For scientists searching the skies for other Earth-like planets—other living worlds—the brightest hope may be a quiet star too dim to be seen with the naked eye, a sedate and solitary red dwarf called LHS 1140 nestled just 40 light-years away in the southern constellation Cetus. There an international team of astronomers has found a world that, although not a twin of Earth, certainly counts as a close cousin.
LHS 1140 b is a “super-Earth,” a planet bigger than ours but smaller than Neptune, and the most common variety of world thought to exist in our galaxy. Many erstwhile super-Earths, however, have proved to be uninhabitable “mini-Neptunes” smothered beneath thick layers of gas. This world is different. At just under 50 percent larger than Earth but more than six times as heavy, its dimensions suggest it must be a ball of rock and metal, potentially with a thin and comparatively Earth-like atmosphere. Its 25-day orbit brings it 10 times closer to its star than Earth ever gets to our sun, but LHS 1140 shines so weakly that its planet soaks up just half the starlight our own world receives—just enough, it seems, to sustain the possibility of life-giving liquid water oceans on its surface. This alien world might well be tidally locked due to its nearness to its star, eternally turning the same face to its sun just as the moon does to Earth, leaving its far side in constant darkness. The planet and star are estimated to be at least five billion years old—that is, about half a billion years older than our solar system.
Most importantly, each orbit sends this temperate, rocky world “transiting” across the face of its star as seen from Earth—a fortuitous alignment allowing astronomers to observe the planet more closely than any other potentially habitable world yet found beyond our solar system. Molecules in a transiting planet's upper atmosphere absorb a fraction of the starlight passing through, forming a tenuous ring of light around the globe that astronomers can study to learn what is in its alien air. In coming years astronomers will use this and other techniques to seek out any biosphere that might exist on LHS 1140 b, potentially revealing signs of oxygen and other atmospheric gases that, on Earth, constitute the literal breath of life. The planet’s discovery is detailed in a study published in Nature.
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“LHS 1140 b is the best candidate to look at for signs of life in the near future,” says study co-author David Charbonneau, an astronomer at Harvard University who leads the MEarth Project, a global network of small telescopes that first observed the transiting planet. (The “M” in “MEarth” stands for “M dwarf,” a technical term for those red dwarf stars that are about 30 percent or less the mass of the sun. Such stars are by far the most common variety in our galaxy, and the most amenable to studies of planets.) “This is the first time we’ve found a rocky planet that gives us the opportunity to look for oxygen,” Charbonneau adds This really is the one we’ve been hunting for.”
Long sought, the planet was also one that almost got away. MEarth’s array of telescopes in the Southern Hemisphere, located at the Cerro Tololo Inter-American Observatory in Chile, first picked up tentative signs of LHS 1140b’s transit in September 2014. MEarth team member and lead study author Jason Dittmann, then a graduate student at Harvard University, spearheaded the effort to confirm and study the potential planet. The case for the planet slowly grew over the next two years, as the MEarth team enlisted help from a second group of astronomers operating the European Southern Observatory’s HARPS instrument in Chile—the world’s premiere planet-hunting spectrograph. Rather than look for transits, HARPS finds planets by the periodic gravitational wobbles they impose on their stars. This slow, painstaking technique allows a planet’s mass to be estimated. “MEarth detected a transit event, but only one, and it was low signal to noise so they were not completely sure it was real,” says study co-author Xavier Bonfils, an astronomer at the University of Geneva who helms the HARPS survey of red dwarf stars. “But they have never passed us a false positive, so we considered this a quite reliable candidate and began an intensive observing campaign.”
By combining HARPS and MEarth observations, the teams eventually predicted a transit for the putative planet would be viewable from facilities in Hawaii and Australia on September 1, 2016. But on the appointed night, poor weather prevented five of the six telescopes from observing the star. Only one observer, amateur astronomer and study co-author Thiam-Guan Tan, successfully watched the transit using a small telescope in the suburbs of Perth, Australia. That night, Tan sent the MEarth team a terse e-mail reporting his success: “Transit egress seen at ~HJD +7633.12. Depth about 5 mmag.” That is, Tan had recorded LHS 1140 dimming by just half of 1 percent from a transiting planet—equivalent, he says, to “observing the dimming of light caused by a grain of sand moving in front of a candle placed 400 kilometers away.”
