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LHS 1140 planet
An artist’s impression shows a super-Earth passing across the disk of the faint red star known as LHS 1140. (CfA Illustration / M. Weiss)

It’s not the closest potentially habitable planet, but astronomers say a world 40 percent wider than Earth could be one of the best places to target in the search for life beyond our solar system.

“This is the most exciting exoplanet I’ve seen in the past decade. We could hardly hope for a better target to perform one of the biggest quests in science – searching for evidence of life beyond Earth,” Jason Dittmann, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said today in a news release.

Dittmann is the lead author of a paper published by the journal Nature describing the exoplanet, known as LHS 1140 b.

The super-Earth orbits a star dubbed LHS 1140, 40 light-years from Earth in the southern constellation Cetus. The planet was discovered in 2014 with the MEarth-South telescope array at Cerro Tololo Inter-American Observatory in Chile, using a method that measured the faint dimming of starlight as it passed across LHS 1140’s disk.

An analysis of the resulting data determined that the planet orbits the star every 25 days, at a distance well within Mercury’s orbit around our own sun (roughly 7.4 million miles). But because LHS 1140 b is so much dimmer than our sun, the planet is thought to be in a habitable zone where water can exist in liquid form.

Further observations from the High Accuracy Radial-velocity Planet Searcher, or HARPS, led the researchers to estimate the planet’s mass at 6.6 times that of Earth. That suggests the planet is dense enough to be rocky.

Over the past year, other astronomers have detected potentially habitable exoplanets that are closer, including Proxima Centauri b and the TRAPPIST-1 worlds. Those worlds, however, present challenges for further study.

Proxima Centauri b may be a mere 4.2 light-years from Earth, but scientists suspect that it doesn’t make a transit across its parent star. That would rule out the prospect of analyzing how starlight is altered as it passes through any atmosphere – which is currently considered the leading strategy for hunting down signs of habitability and alien life.

The seven known planets in the TRAPPIST-1 system make transits, which fortunately allows for further study. But some scientists worry that the system’s young red dwarf star is so active that high-energy radiation has sterilized any environment where life might otherwise exist.

The star LHS 1140 is also a red dwarf, but it’s old enough to have settled down.

“The present conditions of the red dwarf are particularly favorable – LHS 1140 spins more slowly and emits less high-energy radiation than other similar low-mass stars,” said the Geneva Observatory’s Nicola Astudillo-Defru, a co-author of the Nature study.

He and his fellow researchers say LHS 1140 b’s atmosphere might have weathered the star’s early outbursts due to the planet’s size, and the fact that it was probably farther away from the star back then. As the planet heated up, a steaming ocean of lava conceivably provided water vapor to replenish the atmosphere.

For now, much of the speculation about the planet’s atmosphere is little more than hand-waving.

“Right now we’re just making educated guesses about the content of this planet’s atmosphere,” Dittmann acknowledged. “Future observations might enable us to detect the atmosphere of a potentially habitable planet for the first time. We plan to search for water, and ultimately molecular oxygen.”

First, astronomers will use the Hubble Space Telescope to gauge more precisely how much high-energy radiation LHS 1140 b is currently getting, Then they’ll turn to NASA’s James Webb Space Telescope, which is due for launch next year. The European Southern Observatory’s Extremely Large Telescope in Chile, scheduled to enter service in the mid-2020s, could also be employed in the quest.

Dittmann and Astudillo-Defru are among 24 authors of the Nature study, titled “A Temperate Rocky Super-Earth Transiting a Nearby Cool Star.”

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