Earth-sized planets found orbiting star could hold life
Astronomers have discovered a system of seven Earth-sized exoplanets orbiting a nearby star, at least three of which sit in the temperate “habitable zone” where liquid water could exist.
The proximity of the planets’ host star, known as TRAPPIST-1, to our own — just 39 light-years away — immediately makes this system one of the best places to look for life outside our solar system.
And a Canadian instrument aboard the next-generation James Webb Space Telescope, set to launch next year, will be positioned to do just that.
“We’re very excited here in Canada,” said Université de Montréal professor René Doyon, the principal investigator of the instrument, NIRISS, which was given to the telescope project by the Canadian Space Agency.
“We will be among the first to actually study these habitable-zone exoplanets,” Doyon said, using the term for planets that orbit stars outside our solar system.
TRAPPIST-1 is an ultracool dwarf star. Small, dim stars like this are great candidates for detecting new Earth-sized planets, because when one passes in front of its star, temporarily blocking it — a “transit” — the starlight reaching Earth dips more dramatically than it would for a very big, bright star like the sun.
Using a ground-based telescope in Chile, a team led by Michaël Gillon, an astronomer at the Université de Liège in Belgium, reported last year that TRAPPIST-1 had three Earth-sized exoplanets. But when they followed up and looked more closely at the system using NASA’s Spitzer space telescope and other instruments, they discovered the dwarf star actually hosted seven.
“It’s the first time that so many planets of this kind are found around the same star,” Gillon said. A paper describing the system was published in the journal Nature.
TRAPPIST-1may be small and dim, but because the planets all orbit very close to it — their orbital periods range from1.5 to about 20 days — the six innermost planets could have surface temperatures between zero and 100 C. The planets are so close to each other that astronomers believe a person standing on one could see geological features on the next, the same way we can see craters on the moon. At least three sit in the habitable zone, where liquid water — and life — are possible.
Data from the Spitzer telescope also allowed the team to measure the size of the planets and estimate the mass of the six closest to the star. Calculating the density of those six showed they were all likely to be rocky, terrestrial planets like Earth.
“What’s exciting about this is that it’s a very compact, well-stocked planetary system. It’s pretty much a cornucopia of planets,” said Ray Jayawardhana, a York University astronomer who studies planetary origins and diversity. He was not in- volved in the research.
What excites astronomers the most is the potential to characterize these planets’ atmospheres. Because the planets transit their star, because there are seven of them and because they are so close to Earth, astronomers have an opportunity to analyze the sunlight filtering through any atmosphere the planets might have and identify chemical signatures that could either rule out or suggest the existence of life.
NIRISS, the Canadian Space Agency instrument aboard the James Webb Space Telescope, is designed for precisely this task. Doyon and Gillon’s teams have already discussed using the instrument to search for water in the atmospheres of the TRAPPIST-1 planets, while other instruments aboard the telescope can be used to search for “biosignatures,” like methane and ozone.
“Of course, we don’t know what nature will give us. But the expectation is that it should be possible to detect water with NIRISS,” says Doyon. “We will certainly try.”
The James Webb telescope will launch in October 2018 and its first observation period is scheduled for summer 2019.
“We can expect that, within a few years, we will know a lot more about these planets and, we hope, if there is life there, maybe within a decade,” said Amaury Triaud, an astronomer at the University of Cambridge and a co-author on the Nature paper.
In the meantime, Gillon’s team is following up with existing tools. They have already used the Hubble space telescope to search for hydrogen envelopes around two of the seven planets and did not detect one — an encouraging sign, since such a heavy atmosphere would trap greenhouse gases and lower the chances of habitability.
Last year, astronomers announced that they had discovered a planet with a mass similar to Earth’s orbiting the habitable zone of Proxima Centauri, the star closest to ours. Today, we know there are seven orbiting another nearby dwarf star. While 25 years ago astronomers didn’t know whether exoplanets existed, these discoveries suggest the Milky Way is teeming with terrestrial Earth-size planets, even around dwarf stars like TRAPPIST-1.
“Dwarf stars are the most common type of star in the galaxy,” said Jayawardhana. “So if they produce terrestrial planets in abundance, that means yet more opportunities or locales for many, many more potentially habitable worlds in the galaxy.”