Our Milky Way galaxy may contain thousands of examples of an exotic class of planets unlike anything found in our own solar system, astronomers say.
What makes them different from any world orbiting our sun is that their atmospheres consist mostly of helium, researchers at NASA's Jet Propulsion Laboratory say.
The existence of warm, Neptune-sized planets shrouded in an atmosphere of helium is suggested in data collected by NASA's Spitzer Space Telescope, they say.
"We don't have any planets like this in our own solar system," says Renyu Hu, a NASA Hubble scientist at JPL. "But we think planets with helium atmospheres could be common around other stars."
Hu is the lead author of a study that will appear in the Astrophysical Journal.
A surprising number of these "warm Neptune" exoplanets exist in our Milky Way galaxy, the researchers say, mostly orbiting their host stars in orbits closer than our own neighbor Mercury revolves around our sun.
Spinning around their stars in a matter of days, these planets can attain temperatures of more than 1,300 degrees Fahrenheit.
That may be one factor behind how they've developed atmospheres rich in helium, Hu says.
"Hydrogen is four times lighter than helium, so it would slowly disappear from the planets' atmospheres, causing them to become more concentrated with helium over time," he says. "The process would be gradual, taking up to 10 billion years to complete."
That would make warm Neptunes much older than the 4.5-billion-year age of the Earth.
Such planets are believed to have rocky or liquid cores surrounded by clouds of gas, which, if predominantly helium, would cause them to appear grayish or even white.
Although that can only be inferred for now, future instruments such as NASA's planned James Webb Telescope might someday directly detect atmospheric helium, the researchers say.
Helium planets are just one type of possible exoplanets in a long list that has continued to surprise and amaze astronomers, says study co-author Sara Seager of MIT, who also works with JPL.
"Any planet one can imagine probably exists, out there, somewhere, as long as it fits within the laws of physics and chemistry," she says. "Planets are so incredibly diverse in their masses, sizes and orbits that we expect this to extend to exoplanet atmospheres."
