LOS ANGELES, June 11-- Planets having atmospheres rich in helium may be common in our Milky Way galaxy, according to a new theory based on data from NASA's Spitzer Space Telescope.

"We don't have any planets like this in our own solar system...but we think planets with helium atmospheres could be common around other stars," NASA Hubble Fellow and lead author of a new study Renyu Hu said Thursday.

Using data from NASA's Spitzer Space Telescope, researchers have found hundreds of planets, which are the size of Neptune or smaller, with tight orbits that are closer to their stars than our own sizzling Mercury to our sun. Radiation from the stars would boil off hydrogen in the planets' atmospheres, scientists say in the new study.

Both hydrogen and helium are common ingredients of gas planets like these. Hydrogen is lighter than helium and thus more likely to escape.

"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," said Hu in a statement on Thursday.

After billions of years of losing hydrogen, the planets' atmospheres would become enriched with helium, leading scientists to predict the planets would appear covered in white or gray clouds.

"The process would be gradual, taking up to 10 billion years to complete," Hu said. Our planet Earth is about 4.5 billion years old.

According to NASA, the next step to test this theory is to look at other warm Neptunes for signs of carbon monoxide and carbon dioxide, which are indicators of helium atmospheres.

These findings were accepted for publication in the Astrophysical Journal.