PHOENIX - Scientists at Flagstaff's Lowell Observatory are part of a team that has announced the discovery of the largest-known planet outside our solar system.
The planet TrES-4 is 20 times the diameter of Earth and 1.7 times the diameter of the largest planet in our solar system, Jupiter.
Scientists believe the planet is composed mainly of hydrogen and with a temperature of 2,300-degrees Fahrenheit, it's unlikely anything lives there.
"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 and lead author of an article announcing the finding in the peer-reviewed Astrophysical Journal Letters.
Lowell, along with the California Institute of Technology's Palomar Observatory in San Diego County and telescopes operating in Spain's Canary Islands, discovered the planet circling a star 1,400 light-years away in the constellation Hercules.
Lowell scientists first spotted the new planet and a smaller one in spring 2006.
Scientists at Caltech, Harvard University and the W.M. Keck Observatory in Hawaii later confirmed the discovery.
In May, scientists announced the discovery of the smaller planet, TrES-3.
"It's very solid stuff," astronomer Alan Boss at the Carnegie Institution of Washington said of the discovery of TrES-4, marveling at the planet's enormously low density - about half that of Saturn in earth's solar system.
"It's really amazing when you think that none of the planets in our solar system" have nearly that low density, Boss said. "It's just letting us know that nature has some surprises for us ... a much wider range of possibility than we could imagine."
Boss said scientists "can't understand why these so-called fluffy planets are so fluffy. It really is a mystery, just how they can be so low-density."
The puzzle is both "a major concern and a problem" for scientists, but also as "an opportunity for us to learn something new," he said.
"The fact that we're seeing things with a range of radii, that means that the planets are not as simple as we would have thought based on our solar system."
Scientists are also working on the possibility of another planet in the same constellation. "It's tough," Mandushev said. "We're not really sure what's going on there. There might actually be another planet in this field, which would be incredible."
The participating Lowell telescope is housed on top of Anderson Mesa, about 15 miles south of Flagstaff.
Lowell is best known for the 1930 discovery of Pluto, which since has been demoted from planet status.