In our solar system, we have four gas giants, which are large planets composed mostly of gases and containing small, rocky cores. These include Jupiter, Saturn, Uranus, and Neptune.
But that’s not where the concept of gas giants ends. Though scientists have come to learn various things about each one, recent observations have led them to believe the Neptune-like exoplanet HAT-P-26b has an atmosphere less rich in heavy elements that once thought. This means it was likely formed close to its star.
Though a gas giants’ metal content can be easily organized on a straight line, where the lighter a gassy planet is, the more heavy elements its atmosphere contains, studying the atmosphere of planets around other stars is more complex. In this case, the planet needs to pass between its star and us, and the star must be bright enough for changes in its light to shine through the planet’s atmosphere. Such conditions make it a challenge to determine if the heavy element connection is valid for gas giants outside our solar system.
But why does it matter what’s going on outside our solar system exactly?
“We really need to learn how other solar systems can form in order to put our own solar system in context,” explains 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.”
Using the Hubble Space Telescope, Wakeford and her colleagues looked at exoplanet HAT-P-26b and were happy to discover water in its atmosphere. Thought to be 440 light years away from our solar system, the exoplanet shares a mass similar to Neptune’s.
“Seeing this beautiful signature of water in the atmosphere was perfect for us,” says Wakeford. “It’s a great planet.”
Though it’s not the first time water has been found in an exoplanet atmosphere, it does offer some intriguing insights, mainly that the researchers could use the water signal to better pinpoint the amount of heavy elements contained on HAT-P-26b.
The team discovered fewer heavy elements than anticipated, having implemented data from other gas giants.
“This is the first step away from the line that we’ve ever seen,” says Wakeford.
The thought is that if HAT-P-26b is out of line now, it likely has been since the beginning.
The fact HAT-P-26b is missing some of the heavy elements that form planetary atmospheres reveals the possibility that it formed closer to its star than the gas giants in our solar system.
This may reveal in turn that we know less about how Neptune-like worlds form than previously assumed.
“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.”
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