The best place to look for nearby Earth-size planets might be the smallest, coolest stars — but these stars could also be bad for creating Earth-like planets.
In the hunt for exoplanets, some astronomers are focusing on cooler, smaller stars, particularly those called M dwarfs. These stars are roughly one-fourth the Sun’s mass and put out a tiny fraction of the Sun’s radiation — like comparing a single Christmas-tree light with a 1000-watt bulb — which makes it easier to detect planetary systems. M dwarfs also make up about 75% of our galaxy’s stars, so if you want to find an exoplanet, these stars are good places to look.
Two new studies bear on astronomers’ rising interest in cool red dwarfs and the search for Earth-like planets. The first, by Helmut Lammer (Austrian Academy of Sciences, Austria) and his colleagues, looked at seven confirmed planets around three different stars — a G star like the Sun, a slightly cooler K star, and an even cooler M star. These seven planets are super-Earths, with diameters about 2 to 4.5 times Earth’s and masses about 2 to 13 times Earth’s. They orbit very close to their stars, much closer than the habitable zones where liquid surface water could exist, and their masses and diameters reveal that the worlds have bloated atmospheres.
Such bloated atmospheres are likely hydrogen-rich primordial envelopes, either gas gathered from the disk the planet formed from or material outgassed or delivered by impacts early in the planet’s formation.
These atmospheres are quite different from those seen around the inner, rocky planets of our solar system: the extreme ultraviolet and X-ray radiation pouring off an infant star blows off inner planets’ protoatmospheres. So why do these seven super-Earths have their heavy shrouds? Lammer and his colleagues calculated how stars’ radiation would heat and blow off their planets’ hydrogen-rich envelopes. They found that, while planets do lose a whole lot of mass, the mass loss of these seven super-Earths is one to two orders of magnitude lower than that for hot Jupiters (possibly because the smaller planets are denser), meaning that the planets couldn’t lose their big atmospheres and will likely persist as “mini-Neptunes,” bodies with big atmospheres around small, rocky cores.
So what? If these super-Earths, which closely orbit their stars, can’t lose their protoatmospheres, then similar planets farther out — say, inside the habitable zone — would have an even harder time doing so, says Lammer. That’s because the star’s radiation is weaker at larger distances. On the other hand, smaller planets might lose too much atmosphere, winding up like Mars or Mercury. Earth and Venus seem to be the Goldilocks size for the Sun, but different sizes and distances might be the right fit for other types of stars.
Young M dwarfs put out a lot more extreme radiation than young Sun-like stars do. These levels can persist for a few hundred million years to a couple of billion years. The M-dwarf planet Lammer and his colleagues studied, GJ 1214b, is only 1.4% the Earth-Sun distance from its star and receives 470 times as much X-ray and extreme ultraviolet radiation as Earth. Yet at 2.7 times Earth’s girth (and more than 6 times Earth’s mass), it holds on to its puffy atmosphere pretty well.
“I expect that it should be possible to find Earth-like planets within M-star habitable zones, but I think that the fraction of such planets is smaller than most people think,” Lammer concludes.
Finding Earth-like planets
A few potential members of that small group were announced this week in a study by Courtney Dressing and David Charbonneau (Harvard-Smithsonian Center for Astrophysics). Dressing and Charbonneau took a new look at 3,897 cool K and M red dwarf stars observed by NASA’s Kepler spacecraft. Kepler’s list of stellar parameters is quite accurate for stars similar to the Sun, but the catalog overestimates the temperatures and sizes of smaller, cooler stars. Because the sizes of Kepler’s planet candidates are determined by the sizes of their stars, the catalog’s errors spill over into planet characteristics, making them look larger than they really are.
Dressing and Charbonneau’s careful recalibration downsized the typical star’s temperature by 130 kelvins and its diameter by 31%, which in turn dropped the average planet size by 29%. The sample of stars includes 95 planet candidates around 64 stars, including three planet candidates — KOI 1422.02, KOI 2626.01, and KOI 854.01 — which are in the habitable zone and (now) range from 0.9 to 1.7 times Earth’s size.
Extrapolating out from how many transiting planets they did see, how many might be there but orbiting at the wrong angle to be visible from our point of view, and how many they should have been able to detect overall, Dressing and Charbonneau conclude that 60% of the warmest dwarfs have planets between 1.4 and 4 times Earth’s size. Overall, roughly one of every two dwarfs has a planet 0.5 to 1.4 times Earth’s diameter with an orbital period less than 50 days. (All three habitable-zone candidates have orbital periods around that value or less.)
They also estimate that about 6% of red dwarf stars have an Earth-size planet in the habitable zone. Given the large number of cool dwarfs in the galaxy, that could mean the closest Earth-size planet is just over 20 light-years away; best guess, about 13 light-years, says Dressing.
“If we scaled the Milky Way Galaxy down to the size of the United States . . . and we stood on one side of [New York City’s] Central Park, we’d find that the nearest Earth-like planet is just across the park,” Dressing explains.
Charbonneau says that the three habitable-zone planet candidates he and Dressing studied are not subject to the same bloated-atmosphere problem as the super-Earths in Lammer’s study. “Courtney was extremely careful to only pick the very small planets that are truly 1 Earth radius,” he says. “We really think it’s very unlikely that these objects have big envelopes.” Still, he adds, given the possible caveats — such as red dwarfs’ high radiation outputs — astronomers will need to directly observe these planets and others like them before knowing for sure what they’re like and what kind of atmosphere (if any) they have.
H. Lammer et al. “Probing the Blow-Off Criteria of Hydrogen-Rich ‘Super-Earths.’” Accepted to Monthly Notices of the Royal Astronomical Society.
C. Dressing and D. Charbonneau. “The Occurrence Rate of Small Planets Around Small Stars.” Accepted to Astrophysical Journal.