Bruce Fegley Jr., a Washington University professor of Earth and planetary sciences, said the farther out you go in the solar system, the more water you find.

"The theory about the gas giant planets (Jupiter, Saturn, Uranus and Neptune) is they have primary atmospheres, which means their atmospheres were captured directly from the solar nebula during accretion of the planets," said Fegley.

"On the other hand, the terrestrial planets (Venus, Earth and Mars) have secondary atmospheres formed afterward by outgassing -- heating up the solid material that was accreted and then releasing the volatile compounds from it. That then formed the earliest atmosphere."

He and collaborator Laura Schaefer said with new theoretical models they are able to surmise the outgassing of materials that went into forming the planets and make predictions about the atmospheres of extrasolar planets.

Fegley presented his research earlier this year in Chicago during the 233rd national meeting of the American Chemical Society.