“Our own TV signals,” he said, “have already passed this star.”

Close but it seems no one's listening.

In May, Zachory Berta, a first-year graduate student of Dr. Charbonneau, called the group’s attention to blips in the Ophiuchus star that seemed to be happening every 1.6 days. If he was right, Mr. Berta said, the next transit would occur at 6 a.m. on May 13.

Dr. Charbonneau was in Washington later that day preparing for a State Department dinner when he got a group e-mail message that began: “We have a winner. Congrats Zach!”

From the drop in starlight, the astronomers could calculate the diameter of the Ophiuchus planet, known now as GJ 1214b. Then they used a sensitive spectrograph on a 3.6-meter telescope in Chile to measure its gravitational tug on the star, thus deriving the planet’s mass. Using those two numbers, Dr. Charbonneau and his colleagues could calculate the density of the planet, about one-third that of Earth.

“What we probably have here is a water world,” Dr. Charbonneau said.

Dr. Charbonneau said the weight of the new planet’s presumptive atmosphere kept the water liquid rather than just boiling into space. He acknowledged that a different recipe, with more rock and a very puffy atmosphere, would also fit the data. That is unlikely, he and other planet experts say, but the steam-world theory may be soon tested.

The new planet is close enough to be studied directly by telescopes on or near Earth. Indeed, Dr. Charbonneau said his team had already applied for observing time on the Hubble Space Telescope...