It's a staple of science fiction: A city-size starship leaves our solar system, its passengers propagate on a trip that spans multiple generations and settle a planet several light-years from home. Alpa Centauri, the nearest star system, is often featured in these stories. The scifi author Kim Stanley Robinson made Tau Ceti, nearly three times as far away, the destination for a generation ship in his 2015 novel, Aurora, because he thought the system was more likely to have a habitable environment.
Life may not imitate art in this case, but they do kind of orbit each other. A team of international scientists, including a NASA researcher, has identified three Earth-size planets that might be candidates if real-life future humans ever set out to reach a habitable exoplanet, or worlds orbiting other stars.
The newly discovered system, announced in Nature on Monday, is about 40 light-years from Earth. That's close enough, astronomically speaking, to be considered "nearby." Our local galactic neighborhood might include the 3,000 or so nearest stars, says Michaël Gillon, an astronomer at Belgium's University of Liège and the paper's lead author. "So 100 parsecs"—more than 325 light years—"is still pretty nearby at the galactic scale."
The planet hunters were looking for so-called ultracool dwarf stars, which are too weak to show up in the usual searches for exoplanets. They came across two Earth-size planets that take just two days to zip around their Jupiter-scale dwarf star. The planets receive about two to four times our solar radiation, and virtually none of it is in the visible spectrum. The sun is probably too close for the planets to be entirely habitable, but the authors suggest there might spots where humans could survive. The planets are tidally locked to their star, just as the moon is to Earth, meaning the same side always faces the light.
Researchers caught a third planet transiting the dwarf star but didn't have enough information to characterize it as well as the other two. Astronomers hunt planets by measuring changes in the brightness when something passes between a star and Earth. For this project the scientists used the "Transiting Planets and Planetesimals Small Telescope," abbreviated as Trappist. Yes, Gillon explains, the acronym was picked for a reason: "Except for a few monks, the word 'Trappist' in Belgium calls to mind excellent beers."
Gillon and his colleagues will need help from the Hubble Space Telescope and the James Webb Space Telescope to reveal what's really in the atmospheres of these newly found planets. It's certainly too soon to build a Noah's space ark—but it's never too early to think about it. This type of speculation isn't exclusively the domain of science fiction. When NASA discovers an enticing planet, it often asks artists to go to work on a geoscience-guided mock-up.
But science-fiction writers tend to be best at thrilling speculation, so Kim Stanley Robinson agreed to review the new Nature paper for Bloomberg. He has already spent a lot of time imagining life on an exoplanet and consulting NASA scientists on the topic.
"Looks to me like the star is so different from our sun, and its planets so close to it, that it would be extremely different for Terran life-forms to try to exist there. Two to four times as much irradiation but from different parts of the spectrum, too," Robinson says.
It would also be a very long trip, if you assume (as Robinson does) that traveling faster than the speed of light will never be possible. A voyage there at typically postulated starship speeds would be "600 or 800 years," he notes. "Very hard to sustain."
Research like the Nature paper helps scientists figure out what questions to ask next and helps authors shape realistic fiction. "There's a feedback loop," says Robinson. "Many scientists, when young, read science fiction and get inspired to work in their chosen fields." Gillon, for one, has always been a fan of the genre.
The discovery of these Earth-like planets offers new data and raises new questions. Robinson's assessment is something scientists would almost certainly agree with: "For sure it would be weird."