Astronomers say they've pinpointed a potentially habitable planet that might offer our best chance of characterizing an alien atmosphere in the near future.
The planet LHS 1140b, described this week in the journal Nature, provides a tempting target for astronomers looking to probe an exoplanet's thin but essential shell of air, which could offer clues about whether such a world could host life.
Scientists have been on the hunt for a kind of "Goldilocks" planet: a body that's small enough to be rocky and not gaseous, far (but not too far) enough from its star to hold liquid water, and bearing an atmosphere that could protect any possible life present.
Recent finds have checked off the first two of those boxes, but the third has remained a challenge. That's in part because it's exceedingly difficult to study the thin rind of air around an already small planet.
One way researchers discover exoplanets is by waiting for them to pass in front of their host star, causing a dip in the starlight (a technique used to great effect by the Kepler spacecraft). They then study the light that filters though the atmosphere in order to determine what that atmosphere is made of — a technique known as transmission spectroscopy.
The closest Earth-sized habitable-zone planet to us, circling Proxima Centauri, lies only 4.2 light years away, but does not transit in front of its star relative to our point of view, so we cannot see the planet or its atmosphere. The super-Earth Gliese 1132b, which may have a watery atmosphere, is far too close to its star to host life. The TRAPPIST-1 system, 39 light-years away, holds a whopping seven Earth-sized worlds, several of which are in their star's habitable zone — but according to the Nature study authors, their masses and densities are too "poorly constrained" to know for sure whether they are rocky bodies.
LHS 1140b, then, could be one of the best targets for an atmospheric study.
The planet also lies about 39 light-years away, circling a red dwarf star in the constellation Cetus. The astronomers discovered it using the MEarth-South telescope array in Chile, which focuses on finding a type of red dwarf star called an M dwarf. There's a good reason for that, the researchers pointed out.
"M dwarf stars, which have masses less than 60% that of the sun, make up 75% of the population of the stars in the galaxy," the study authors wrote. On top of that, "statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs [or 34 light-years] away."
The scientists learned more about their find with the European Southern Observatory's High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph in Chile. An orbiting planet's tiny gravitational tug causes its star to wobble — and that wobble can be used to calculate the planet's mass.
The planet turns out to be packing a whopping 6.6 Earth masses into a body with a radius only 1.4 times that of Earth. This planet is so dense that it must be rocky, the thinking goes.
And even though LHS 1140b is so close to its star that it completes an orbit every 25 days, its star is much cooler, dimmer and smaller — only a fifth the size of the sun. This puts the planet potentially in the habitable zone.
This red dwarf, however, probably had a much more active youth. If so, it would have bathed the planet in harsher ultraviolet light, potentially stripping water from any existing atmosphere and contributing to a runaway greenhouse effect. (This is what happened in Venus' carbon-dioxide-rich atmosphere, helping to make the planet the hottest in the solar system.)
However, it's possible that a planet like LHS 1140b would have had a magma ocean for millions of years, which would have released water into the air and replenished the atmosphere long after the star had calmed down. If that's the case, that atmosphere could still be there.
"Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth," the study authors wrote. "Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future."
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