Planet hunters have found two worlds roughly the mass of Neptune, each orbiting a star within 30 light-years of our solar system. The planets are likely gaseous or mixtures of ice and rock, but they might be barren rock worlds like Mercury.

The announcement today by a U.S. team comes just a week after a competing European group revealed a similar discovery of a slightly less massive planet that most likely has a rocky surface and was billed as a "super Earth."

The discoveries pushed the limit of current search technology and reveal worlds of a sort never seen outside our solar system. Together they suggest is it just a matter of time before objects much like Earth are detected.

The planets were found using a Doppler-shift method that notes a wobble in a star caused by the gravity of the orbiting planet. No pictures are available. The two newest discoveries were led by Geoff Marcy of the University of California, Berkeley and Paul Butler from the Carnegie Institution of Washington, this globe's most prolific planet-hunting duo, and will be discussed in papers to be published in the Astrophysical Journal.

Hot places

Prior to today's announcement at a NASA press conference, Marcy detailed the pair of discoveries for

One of the planets orbits the star 55 Cancri, already known to host three gas giant planets. Its fourth known world is 18 times as massive as Earth, just slightly more massive than Neptune, Marcy said. It completes a year in a mere 2.81 Earth-days, circling just 3.5 million miles (5.7 million kilometers) from the star.

"It could be made of gas, rock and iron, or rock and ice, and there may or may not be an atmosphere," Marcy said. "We don't know."

The discovery resulted from measurements taken by Lick Observatory. Barbara McArthur at the University of Texas helped pin it down with more observations from the ground-based Hobby-Eberly Telescope and the Hubble Space Telescope.

The other newest known planet is about 25 times as hefty as Earth. Its circular orbit is tight, too, a mere 2.64 Earth-days long around the star Gliese 436. Marcy speculated that this world, too, could be gaseous like Saturn or Jupiter.

"But with a mass near that of Neptune, it could have a rock-ice core and a thick envelope of hydrogen and helium gas," he said. "Alternatively, it could be made of only rock and iron, like Mercury."

The Gliese 436 planet is probably tidally locked to the star, Marcy said, always showing it the same face -- just as our Moon does with respect to Earth. If rocky and barren, the lit surface would be about 710 degrees Fahrenheit (377 Celsius), and the back side far below zero. If the planet has a thick atmosphere like Venus, however, then the entire surface would be hot.

The discovery was made with the Keck Observatory in Hawaii.

Next up: Other Earths

The Doppler method was used to find the first planet beyond our solar system in 1995. Initially it found only Jupiter-sized planets very close to stars, because those had the greatest gravitational influence on the stars being surveyed. The technique was later refined to spot large planets in more distant, Jupiter-like orbits and also less massive, Saturn-sized objects.

"We had found Jupiters and Saturns, and now we've found Neptunes," Marcy said. "The next destination is other Earths."

Comparing Mass

The mass of select planets, including the new discoveries around three stars, compared to Earth.

Planet Earth Masses
Neptune 17.2
Saturn 95
Jupiter 318
mu Arae 14
55 Cancri 18
Gliese 436 25

It is not clear just how long that breakthrough will take or who will do the finding, however.

The "super Earth" announced last week, by a competing team based in Switzerland, is just 14 times the mass of Earth and also in a tight orbit, around a star called mu Arae. It is almost surely rocky, experts say. The Europeans may have an edge in finding something smaller.

The European program "should be able to push down to masses of, say, eight Earth masses -- twice Earth-size -- for the shortest period orbits, those of just a few days," said Alan Boss, a planet-formation theorist at the Carnegie Institution in Washington. "There could be claims of planets with masses of about eight Earth masses within the next year or less."

Migrating planets

All three discoveries illustrate a wide range of solar systems.

The mu Arae planet found by the Europeans is bounded well to the outside by a Jupiter-mass planet. It must have formed inside the larger planet, and theory suggests that it would have developed as a rocky world. It might have a thin atmosphere.

Boss does not think the two planets found by the U.S. team are made primarily of rock. They likely formed much farther out and migrated inward to their present orbits, Boss explained in an e-mail interview prior to the press conference.

"Hence the Gliese 436 and 55 Cancri Neptune-mass planets might actually be more like Neptune in composition as well as mass, with significant ice and gas in addition to rock," Boss said.

All this depends on how planets form and migrate, two things astronomers know surprisingly little about. Inward migration has also been used to explain the dozens of "hot Jupiters," the most massive gas planets found in orbits that last just a few days.

"Understanding planetary migration is now a major challenge for theorists," Boss said.

Prior to today's announcement, the 55 Cancri system was seen as one that would, mathematically speaking, allow the presence an Earth-sized planet in a habitable, Earth-like orbit. That possibility seems less likely now. While such a planet could have a stable orbit, Boss said, "it is highly unlikely that it would have survived the orbital migration of the inner planets through this region."

The discovery of potentially habitable planets -- roughly Earth-sized and in wider orbits -- will require a leap of technology, one that's already planned. A pair of soon-to-launch space observatories will soon race to find planets just like Earth in size and orbit.

NASA's Kepler observatory is slated for launch in 2007. The European Space Agency's COROT mission will launch in 2006 under current plans. Both missions will survey large numbers of stars and are expected to detect several rocky planets, at least.

The newest discoveries suggest these space telescopes could find "hoards of close-in Earth-size planets," Boss said.

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