Water vapor has been detected in the atmosphere of a Neptune-size exoplanet about 150 light-years from Earth, according to research presented Thursday by astronomers at the University of Kansas.
The exoplanet is a gas giant in a close orbit around its star, which means it's not thought to be a habitable place that can support alien life. But the scientists said their findings demonstrate how studying the chemical makeup of exoplanet atmospheres can help astronomers learn how the planets came to be and how they compare to more familiar planets closer to home.
"This planet is a little bigger than our Neptune, but it's very, very different," said Jonathan Brande, a doctoral student in physics and astronomy at the University of Kansas, who led the research. "So the question is: How did this planet form and get to where it is now, and how did our Neptune form and get to where it is?"
The exoplanet, known as TOI-674 b, is about 1.3 times the size of Neptune in our solar system, but it is in a much tighter orbit, taking just under two days to circle its host star, compared to the roughly 165 years it takes for Neptune to journey around the sun.
It's not the first time water vapor has been discovered in the atmosphere of an exoplanet. In 2019, the Hubble Space Telescope found water vapor on a distant planet known as K2-18b, which is in the "habitable zone" of its parent star, where conditions are such that liquid water could exist on the planet's surface.
But Brande said it's unusual to see a Neptune-size planet with a detectable atmosphere so close to its parent star, which is likely to indicate that the exoplanet was born farther out before it migrated to its current position.
The research hasn't yet been published in a peer-reviewed journal. Brande said the findings have been submitted to The Astronomical Journal.
Scientists study exoplanets by observing them as they pass in front of, or transit, their host stars. Measuring the spectrum of light that shines through an exoplanet's atmosphere during transits can reveal the presence of water vapor or other telltale chemical signatures, such as methane or carbon dioxide.
In this case, TOI-674 b's size made it an ideal target. Every time the exoplanet passes in front of its star, it blocks 1.3 percent of the star's light, Brande said.
"That makes it pretty easy to study in transit," he added.
Brande and his colleagues observed the atmosphere of TOI-674 b using instruments aboard the Hubble Space Telescope. They combined the observations with data from NASA's Transiting Exoplanet Survey Satellite, which was launched in 2018, and the agency's Spitzer Space Telescope, which was decommissioned last January after a 16-year mission.
NASA's newly launched James Webb Space Telescope will be able to study exoplanet atmospheres in much greater detail once it officially comes online later this year. The $10 billion observatory recently completed a complex unfurling process in space, and it is about midway through a 1 million-mile journey to its final orbital perch.
Ian Crossfield, an assistant professor of physics and astronomy at the University of Kansas, said the water vapor discovery offers "just a foretaste" of what astronomers will be able to learn about exoplanet atmospheres with the Webb telescope.
The researchers plan to continue studying TOI-674 b and hope to gain more insights into its atmosphere.
"It's exciting for so many reasons," Brande said, "and now I have an excuse to write a proposal that will hopefully get accepted for time to work with James Webb."