By Dennis Overbye,
New York Times Staff Writer, Wednesday, January 25, 2006

Now you see it, now you don't.

Astronomers said today that, by virtue of the ceaseless shifting of the billions of stars in the Milky Way and a trick of Einsteinian physics, they had briefly glimpsed the most Earthlike planet yet to be discovered outside the solar system. It is a ball of rock and ice only about 5.5 times as massive as the Earth, orbiting a dim reddish star 21,000 light years from here.

The discovery, they said, suggests that rock-ice planets like our own are predominant in the cosmos. That bodes well for future planet hunting missions from space like NASA's Terrestrial Planet Finders. The distant planet manifested itself as a brief flash as it passed in the night in front of an even more distant star, and the planet's gravity focused and momentarily brightened the light from the even more distant star. It was all over in less than a day, a cosmic eye blink.

"It was the blip in the night that we have been waiting for," said Jean-Philippe Beaulieu of the Institut d'Astrophysique de Paris, who led a collaboration of 73 astronomers from a variety of countries and institutions. Before reporting their findings in Thursday's issue of the journal Nature, they talked about them at a news conference today in Washington.

The discovery was hailed as "a big one" in an e-mail message from Alan Boss, a planetary theorist at the Carnegie Institution of Washington. And Geoffrey Marcy of the University of California, Berkeley, said, "The result looks solid to me and perhaps the planet is, too."

Dr. Marcy was co-discoverer last year of the previous record holder, a ball of rock seven times the mass of the Earth, but like most of the other known exoplanets, that planet hugs its home star in a lethally hot and close orbit.

The new planet, dubbed OGLE-2005-BLG-390Lb, is smaller than Neptune and resides about 234 million miles from its star, about the distance of the asteroid belt in our own solar system. At that distance its surface temperature would be a cool -- 370 degrees Fahrenheit, Dr. Beaulieu said.

As he explained in an e-mail message, it would have been much easier to see a big gas giant sort of planet. The unlikelihood of detecting such a small planet argued for its commonness. "If only a small fraction of the stars had such planets, we would have never detected this small planet," Dr. Beaulieu said.

Scott Tremaine, a theorist at Princeton, said, "the results suggest that rock-ice planets must be more common than gas giants."

The work is largely the result of two large teams who have built far-flung observing networks to exploit one of the features of Einstein's General Theory of Relativity, which explains gravity as the bending of space. The theory says that a massive object can act as a gravitational lens, bending and magnifying the light from more distant objects in space.

The Optical Gravitational Lensing Experiment (Ogle), led by Andrzej Udalski of Warsaw University, have been set up to monitor the brightness of millions of Milky Stars every night from a telescope in Chile, to catch fluctuations caused by the passage of intervening objects of various kinds -- dim stars, so-called dark matter objects, or planets.

Last July, alerted by the Ogle team that such a lensing event was under way, Dr. Beaulieu's Planet (for Probing Lensing Anomalies NETwork) team sprang into action to do high-resolution observations. On Aug. 9, they recorded a tiny blip on the much larger blip caused by the passage of the unseen star, the planet's parent. That tiny blip was the planet itself.

Dr. Beaulieu's observations did not fix the mass of the planet directly, only the angular separation ratio of masses between it and its parent. The astronomers then had to resort to a statistical analysis of the distribution of stars in the galaxy to derive a mass and distance.

Unfortunately, as Dr. Marcy pointed out, the result cannot be checked because the host star, along with its planet, has already disappeared into the darkness from which it came.

Asked if this was frustrating, Dr. Beaulieu said that his team knew which way it went, and that such stars might be detectable in five to seven years with better and bigger instruments.

"So it is not frustrating, it is cool!" he said. "We know we will not observe it again, but we managed to catch it!"

©2006 New York Times Company