Journey to the Center of Geek

There's actually one additional interesting point not made entirely clear in the article.

Two newly discovered planets outside our solar system have roughly the size of Earth and the mass of Neptune, US scientists announced Tuesday.

"We found two such planets both 15 to 20 Earth masses," Geoffrey Marcy, of the University of California at Berkeley, told journalists.

"Both planets orbit very nearby stars in our solar neighborhood, and indeed they are only about 35 light-years away." ...

One of the two planets orbits the star 55 Cancri, already known for its three gaseous giants. The new planet's mass is 18 times that of Earth and slightly more than Neptune's.

The other new planet orbits star Gliese 436, and has a mass 20 times Earth's.

"These two new planets have sizes - diameters - that are in fact comparable to that of the Earth," March said at the press conference organized by NASA. "They are only two or three times the diameter of our Earth."

"With these 15-Earth-mass planets that we are now discovering, I think it is fair we are poised unexpectedly for the next step in planet discovery, namely finding truly Earth-mass planets," he said. ...

Last week, European astronomers also announced the discovery of two planets beyond our solar system. One, detected orbiting a star in the Autel constellation, mu Arae, has a mass 14 times that of Earth, making it the smallest planet ever found outside the solar system.

More than a hundred planets have so far been discovered beyond the nine planets around our Sun.

The additional interesting thing is that volume is related to the cube of the radius, or half the diameter. So if a planet's diameter is three times that of Earth, so is it's radius, and it's volume is thus 3x3x3 = 27 times that of Earth. So what we have here are two planets with volumes of 8 to 27 times that of Earth with masses 15 to 20 times that of Earth. That means that their densities are about the same as Earth. These are not gas giants or even gas planets; they are much more Earth-like. And that is a significant find.