In a major milestone in the search for life on other planets, astronomers have detected, for the first time, an atmosphere surrounding an Earth-like, rocky planet orbiting within the habitable zone of another star.
The finding provides the strongest evidence yet that worlds with conditions similar to Earth in composition and temperature, with the potential to support life, could exist beyond our solar system.
"An atmosphere is essential for a planet to support life as we know it," said lead author Collin Cherubim, who recently earned his Ph.D. in Earth and Planetary Sciences from Harvard University.
"This is the first time anyone has found an atmosphere on a rocky planet in the habitable zone of another star."
Published today in Science, the study reports observational results detecting helium escaping from the atmosphere of LHS 1140 b, a rocky exoplanet about 48 light-years from Earth. Motivated by theoretical predictions, the discovery provides evidence that the planet possesses an atmosphere.
The planet orbits a red dwarf star within the star's habitable zone, or the region where temperatures and environmental conditions are within the range that could support liquid water on the planet's surface.
Astronomers have discovered thousands of exoplanets, including a few rocky worlds within their stars' habitable zones, but determining whether those planets have atmospheres has remained a great challenge.
"Twenty years ago we wondered whether other terrestrial-type planets even existed," said Robin Wordsworth, Gordon McKay Professor of Environmental Science and Engineering and Professor of Earth and Planetary Sciences at Harvard and one of Cherubim's dissertation advisors. "Then we learned they're common, and found some in the habitable zone. The next question was whether any of them had managed to keep an atmosphere. Now we know at least one has."
Although other studies have found rocky planets in the habitable zones of their stars, this study is the first to clearly demonstrate the presence of an atmosphere, one that has existed for billions of years.
Cherubim and his colleagues' theoretical model predicted that LHS 1140 b has an upper atmosphere rich in helium that is slowly escaping into space.
To test their prediction, the team used the Warm Infrared Echelle (WINERED) Spectrograph on the Magellan Observatory in Chile. They observed a rare alignment, where LHS 1140 b and another planet transited their star on the same night.
Although one planet showed no evidence of an atmosphere, the other, LHS 1140 b, showed helium escaping from around it, confirming that it retains an atmosphere.
Cherubim's joint advisor David Charbonneau, head of the Harvard Department of Astronomy and astronomer in the Center for Astrophysics | Harvard & Smithsonian, was initially skeptical of Cherubim's plan because it was the product of a mathematical calculation and had never been observed before for a rocky world.
But when the results came in, he was convinced.
"Collin analyzed the planets we knew about and predicted that this one would have a helium atmosphere," Charbonneau said. "Then he organized telescope time, got the data, and the detection was statistically rock solid."
The findings suggest that ground-based observations searching for escaping gases may become an important tool for studying atmospheres on rocky exoplanets.
The planet's atmosphere has likely survived for more than three billion years, the astronomers say, making it a valuable target for future observations.
Cherubim said he'd like to determine the atmosphere's full composition and eventually investigate whether the planet has surface oceans or other characteristics associated with habitability. He and his colleagues will also use his model to search for similar worlds.
"This has been a model validation, and hopefully it's just the first of many more observations to come," he said.
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