With the planet’s orbital period in hand, subsequent observations with MEarth and HARPS quickly firmed up estimates for its size and mass, revealing it to be a giant, rocky and very noteworthy world.
Planetary Pay Dirt
One could be forgiven for thinking planet hunters are somehow confused. With every passing month a new prime candidate for “Earth 2.0” seems to emerge. But not all potentially habitable worlds are equally promising for follow-up study.
For example, since its launch in 2009 NASA’s Kepler space telescope has discovered about a dozen potentially habitable worlds transiting other stars in our galaxy. Yet Kepler’s finds are thousands of light-years away—too far to be scrutinized for more nuanced signs of habitability and life. Conversely, last year astronomers discovered a potentially habitable Earth-size planet, Proxima b, around the sun’s nearest neighboring star—the red dwarf Proxima Centauri, scarcely more than four light-years away. But like most other known nearby worlds, Proxima b does not appear to transit, meaning deeper studies may be delayed for years as astronomers develop the technology to actually snap its picture.
Earlier this year, planet hunters hit pay dirt with a system of at least seven Earth-size planets transiting another red dwarf, TRAPPIST-1, which like LHS 1140 is about 40 light-years away. Researchers carefully studied each transiting planet’s shadow to determine its size, and even managed to estimate some of their weights by watching how the orbiting planets tugged on one another to subtly alter the timing of their transits. These studies, however, yielded mixed results—the worlds of TRAPPIST-1 could be rocky, Charbonneau says, or they could be drowned or smothered beneath thick layers of water, ice or gas. Even so, because they do transit, astronomers using NASA’s upcoming infrared James Webb Space Telescope or under-construction ground-based telescopes with 30-meter mirrors will be able to learn much more about the planets of TRAPPIST-1 by studying the makeup of their atmospheres. But although TRAPPIST-1 is the same distance from Earth as LHS 1140, it is a much smaller and dimmer “ultracool” red dwarf—as small and dim as a star can be, in fact, while still qualifying as a star. The meager trickle of light it shines toward Earth is insufficient to support a robust search for atmospheric oxygen.
Even if TRAPPIST-1 were bright enough to allow its planets to be studied for signs of oxygen, the star presents other problems for life-seeking astronomers. Like all red dwarfs, it experienced a tempestuous youth during which it shined far brighter as it slowly contracted to its current size. This formative period lasted for perhaps a billion years, and may well have left its retinue of worlds scorched and airless—or wreathed in a crushing, arid atmosphere of almost pure carbon dioxide, due to a Venus-style runaway greenhouse effect. Even today the star is highly active, bathing its planets in atmosphere-eroding x-ray and ultraviolet radiation. LHS 1140, by contrast, is thought to have had a much briefer formative phase of just 40 million years, and is now a relatively quiescent star. “That’s the big question now: ‘Which planet is going to retain its atmosphere against stellar heating and erosion?’” Bonfils says. “And the chance seems higher around a quiet star like LHS 1140.”
The great bulk of LHS 1140 b, its discoverers say, could offer additional advantages. The planet’s hefty gravitational field may have allowed it to retain more of its air against stellar insults. And even if it did lose its primordial atmosphere or suffer a runaway greenhouse effect during its star’s initial 40 million years of planet-scorching brightness, back then its crust and mantle were likely still molten, forming a planetary magma ocean that could act as a reservoir for volatile gases. As the magma cooled, it could release those gases to replenish the planet’s atmosphere and inventory of water.
