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Science

Water vapor found in the atmosphere of a hot, Neptune-sized planet

Astronomers have found lots of water in the atmosphere of a Neptune-sized exoplanet — a planet far outside of our Solar System. It’s just one of a handful of times that scientists have been able to detect water in the atmosphere of an exoplanet of this size and mass. And they’re using these measurements to piece together how this world may have formed.

The planet, dubbed HAT-P-26b, was first spotted in 2011 tightly orbiting a star 430 light-years away. It was found because it’s a transiting planet, meaning it periodically passes between its star and Earth on its orbit. During these passes — or transits — the planet periodically dims the star’s light, making enough of a change that can be seen from Earth. On each pass, this light also filters through the planet’s atmosphere. Using NASA’s Hubble Space Telescope, astronomers observed this filtered starlight during four different transits of HAT-P-26b, and were able to figure out just how much water is in its atmosphere. “Each molecule absorbs light in a different way,” says Hannah Wakeford, a postdoctoral researcher at NASA’s Goddard Space Flight Center and lead author on a study about HAT-P-26b published today in Science. “So we were looking for those signatures of the different molecules.”

It’s not the first time water has been found in the atmosphere of another planet. In fact, water is fairly common in the atmospheres of planets in our own Solar System; all of the gas giants in our neighborhood — Jupiter, Saturn, Uranus, and Neptune — have water surrounding them. But finding water in the atmospheres of exoplanets can be tough, since they’re so far away. Usually, most of our atmospheric data comes from exoplanets that are bigger, about the size of Jupiter, since those are easier to see. But today’s study is one of the most detailed about a “warm Neptune” — an exoplanet the size and mass of Neptune that orbits much more closely to its parent star.

Like Neptune, HAT-P-26b is a gas giant, so it’s probably not going to support life, despite the water in its atmosphere. But knowing the water content is helping Wakeford and her team figure out what else is circulating in the planet’s atmosphere. Based on the Hubble measurements, there seems to be heavy elements — or elements heavier than hydrogen and helium. This abundance of heavy elements is known as metallicity, and it tells us what conditions were like when a planet was forming. Based on this assumed metallicity, Wakeford argues that HAT-P-26b probably formed even closer to its star than it is now.

 Photo: NASA
NASA’s Hubble Space Telescope.

Astronomers often use our Sun to describe how a rich a planet’s atmosphere is in heavy elements. If a planet has a lot more heavier elements than the Sun, the world is said to have high metallicity. If a planet doesn’t have substantially more heavier elements than the Sun, that equals low metallicity. In the case of HAT-P-26b, the exoplanet only has 4.8 times the amount of heavy elements than what’s found in the Sun. That’s considered a fairly low metallicity by our Solar System’s standards.

That came as a bit of a surprise, Wakeford tells The Verge. The large gas giants in our Solar System — like Jupiter and Saturn — don’t have many more heavier elements than the Sun: Jupiter’s metallicity is five times greater than the Sun and Saturn’s is 10 times greater. Meanwhile, smaller gas giants like Neptune and Uranus have metallicities that are about 100 times greater than the Sun. “We were expecting [HAT-P-26b] to have a very high metallicity, but what we found is it’s actually closer to Jupiter in the amount of heavy elements it has in its atmosphere,” says Wakeford.

This gave Wakeford and her team the idea that maybe HAT-P-26b was really close to its star when it was forming. When our Sun was born, it was surrounded by a giant rotating disc of hot gas and debris. But parts of the disc farther out from the Sun were much colder than the inner regions. With cold enough temperatures, the disc’s material would have started to freeze into ices — and these ices would have been rich in heavier elements, according to Wakeford. So the gas giant planets that formed far enough out, like Uranus and Neptune, would have scooped up more of these metal-rich ices during formation. Jupiter and Saturn probably formed closer to the Sun, where fewer ices were present. That’s why ultimately the researchers think HAT-P-26b formed pretty close to its parent star. That would explain why HAT-P-26b bucks the trend.

However, this is just a theory based on the amount of water seen in the exoplanet’s atmosphere. And one expert says he would have been slightly more cautious on how he would have interpreted the planet’s metallicity based on those water measurements alone. “I could take the same data and interpret it 10 different ways,” Kevin Heng, an exoplanet expert at the University of Bern in Switzerland, who was not involved in this study, tells The Verge. “I’m not saying they’re wrong, but it’s not convincing to me that this interpretation is unique.”

Still, Heng says the water measurements are robust, and that’s good news for the study of exoplanets moving forward. Studying atmospheres in detail is key to figuring out what it’s like on the surface of an exoplanet. HAT-P-26b may not be suitable for life, but other planets might be. And the better we get at studying atmospheres, the closer we get to finding an exoplanet with an atmosphere and composition suitable for life.

“First we learned how to find exoplanets — now that’s routine,” says Heng. “Then the next step is to find out if these things have atmospheres. We’re at the point where doing it for big Jupiter-like planets is routine, and the next step is to do it for smaller things.”

And soon, we may have a very powerful tool that can help us study atmospheres in more detail. Next year, NASA is launching the James Webb Space Telescope, considered the successor to Hubble. The new observatory will be the most powerful space telescope in operation once it launches, capable of observing exoplanets in unprecedented detail. The James Webb Space Telescope should be able to do follow-up observations of this planet, helping to determine exactly what is in its atmosphere. “It’s just going to revolutionize how we measure these things,” says Heng. “With Webb, it would confirm the detection of water; it should be quite easy to do.”

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