A New Exoplanet May Be Most Promising Yet in Search for Life
At left is an artist’s impression of the newly discovered rocky exoplanet. The exoplanet is close enough that astronomers are hopeful that with the next generation of big telescopes, they will be able to probe its atmosphere for signs of water or other evidence of suitability for life.
A prime planet listing has just appeared on the cosmic real estate market, possibly the most promising place yet to search for signs of life beyond the Solar System, the astronomers who discovered it say.
It is a rocky orb about one and a half times the size of Earth, about 40 light years from here. It circles a dwarf star known as LHS 1140 every 25 days, an orbit that puts it in the “Goldilocks” zone where temperatures are conducive to liquid water and perhaps life as we know it.
It is close enough that astronomers are hopeful that with the next generation of big telescopes, they will be able to probe its atmosphere for signs of water or other evidence of suitability for life.
“This planet is really close to us: If we shrank the Milky Way to the size of the United States, LHS 1140 and the Sun would fit inside Central Park,” David Charbonneau, of the Harvard-Smithsonian Center for Astrophysics, said in an email.
His colleague Jason Dittmann, who led the discovery team and is lead author of a paper published on Wednesday (April 18, 2017) in Nature, said in a statement, “This is the most exciting exoplanet I’ve seen in the last decade.”
The planet was discovered by the MEarth-South survey at the Cerro Tololo Inter-American Observatory in Chile, an array of small telescopes that looks for the dips in starlight when planets pass in front of nearby stars.
The depth of the dip told them how big the new planet is. Then they determined that it was about six times as massive as Earth by using a spectrograph called Harps, for High Accuracy Radial velocity Planet Searcher, at the European Southern Observatory, also in Chile, to measure how much the planet perturbed its home star. The resulting density puts the little world into a rapidly growing class called “superEarths.”
The star LHS 1140 is about one-fifth the size of our Sun. In its close orbit, the planet receives about half as much energy as Earth does from its own Sun, enough for a microbe or something more complicated to make a living.
This discovery continues a recent run of promising new planets circling nearby dwarf stars. Last summer there was the discovery of Proxima b, the nearest star to us, only 4.2 light years from here.
In February astronomers discovered a system of seven Earth-size planets circling a dwarf star known as Trappist-1.
According to Dr. Charbonneau, who originated the MEarth system, red dwarf stars outnumber stars like our Sun by about 10 to 1 in the 30-light-year bubble that constitutes our “block” in the cosmos.
About one in four of them have rocky planets in their habitable zones, according to work by Dr. Charbonneau’s former student Courtney Dressing, now at the California Institute of Technology.
Once upon a time, such planets were not looked upon favorably in the extraterrestrial life sweepstakes, because they were almost undoubtedly tidally locked, keeping one side faced to its star and the other facing out in space. That would result in a burning hell on one side and eternal frostbite on the other, neither side suitable for life.
But recently astronomers have determined that if these planets have thick enough atmospheres, winds can distribute the heat around both hemispheres and make them livable.
“Now we love them,” Sara Seager, a planetary expert at the Massachusetts Institute of Technology, said at a recent meeting on the origins of life sponsored by Harvard at the American Academy of Arts and Sciences in Cambridge. “If they have an atmosphere, they can harbor life.”
Astronomers said that the new planet offered the best hope so far to test that proposition. When the planet crosses in front of LHS 1140 the atmosphere acts like a filter, leaving an imprint on the star’s light that could betray the presence of water and other molecules important for life.
This will be a job for powerful new telescopes like the James Webb Space Telescope, due to be launched next year, or giant ground-based telescopes like the Giant Magellan and European Extremely Large telescopes, now being built in Chile, Dr. Charbonneau said.
Whether such planets actually have atmospheres is still controversial, however. When red dwarf stars are young, Dr. Charbonneau pointed out, they are ferociously luminous and might have blown away the planets’ atmospheres or caused a runaway greenhouse, leaving them barren. But the LHS 1140 planet is heavy enough, he said, that it might have been able to retain its atmosphere or regenerate it by volcanic activity later on.
“But the key point is yes, these are really exciting ideas to test,” he continued. “Do temperate, rocky M-dwarfs planets retain their atmospheres, and do they have life? This world enables those studies.”
 Dennis Overbye, “Promising Target in the Search for Extraterrestrial Life”, New York Times (April 20, 2017), p. A19. A version of this article appeared online at the New York Times (April 19, 2017).