Davis Mountains southeast of El Paso.

The star already had three known gas giant planets looping it in orbits that take anywhere from 14 to 4,520 days. The new planet is the innermost of the quartet, zooming around the star in 2.8 days from a distance of about 3 million miles.

Researchers acknowledged there probably are several different types of solar systems orbiting distant stars. But for now, the 55 Cancri system bears the closest resemblance to ours.

"This star is the premier lab for study of the formation and evolution of planetary systems," said astronomer Barbara McArthur, who led the Texas team.

The other new planet discovered by American scientists orbits a star called Gliese 436, that lies about 33 light-years from Earth in the direction of the constellation of Leo.

This Neptune-sized planet also sits 3 million miles from its star and whips around in a tight circular orbit once every 2.64 days.

It was discovered by a team led by Marcy and Paul Butler of the Carnegie Institution using one of the twin telescopes atop Mauna Kea in Hawaii.

"Our ability to find planets in the Neptune-mass range tips the scales for finding other Earths sooner rather than later," Butler said.

Together, Marcy and Butler have spotted about half of the known exoplanets. They studied Gliese 436 for four years beginning in 2000.

Besides the exoplanet's size, what makes the discovery remarkable is that Gliese 436 is a red dwarf star that produces only 2 percent or 3 percent as much light as the Sun. Stars in this category account for 70 percent of the stars in the Milky Way galaxy, but until now, astronomers had not believed that such faint stars would yield new planets.

Now they are "revamping" their detection strategy to look at more dwarf stars.

"We estimate there is something like 20 billion planetary systems existing in our Milky Way galaxy alone," Butler said.

Meanwhile, the European team on Aug. 25 described its new object as a "super-Earth."

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.

To find the new planets, both the U.S. and European teams relied on new instruments that more precisely measure how much a star wobbles when planets orbit, exerting a gravitational tug.

Marcy described the competition as "stiff, but friendly."

Two more exoplanets -- both gas giants -- also were spotted in the past week. One was by the same European team using a telescope in Chile.

The other was discovered by a U.S.-based team using a network of small telescopes on California, Texas and the Canary Islands. Instead of measuring star wobbles, they measured the tiny amount that a star dims when a planet passes by in its orbit.