Astronomers have so far confirmed the existence of 3,475 worlds outside the solar system. Many of these exoplanets lie in the habitable zone of their stars — the area warm enough for worlds to possess liquid water on their surfaces. Given that life exists virtually wherever there is liquid water on Earth, these discoveries raise the possibility that some of these exoplanets might be homes to alien life.
Now scientists have discovered a “super-Earth” named LHS 1140b in the habitable zone of a nearby star. If life does exist there, it faces radically different conditions than Earth’s — for instance, gravity on its surface is more than three times stronger than Earth’s.
“LHS 1140b is slightly bigger than the Earth, but smaller than Neptune and is made primarily of rocks instead of gas. We don’t have any planets like that in the solar system,” said study lead author Jason Dittmann, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
This artist’s impression shows the exoplanet LHS 1140b, which orbits a red dwarf star 40 light-years from Earth and may be the new holder of the title ‘best place to look for signs of life beyond the Solar System.’ Credit: ESO/spaceengine.org
Astronomers investigated a red dwarf star named LHS 1140 located some 40 light years from Earth and just 14.6 percent the sun’s mass. Red dwarfs, also known as M dwarfs, are small, dim stars less than 60 percent the sun’s mass and up to 50 times fainter. These stars comprise up to 75 percent of the stars in the galaxy, and NASA’s Kepler spacecraft has discovered that at least half of these stars host rocky planets.
One reason red dwarf planets are potentially key places to search for life as we know it is because there are so many of them. Another is the incredible longevity of red dwarfs — while the sun will die in a few billion years, red dwarfs will take trillions of years to burn through their fuel, significantly longer than the 13.8-billion-year age of the universe. This longevity potentially gives red dwarfs an extraordinary amount of time for life to evolve around them.
Using the MEarth-South telescope array at the Cerro Tololo Inter-American Observatory in Chile, researchers detected an exoplanet around LHS 1140. Their findings suggest this world, LHS 1140b, lies within its star’s habitable zone. Although the exoplanet is more than a dozen times closer to its star than Earth is to the sun, LHS 1140 is roughly 300 times dimmer than the sun, meaning that worlds can get close to the star while staying habitable.
Super-Earth exoplanet LHS 1140b is located in the habitable zone surrounding its parent star, LHS 1140. Credit: M. Weiss, CfA.
This newfound planet is about 40 percent wider than Earth and 6.6 times more massive. Although no planet the size of LHS 1140b exists in our solar system, “results from Kepler have shown that this size planet is pretty common,” Dittmann said. “So understanding this planet provides us insight into a very common type of planet in the galaxy.”
Previously, astronomers had discovered a number of exoplanets in the habitable zones of nearby red dwarfs. For instance, Proxima Centauri, the red dwarf closest to the sun, has one planet in its habitable zone, while TRAPPIST-1 about 40 light years away has three. However, the masses of these planets remains uncertain, which makes it difficult to pinpoint their densities and thus their likely compositions — for example, instead of rock, they could be mostly ice or even gas. “In our case, we can definitely tell that LHS 1140b is made up of primarily rocky material like the Earth,” Dittmann said.
LHS 1140b was found using the MEarth-South Observatory in Chile. Credit: The MEarth Project
Prior work also suggested that even if worlds in Proxima Centauri and TRAPPIST-1’s habitable zones were rocky, radiation from those stars could disrupt their atmospheres. “What good is liquid water on the surface of the star if the star’s X-ray radiation kills you anyways?” Dittmann said.
In comparison, “LHS 1140 is relatively inactive, which may be helpful for maintaining an atmosphere and for the stability of organic compounds,” Dittmann said. LHS 1140b’s powerful surface gravity may also have helped it hang onto its atmosphere, he added.
Since LHS 1140b is nearby and regularly crosses in front of its star, telescopes currently under construction, such as the Giant Magellan Telescope in Chile and NASA’s James Webb Space Telescope, might be able to analyze starlight passing through its atmosphere, if it has one. These telescopes could search for specific gasses linked with the potential for life — for instance, oxygen.
The scientists detailed their findings online April 19 in the journal Nature.