Welcome to LHS 1140b: A Super-Earth in the Habitable Zone

The MEarth exoplanet survey has nabbed a rocky super-Earth transiting its host star just 41 light-years away from Earth.

We're getting our first good characterizations of terrestrial exoplanets lately. First, news broke of a possible planet around the nearest star to our Sun, Proxima Centauri. Then, we explored TRAPPIST-1, a mini solar system just 39 light-years away. Now, researchers from the Harvard-Smithsonian Center for Astrophysics announced the discovery today of a possible super-Earth orbiting an M-dwarf star just 41 light-years away. The discovery was published in the April 20th Nature.

LHS 1140b is a tantalizing find. Its cool, red host, LHS 1140, contains only 15% the mass of our Sun and is at least 5 billion years old. The planet passes in front of its star once every 25 days as seen from Earth. Jason Dittmann (Harvard-Smithsonian Center for Astrophysics) and team combined discovery data from the MEarth project with radial-velocity measurements from the High Accuracy Radial velocity Planet Searcher (HARPS) survey.

The high-resolution followup observations enabled researchers to calculate the planet's orbital parameters and physical characteristics to a high degree of precision: The super-Earth, containing between 4.8 and 8.5 times Earth's mass, orbits just 0.09 astronomical units from its primary (almost a quarter of the average distance between the Sun and Mercury). The planet spans around 1.4 Earths. Combine its mass and radius and you'll calculate an incredibly dense planet between 9.1 and 15.9 g/cm³ — more than twice Earth's average density!

Though red dwarfs are often tempestuous flare stars — a strike against life on any orbiting worlds — they're also long-lived and miserly in terms of energy output. These cool stars are expected to shine for trillions of years, longer than the present age of the Universe. That's a plus in that it gives ample time to get the engine of evolution going.

And LHS 1140 may prove to be special compared to the TRAPPIST-1 system, which also features a red dwarf star, Dittmann argues. TRAPPIST-1 rotates quickly, about once every three days, and that rotation powers stellar activity at high-energy wavelengths, which can dessicate planets and wipe out life. LHS 1140, on the other hand, appears to be inactive. "We don't see any flares in our years of monitoring it," Dittmann says. That's perhaps unsurprising, given the star's rotation period of 130 days. While red dwarf stars have long been maligned for their inhospitability, placid LHS 1140 may prove to be the exception to the rule.

The 14th-magnitude star is located near right ascension 00^h 45^m, declination -15° 14′, in the constellation Cetus the Whale.

Scanning Red Dwarfs for Rocky Worlds

The MEarth Project (pronounced “mirth,”) is designed to hunt for Earth-sized planets around nearby M-dwarf stars (the “M” in “MEarth”). Launched in 2014, the MEarth exoplanet survey consists of two arrays of eight 16-inch telescopes (each about the size of a large amateur telescope). One group is located in the Southern Hemisphere, at the Cerro Tolodo Inter-American Observatory in Chile, and the other at the Fred Lawrence Whipple Observatory south of Tucson, Arizona. The MEarth project uses these groundbased observations to look for exoplanets transiting nearby M stars, noted by the telltale dips in brightness when the planet passes in front of its host star. To date, MEarth has discovered three exoplanets (the other two were GJ 1214b and GJ 1132b).

All three MEarth planets are super-Earths. But just what a super-Earth looks like up close is still a matter of conjecture. In our own solar system, Earth dominates the terrestrial worlds; the distant ice giants, Uranus and Neptune, are the next up in size. But planets between Earth and Neptune in size appear to rule the galaxy. Researchers are still ferreting out whether these planets are rocky like Earth or gaseous like Neptune on a case-by-case basis.

LHS 1140b receives about half as much of its sun's energy as Earth does from our Sun. So while it's certainly massive enough to retain an atmosphere, and possibly even liquid water on its surface, any claims of "Earth-like" (versus the simpler "Earth-sized") come with the ready caveat: terrestrial planets appear to come in more types than we yet realize. From a distance, even Venus appears "Earth-like," but I wouldn't bet on an interstellar ark headed there.

Still, nearby worlds such as LHS 1140b make great potential targets for future missions such as the James Webb Space Telescope (JWST) and the Transiting Exoplanet Survey Satellite (TESS), both of which launch next year.

 

Next up: obtain a spectrum of LHS 1140b and see what sort of atmosphere it has — and if its atmosphere contains signatures of biological activity.

"LHS 1140b is the best terrestrial exoplanet for follow-up studies," says Elisabeth Newton (MIT). "Proxima b doesn't transit its host star, which means that we can't use JWST to study its atmosphere. The TRAPPIST-1 star is a much fainter star, which means that measuring the mass of the planets through radial velocities is extremely challenging."

We're getting to know our own exoplanet neighborhood, and brave new worlds such as LHS 1140b, a little at a time.