Neptune-like exoplanet spotted that has a watery atmosphere

By Leah Crane

Not all gas giants are created equal. Observations of the Neptune-like exoplanet HAT-P-26b show that its atmosphere is less rich in heavy elements than expected, meaning it probably formed close to its star.

In our solar system, the gas giants’ metal content can be plotted on a straight line: the lighter a gassy planet is, the more heavy elements its atmosphere contains. Jupiter’s atmosphere has less heavy elements than Neptune’s, for example.

But in order to study the atmospheres of planets around other stars, they need to meet very particular conditions: the planet must pass between its star and us, and the star must be bright enough for us to discern changes in its light as it shines through the planet’s atmosphere. That makes it difficult to tell if the heavy element relationship holds true for gas giants outside our solar system.

“We really need to learn how other solar systems can form in order to put our own solar system in context,” says Hannah Wakeford at NASA’s Goddard Space Flight Center. “What we’re trying to learn ultimately is how easy it is to form a solar system like our own.”

Wakeford and her colleagues used the Hubble Space Telescope to look at HAT-P-26b, an exoplanet about 440 light years away that has a similar mass to Neptune, along with an extensive but relatively tenuous atmosphere. As the world passed in front of its star, they saw distinct signatures of water in its atmosphere.

“Seeing this beautiful signature of water in the atmosphere was perfect for us,” says Wakeford. “It’s a great planet.”

Out of line
This isn’t the first time signs of water have been spotted in an exoplanet atmosphere, but it does point to something surprising. Because oxygen is heavier than helium – which astronomers consider the cutoff for so-called heavy elements – and water has oxygen in it, the researchers could use the water signal to determine the abundance of heavy elements in the atmosphere of HAT-P-26b.

They found less heavy elements than they would have expected, given data from the few other gas giants inside or outside our solar system whose atmospheres have been probed. “This is the first step away from the line that we’ve ever seen,” says Wakeford.

If HAT-P-26b is out of line now, it must have been strange from the start.

When planetary atmospheres are formed from a star’s disc of dust and gas, they acquire their heavy elements from pockets of ice and debris that are more common further from the star. HAT-P-26b’s missing heavy elements may indicate that it formed closer to its star than the gas giants in our solar system.

This could point to a surprising diversity in how and where Neptune-like worlds are formed. But since so few of these worlds have been analysed, it is impossible to say for certain that HAT-P-26b isn’t just an anomaly.

“The exciting thing about this field is that we have no idea if this is an unusual or typical planet, because it’s the lowest-mass planet for which we have this good of a measurement,” says Ian Crossfield at the University of California Santa Cruz. “This puts us on the road towards being able to study planets more like Earth.”

Journal reference: Science, DOI: 10.1126/science.aah4668