Watery exoplanet’s skies suggest unexpected origin story

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 closer to home, then those exoplanets were probably created in similar ways.

In Earth’s solar system, more massive planets have a lower abundance of elements heavier than hydrogen and helium. Neptune’s abundance of heavy elements is 100 times that of the sun. Jupiter, 18 times as massive as Neptune, has only five times the solar abundance of heavy elements. Neptune’s abundance is thought to be higher because it formed farther outward, toward the edge of the disk of dust and gas that circled the young sun. There, icy rocks accumulated, which bombarded Neptune and enriched its atmosphere with heavy elements as the rocks disintegrated.

Previous studies of three exoplanets in separate planetary systems have shown a similar relationship between a planet’s mass and the abundance of heavy metals in its atmosphere.

To study an exoplanet’s atmosphere, researchers watch the planet pass in front of its parent star, blocking some of the star’s light. A fraction of that starlight gets filtered by the planet’s atmosphere, which absorbs some wavelengths of light, giving clues to its composition.  Researchers using the Hubble and Spitzer space telescopes to study HAT-P-26B’s atmosphere found a prominent sign of water in visible and infrared wavelengths. From that signature, the team inferred that the exoplanet’s atmospheric heavy metal abundance is only four to five times as much as the sun’s.

Such a low abundance relative to Neptune’s suggests that HAT-P-26b formed nearer to its star than Neptune did to the sun. That proximity could have protected the exoplanet from bombardment by bits of icy rock. HAT-P-26b, which orbits its star in roughly four Earth days, also drew in its gas directly from the disk in which the star and planets were forming, the researchers say.

Astrophysicist Adam Burrows of Princeton University says the result is a solid estimate of the heavy elements in HAT-P-26b’s atmosphere. But the data don’t yet conclusively show HAT-P-26b developed differently than the ice giants in Earth’s solar system, he says. More data in additional wavelengths would be needed to definitely describe HAT-P-26b’s history.