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Astronomers Found the First Atmosphere on a Planet in Another Star's Habitable Zone
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LHS 1140b apparently has everything it needs to support life. Science hasn't observed any there, yet.
According to a new study [science.org] published in Science, the "special" part of this discovery is not just the habitable zone around a star. It's that it seems to meet extra criteria that scientists care about -- i.e., it's rocky, not a gas giant, and can still hold onto an atmosphere despite harsh conditions. An atmosphere is what helps stabilize surface temperatures, which is essential for life to exist.
"It's very exciting," said Collin Cherubim, lead author on the study and a PhD graduate from Harvard, in an email. "A major goal in the field has been to understand whether any rocky exoplanets at all can retain atmospheres."
Cherubim says that most rocky planets orbit M-class dwarf stars, which "emit high-energy radiation for far longer than stars like the sun." Most rocky planets are stripped of their atmospheres fairly early in their life cycles, thanks to this radiation, the stripping effect of solar wind and other factors, thereby eliminating their chance to host life.
Until LHS 1140b, Earth was the only rocky planet that humans had ever observed with an intact atmosphere.
It wasn't easy to find. Cherubim used a computer model he'd developed during his work that "simulates how exoplanet atmospheres evolve over billions of years." Those models predicted the existence of "helium worlds," or rocky planets with helium atmospheres (or at least mostly helium). Cherubim tested his theory with LHS 1140b and struck proverbial gold.
Mars is still the best chance humans have at discovering life on another planet.
Whether or not LHS 1140b contains life is a complicated question. Cherubim says that science recognizes three main requirements for a planet to support life: an atmosphere, the right temperatures to sustain liquid water, and a mostly rocky planet to make it hospitable. LHS 1140b appears to have all of those.
The primary problem is that LHS 1140b's upper atmosphere is almost entirely helium, which is also "depleted of hydrogen." That doesn't bode well for life, since helium typically can't support life as we know it here on Earth.
Cherubim says the planet is also tidally locked to its star, like the moon to Earth, meaning one side of the planet has permanent daytime and the other permanent night. It's also roughly 70% larger than Earth, which means gravity is nearly twice as strong there. These findings don't rule out life entirely, but if LHS 1140b does host life, it would surely be very different from life on Earth.
Cherubim also admits that researchers don't yet know whether the planet has a rocky surface or is entirely covered by an ocean, the latter of which would be significant for the search for life. Computer models developed by Cherubim predict that the lower atmosphere may even contain gases more conducive to life, such as carbon dioxide, water and even oxygen.
"While we need more data to see what's going on in the rest of the atmosphere, this may be the first known helium world," Cherubim said. "That these rocky worlds can retain helium atmospheres isn't just good for habitability prospects, but also tells us our [computer prediction] models are getting something right."
Until then, Mars is the most likely candidate [cnet.com] for finding signs of life, since researchers have already identified possible biosignatures in its soil.
Another potential candidate for habitability is K2-18b, where scientists have reported tentative evidence for dimethyl sulfide, a compound commonly produced by oceanic phytoplankton. Scientists are also paying attention to the Trappist-1 star system [cnet.com], which has seven Earth-sized planets, three of them in the star's habitable zone.