Studying both planetary systems together, Dittmann says, could yield crucial insights about how potentially habitable worlds can keep—or lose—their atmospheres around red dwarf stars. “Between TRAPPIST-1 and LHS 1140 b we have the opportunity to compare a planet bathed in intense radiation by an active ultracool dwarf star with one around a much calmer, steadier star,” he explains. “That will let us ask—and answer—some fun questions.” In the meantime, he says, the MEarth team’s plans for LHS 1140 “are incredibly simple: We’re going to hit this system with everything we’ve got.”
All Eyes on the Prize
Already, the team is hammering away at the system with additional observations, bombarding the star with HARPS measurements practically every night for several months in hopes of pinning down the planet’s true mass and learning whether other worlds lurk hidden in the system. Observations with NASA’s Hubble Space Telescope are measuring how much ultraviolet light from the star falls on the planet to better understand its prospects for life. Additional, yet-to-be-approved observations with Hubble and another space-based NASA telescope, the Chandra X-Ray Observatory, could reveal just how much high-energy radiation the world receives, further clarifying its capacity to support life.
This fall the team hopes to take over most of the world-class telescopes in Chile for one night, monitoring a transit of the planet on October 26 with the twin 6.5-meter Magellan telescopes as well as three or four of the eight-meter observatories that make up the European Southern Observatory’s Very Large Telescope complex. These observations will seek to detect the planet’s atmosphere—or at least to confirm that it lacks a thick, biosphere-stifling envelope of gas.
But the best information will come later this decade and early in the next with the launch of NASA’s Webb telescope in 2018 and the debut of ground-based 30-meter extremely large telescopes in the 2020s. Operating in the infrared part of the spectrum, Webb could search for signs of carbon dioxide, water vapor, methane and other gases in LHS 1140 b’s atmosphere. A ground-based facility such as the under-construction Giant Magellan Telescope (GMT) could look for atmospheric oxygen in visible light reflected from the planet. Combining data from Webb and the GMT, Charbonneau says, could allow astronomers to distinguish between potentially biological sources of oxygen—such as photosynthetic organisms—and abiotic production routes for the gas, which can be generated in enormous amounts by runaway-greenhouse conditions. “The message is that we really need both Webb and something like the GMT,” Charbonneau says. “The GMT could detect oxygen, which would tell us that there really could be life there. But to understand the source of that oxygen you must go and measure other atmospheric molecules, and those will be in domain of Webb.”
Astronomers preparing Webb for launch are already planning observations of the new planet. “Only time will tell, but I would not be surprised that LHS 1140b will become one of the most-studied planets by Webb in its entire lifetime,” says René Doyon, an astronomer at the University of Montreal and principal investigator for NIRISS, a Canadian-built instrument for Webb that is optimized for studying planetary atmospheres. Doyon has already allocated some of NIRISS’s precious observing time to study the system, which he calls a “dream target” for Webb.
Pondering the prospect of devoting years—decades even—of his scientific career to studying this newfound planet, Dittmann (who has since moved to Massachusetts Institute of Technology, where he is a postdoctoral fellow) occasionally wonders whether the investment will pan out. Red dwarfs and super-Earths are respectively the most abundant stars and planets in the galaxy, and when they come together to form a transiting system they offer astronomers a bonanza of observational possibilities with current or near-future technology. But they are also profoundly alien, presenting myriad unique challenges to observers hoping to understand them and their prospects for life. Studies of more familiar territory—smaller planets around scarcer, larger stars like our sun—are at present far more difficult, with breakthrough results perhaps still decades away.
“We’re being pushed to [red dwarfs] because of their abundance and our available technology. But you know, we go outside everyday and there’s a nice yellow star up there, shining for us,” Dittmann muses. “It is kind of strange, to wonder why we don’t instead orbit one of the most common star types in the universe—and maybe it’s because they’re not so great for life. It’s on the back of everyone’s mind—certainly mine. Then again, maybe life would have no problem around these stars. What’s important is that we’re now at the point where we’re finding and studying these planets, like LHS 1140 b and those of TRAPPIST-1—and more that will come—so that we can confront all these hypotheticals with actual data. So this is where we’re going. In 10 years I may eat my words, but in 10 years I’ll also be eating lots of telescope time.”