The planet is in the habitable zone of a nearby small star
For the first time, astronomers have detected an atmosphere around a rocky planet orbiting in the habitable zone of a nearby star.
These planets have long been considered the best places in the galaxy to search for signs of extraterrestrial life. But until now, “the planets looked like bare rocks,” says astronomer Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany. It wasn’t clear if planets like this could even have atmospheres. This study shows that they can, report the astronomers in Science July 16.
“I now feel more optimistic about the chances of creating habitable environments beyond the solar system,” Kreidberg says.
The planet, LHS 1140b, is approximately 5.6 times more massive than Earth and 1.73 times larger than Earth. These dimensions correspond to an Earth-like composition enveloped in a gaseous atmosphere, or perhaps in a global ocean of water. It also orbits far enough from its star that its surface temperature can support liquid water.
But the host star is a cool, low M dwarfthe most common type of star in the galaxy. These stars produce notoriously bad neighborhoods, emitting intense radiation and brutal stellar flares that could destroy their planets’ atmospheres.
“The open question is: Can rocky exoplanets even retain their atmospheres around M dwarfs? says planetary astronomer Collin Cherubim of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
Cherubim and his colleagues used the Magellan Clay Telescope at Las Campanas Observatory in New Mexico to observe LHS 1140b passing in front of the star and blocking some of its light.
Just before this transit began and for some time afterward, the researchers observed the molecular imprint of excess helium gas absorbing light from the star. This excess is a sign of helium leaking out of the planet’s atmosphere, says Cherubim – which means there is one.
“Really, the most exciting thing here is that the planet has an atmosphere,” Cherubim says.
Previous studies found signs of helium escaping from larger planets as LHS 1140b, called mini-Neptunes. These may be losing their atmosphere and becoming bare rocks.
LHS 1140b is also losing helium. “It’s changing slowly,” Cherubim says. “On astronomical time scales, billions of years from now, the atmosphere will be very different. »
But it probably won’t be lost forever. LHS 1140b is colder than the mini-Neptunes and loses atmospheric mass at a much lower rate. The planet has already retained its atmosphere for more than 3 billion years and could remain stable for a billion more, Cherubim calculates. Even then, the planet could maintain an atmosphere composed of heavier gases like carbon dioxide or nitrogen.
Follow-up observations with the James Webb Space Telescope will help determine other characteristics of the atmosphere, such as how much helium it contains and what other molecules are found there. Another detection would also confirm that the atmosphere is there, Kreidberg says.
A rocky planet with a helium-rich atmosphere is a surprising discovery, she adds. “It’s a kind of planet we’ve never seen before,” she said. Its atmosphere is thicker than expected for a rocky planet, but not thick enough to be a gas giant. “Because it’s such an exotic and weird planet, it never occurred to me to go looking for it.”