They're small and rocky like Earth, and may have an Earthlike atmosphere, too.
In May, scientists said they'd found three potentially habitable worlds. The trio of exoplanets (which are planets that circle stars other than our Sun) are just a cosmic stone's throw—40 light years—away from Earth. They orbit a small red dwarf star, meaning that if you were standing on the surface of one of them, you'd have a dark red sun in the sky, emitting most of its radiation in infrared.
If that wasn't big enough news for anyone hoping to find life elsewhere in the galaxy, now the case that two of these planets could be habitable looks even stronger.
In a new paper out Wednesday in Nature, the same team says that the two innermost of these exoplanets are mostly rocky. (That's in contrast to the bloated gas giant Jupiter, where no life as we know it could survive.) And the atmospheres of these planets, the team found, aren't large and diffuse like Jupiter's, either. They actually seem to be more compact, like the atmospheres on Venus, Mars, and here on Earth. These are promising signs for life.
The atmospheres of Earth-sized exoplanets have never before been analyzed like this.
"These are the first planets that combine a few key properties," lead author Julien de Wit, a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences, told me. For one, they're Earth-sized and temperate. But scientists are also uniquely able to study them because they circle a Jupiter-sized red dwarf star, called TRAPPIST-1, instead of a big bright star more similar to our own Sun.
"What's key is that it's much smaller than our Sun," de Wit said.
To gather hints about an exoplanet's atmosphere, scientists study the tiny bit of light that passes through it when it slips in front of its host star, he explained. A much smaller star provides "a much larger signal," de Wit continued, and that was the case here.
He and his team used NASA's Hubble Space Telescope to check out TRAPPIST-1. They managed to catch the moment when both planets slipped in front of their host star, almost at the same time. They could measure changes in wavelength as the amount of starlight dimmed when planets passed in front of it.
The dips in light, they found, didn't vary a whole lot, indicating a "compact" atmosphere like the ones found on Venus, Mars, or Earth.
Still, we don't think of Venus or Mars as being very friendly places for life—at least, not on the surface, although some scientists have wondered if they could find something living up in the clouds of Venus, or below the freeze-dried surface of Mars.
"If you look at the terrestrial planets in our solar system, they're all very different," de Wit agreed. "And we expect to have lots of other scenarios [on different exoplanets] that we haven't even thought about yet."
He and his colleagues are going to continue working to learn more about the atmospheres of these exoplanets, and to scour the skies around TRAPPIST-1 (and other red dwarf stars, too) to see if more hidden planets are lurking there.
As for whether there's life in this planetary system, de Wit was hesitant to speculate about it, although he agreed that the latest findings "give more weight" to the argument that one or both of these planets might be habitable.
If there's anything currently living on these worlds—or if humans one day manage to leave Earth and set up camp there—it'd be very different from home. The planets are tidally locked, with one side forever facing the red dwarf star.
"There's a day side and a night side," de Wit explained. "The shadows would always be [stretched] in the same direction, at the same length. It is a very different context from Earth."
How we'd get there is another question. It would take many thousands of years to travel to a system 40 light years away, using current technology. Let's hope Stephen Hawking has a better idea.