The Search for Life: Breakthroughs in Exoplanetary Atmospheres
The search for life outside our solar system has taken a significant turn as researchers have discovered an atmosphere around an Earth-like planet 48 light-years away that could have liquid water on its surface. While atmospheres have previously been identified around gas giant exoplanets and “sub-Neptunes,” and signs of such envelopes have been noted around rocky exoplanets outside their star’s habitable zone, this new discovery is distinct. According to Dr. Collin Cherubim, the first author of the study, “This is the first actually observationally confirmed atmosphere on a rocky planet in the habitable zone outside of our solar system.”
The Evolution of Exoplanet Discovery
The search for exoplanets—worlds orbiting other stars—is often described as astronomy’s great Easter egg hunt. It was in 1992 that professional stargazers discovered the first known exoplanet, a distant body orbiting a pulsar 2,300 light-years from Earth. Since then, astronomers using both space-based and Earth-based telescopes have found more than 6,200 exoplanets. This wealth of data has led researchers to conclude that there is at least one planet orbiting virtually every existing star. These planets represent a diverse “grab bag” of worlds, ranging from gaseous giants like Jupiter and Neptune to rocky bodies similar to Earth and Mars.
Understanding these worlds requires knowledge of atmospheric evolution. Most gas in the universe is a mixture of hydrogen and helium, and it is thought that the initial atmospheres of most planets begin with this composition. However, over billions of years, compositions shift. Hydrogen can react with other chemicals, and both hydrogen and helium can be lost to space. Venus, Earth, and Mars are believed to possess “second atmospheres,” as their original hydrogen/helium envelopes were lost or transformed. While lighter elements are lost more easily, hydrogen can be protected by incorporation into molecules like methane and ammonia. Furthermore, a planet’s gravity can help retain molecules, and a magnetic field can limit radiation’s ability to blast material away.
LHS 1140 b: A Monumental Detection
The rocky planet, called LHS 1140 b, is 48 light-years away from Earth. According to new research, it has an atmosphere that contains helium. It is the first rocky planet to have an atmosphere detected directly, and the first to be found with an atmosphere while also residing in the habitable zone. This zone represents the region where the planet is at the right distance from its star for liquid water to potentially exist on the surface. As researchers continue to search the cosmos for habitable environments, this planet checks more boxes than almost any other candidate.
For more on this story, see Astronomers Discover Potentially Habitable Earth-like Exoplanet 25 Light-Years Away.
Modeling Habitability
Scientists have discovered many rocky exoplanets, but identifying which are most likely to be habitable is a complex task. A new model of habitable exoplanets is narrowing the search by focusing on the size and atmosphere of these worlds. This model predicts which planets could possess life-supporting atmospheres. Researchers have identified just under 50 rocky worlds considered most likely to be habitable out of the more than 6,000 discovered to date. This research was published in the Monthly Notices of the Royal Astronomical Society. Professor Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University, and her team of undergraduates have been central to these efforts.
Other candidates continue to emerge. For example, a recently discovered exoplanet only 25 light-years away, dubbed GJ 3378b, orbits inside the habitable zone of a red dwarf star—a star much smaller and cooler than our Sun. While initially promising, its potential for supporting life faced challenges; astronomers suspected it was a “super-Earth” with a mass at least five times that of our planet. Such a world would possess incredible surface gravity and a crushing atmosphere. Similarly, an international team of scientists identified another “super-Earth,” GJ 251 c, which is almost four times as massive as Earth. Astronomers from the University of California, Irvine, discovered that this planet also orbits within the habitable zone of its host star, raising the possibility that its surface could contain liquid water, an essential ingredient for life.
