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Planet Hunters Nab Three More

Astronomers have spotted three Neptune-size planets outside our solar system.

There are other planets around other stars in other solar systems. That's the old news.

Now, new observations have turned up the three smallest, most Earthlike planets ever found outside our solar system. Each one weighs between 14 and 25 times the mass of Earth. That makes them about the size of Neptune.

This illustration by an artist shows a newly discovered Neptune-size planet orbiting the red dwarf star Gliese 436, which lies 30 light-years from Earth.

This illustration by an artist shows a newly discovered Neptune-size planet orbiting the red dwarf star Gliese 436, which lies 30 light-years from Earth.


Until now, none of the extrasolar planets discovered so far has looked anything like Earth. Out of about 135 such planets, nearly all are roughly 300 times Earth's mass. That's the same size as Jupiter, the biggest planet in the solar system. And like Jupiter, they're all big balls of gas around a solid core of rock and ice. Smaller, rocky planets like Earth are much harder to detect.

With advances in technology, however, the view is getting better. The new planets are so small and far away that astronomers still can't see them directly. Instead, they look for tiny wobbles in the motion of a planet's star, caused by the planet's gravity.

Using this technique, researchers from the University of California, Berkeley, found one of the planets around a star called Gliese 436. The planet appears to be between 21 and 25 times the size of Earth. It speeds around its planet once every 2.64 days.

The second planet orbits a star called 55 Cancri, which is similar to the sun. It's probably 18 times the mass of Earth, and its "year" is just 2.81 days long, say researchers from the University of Texas in Austin. Their discovery adds to three Jupiter-size planets already known to orbit the same star.

The third planet, detected by astronomers in Portugal, is at least 14 times as massive as Earth. It orbits a star called mu Arae. Because the new planets are so small and close to their stars, astronomers suspect that the planets are rocky.

The discoveries may help astronomers figure out how planets form. And with Earthlike planets within our reach, the chances of finding life outside our solar system have improved a bit. Someone might be out there, after all.—E. Sohn

Going Deeper:

Cowen, Ron. 2004. Rocky road: Planet hunting gets closer to Earth. Science News 166(Sept. 4):147. Available at http://www.sciencenews.org/articles/20040904/fob1.asp .

Sohn, Emily. 2003. A planet from the early universe. Science News for Kids (July 16). Available at http://www.sciencenewsforkids.org/articles/20030716/Note2.asp .

______. 2003. A planet's slim-fast plan. Science News for Kids (March 19). Available at http://www.sciencenewsforkids.org/articles/20030319/Note2.asp .

You can learn more about the discovery of planets outside the solar system at planetquest.jpl.nasa.gov/ (NASA Jet Propulsion Lab).

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