Two Planets the Size of Neptune Discovered

Astronomers announced Tuesday the discovery of two relatively small planets orbiting stars in the Earth’s galactic neighborhood and speculated that one of them might have a narrow zone where life could survive.

The planets are between 10 and 20 times larger than Earth, or about the size of Neptune, said astronomers from the University of Texas and two other institutions.

Less than a week ago, Swiss astronomers announced that they had found a similar Neptune-sized planet.

Because of the difficulty in locating “extrasolar” planets orbiting stars other than Earth’s sun, the roughly 140 previous discoveries have been “gas giants” like Jupiter and Saturn, several hundred times larger than Earth.

No extrasolar planet has been seen through a telescope. Instead, astronomers identify them through mathematical calculations and tiny motions that their gravity induces in the stars they orbit.

The new discovery encouraged speculation that within another two decades, Earth-sized planets will be found orbiting nearby stars.

“At a minimum, there are some 20 billion planetary systems in our Milky Way galaxy, some of which we now know contain planets 15 to 20 times the size of Earth,” said Paul Butler of the Carnegie Institution. “What’s left to be discovered is if ‘earths’ form as well.”

Geoffrey Marcy of the University of California, Berkeley, who co- discovered one of the two new planets with Butler, said that because of limitations in the power of present-day telescopes, “over 90 percent of the nearby stars could have solar systems identical to ours and we would not know it.”

The body found by Butler and Marcy orbits a small, dim star known as Gliese 436, which is 30 light years from the Earth in the constellation Leo. Less than half as large as the sun, Gliese 436 puts out only about 2 or 3 percent as much light.

The planet is about 2.6 million miles from the star, compared with the Earth’s 93 million miles from the sun.

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