The planets, which were first reported to the public in May, might not turn out to be habitable or particularly Earth-like. But the small, rocky worlds represent a unique opportunity to go looking for conditions that would favor life.
The system's star, TRAPPIST-1, is an ultra-cool dwarf star — one just one-eighth the mass of our sun, and much cooler. It's more similar to Jupiter, a planet so massive it's almost starlike, than it is to our own sun.
We don't know whether life is more or less likely to evolve around stars different from our own. But when it comes to detecting the kind of atmosphere that would support life as we know it, smaller is better: When a planet passes in front of its star, the star's light has to pass through the planet's atmosphere in order to reach our telescopes. Scientists can use the wavelengths of the resulting light to analyze the molecular makeup of the atmosphere, revealing the presence of water and life-giving organic molecules. But big stars shine too bright for small, Earth-like planets to give off a signal that the Hubble can detect. That's why scientists have previously used this method only for studying the atmospheres of gas giants and "super-Earths," which are rocky but too close to their suns to host life as we know it.
That's why researchers from MIT and the University of Liege in Belgium scrambled for time on the Hubble Space Telescope just days after their initial results on the planets were published. The team had been using NASA's Spitzer Space Telescope to better determine the planets' orbits when they realized they were just days away from a fantastic alignment: a double transit, in which two planets would pass in front of their star from the Hubble's perspective at the same time.
"That meant we would be able to gather the first pieces of information about the atmosphere of two planets at once," said Julien de Wit, a postdoctoral researcher at MIT. "I thought that maybe that was a good excuse to reach out to NASA at the last minute." Luckily their proposal — which they put together in 24 hours — was accepted, and they got the data they were hoping for.
As the Hubble gazed upon TRAPPIST-1 during the lucky transit, the system's inner two planets caused starlight to dip and warp as it passed through their atmospheres. According to de Wit and his colleagues, the wavelengths of light didn't vary much as the planets transited, suggesting a compact atmosphere like the ones found on Venus or Earth (as opposed to a sprawling, puffy gas giant atmosphere).
"They're most likely planets we'd call terrestrial," de Wit said. "Not necessarily Earth-like, exactly, but a rocky planet like Earth or Venus or Mars."
There's plenty of work left to do on these promising planets. For starters, the third most distant world in the system — which has yet to be confirmed as a rocky planet — is actually the most probable spot for habitability. The two planets described in Wednesday's study are probably too close to their sun to hold liquid water (though there may be exceptions in a few spots, because half the surface is void of sunlight and that's got to make some weird stuff happen). The third planet may be in just the right orbital spot to have lakes and oceans, but scientists have yet to catch signs of its atmosphere.
Even the two planets that have been most recently studied will require some follow-up. Researchers still have to determine the composition of these compact atmospheres, puzzling out the individual molecules represented by the changing wavelengths of TRAPPIST-1's light during planetary transits. The researchers are continuing to study the system with the Hubble, but they expect to get a much higher-resolution look at the atmospheres in question after the 2018 launch of the new James Webb Space Telescope.
They'll also be on the lookout for more planets in the same star system and other, similar systems nearby. Around 15 percent of the stars near the sun are ultra-cool dwarf stars like TRAPPIST-1, so if this glut of Earth-like planets is "normal" for such a system, we could have lots of potential targets in the search for life.
TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope), the telescope used to discover the planets, is actually a tiny prototype. The researchers have launched an effort dubbed SPECULOOS (Search for habitable Planets Eclipsing ULtra-cOOl Stars), and are building four larger versions of the telescope in Chile. They're also raising money for similar telescopes set up to survey the Northern sky.
"We want to find other stars like this one so we can repeat the exact same process," de Witt said. "That would really increase our chances of finding truly habitable planets."