Planetary system

Watery exoplanet’s skies suggest unexpected origin story

HAT-P-26b illsutration
WET AND WILD The exoplanet HAT-P-26b, illustrated here, has relatively low levels of heavy elements in its atmosphere, compared with those found in Neptune’s atmosphere. As a result, the exoplanet may have had a different origin story than ice giants in our solar system.

A watery world about 430 light-years from Earth may have had a relatively calm origin.

The Neptune-mass exoplanet, HAT-P-26b, has surprisingly low levels of heavy elements in its atmosphere, suggesting that it formed close to its star, researchers report in the May 12 Science. That’s different from how the ice giants in Earth’s solar system, Neptune and Uranus, formed, suggesting possible new insights into different ways planetary systems originate throughout the galaxy.

“With the observations of exoplanets’ atmospheres, we are looking outward to look in,” says study coauthor Hannah Wakeford, an astronomer at NASA’s Goddard Space Flight Center in Greenbelt, Md.

Scientists mostly use computer simulations to try to understand how planetary systems form. These simulations are based, in part, on how the planets in Earth’s solar system coalesced, but it’s unclear how common these types of planetary origins are. Many Neptune-sized worlds, for instance, have orbits vastly different than the ice giants of Earth’s system. But if the abundances of heavy elements in atmospheres of exoplanets in other systems resemble the abundances for planets of similar mass…