A team of astronomers has discovered the largest known planet outside our solar system. The planet, named TrES-4, is 20 times the diameter of Earth and 1.7 times the diameter of Jupiter.

Astronomers at the W.M. Keck Observatory in Hawai'i helped confirm the discovery.

Scientists say the planet probably is composed mainly of hydrogen, and it's unlikely anything lives there as the temperature is 2,300 degrees Fahrenheit. The planet has the density of balsa wood and probably thins out considerably in the upper atmosphere.

"There is probably not a really firm surface anywhere on the planet. You would sink into it," said Georgi Mandushev, a research scientist at Lowell Observatory in Flagstaff, Ariz., and lead author of an article announcing the finding in the peer-reviewed Astrophysical Journal Letters.

The newly discovered planet circles a star 1,400 light years away in the constellation Hercules.

Lowell is part of a three-telescope network that made the discovery. The other telescopes are at California Institute of Technology's Palomar Observatory in San Diego County, Calif., and in Spain's Canary Islands. The network, called Trans-Atlantic Exoplanet Survey, uses 4-inch-diameter telescopes no bigger than the telescopes in people's backyards.

The telescope network looks for "transiting planets" — planets that pass between Earth and their star, causing a slight reduction in light. It's a way of detecting new planets that involves a degree of luck, because not all planets pass between Earth and their star.

Scientists worldwide have identified roughly 250 planets outside our solar system, although it's not possible to predict accurately the size of most because they are so far away.

Lowell scientists first spotted the new planet and a smaller one in spring 2006, and other scientists at Caltech, Harvard University and the Keck Observatory later confirmed the discovery.