clock menu more-arrow no yes mobile

Filed under:

This solar system of seven Earth-sized planets may be the best place to look for alien life

New, 58 comments

Many of the planets have the right temperatures needed for hosting liquid water

TRAPPIST-1 and its planets.
Photo: NASA / JPL-Caltech

Around 40 light-years away, seven Earth-sized planets have been spotted orbiting closely around a small, ultra-cool star. It’s one of the largest solar systems that’s ever been discovered outside of our own, and it’s a particularly enticing find in the ongoing search for extraterrestrial life. Six of the planets in the system may have the right temperatures for liquid water to exist on their surfaces, and astronomers are confident they’ll be able to get a more in-depth look at these seven worlds with future space telescopes.

The solar system, detailed today in a study in Nature, isn’t a completely new find. In fact, the discovery of this system was announced last year by the same researchers. But at the time, they thought they had only found three planets around the star, named TRAPPIST-1. When the researchers took a closer look at the system with more precise telescopes, including NASA’s Spitzer Space Telescope, they found more planets nearby.

“We got plenty of new data, and we went from three to four to five planets,” Michaël Gillon, a research associate for the Belgian Funds for Scientific Research and lead author of the Nature study, tells The Verge. “Then we got this Spitzer data that showed there were, in fact, seven planets.”

A rendering of the TRAPPIST-1 planets.
Photo: NASA / JPL-Caltech

Since these planets are roughly the same size as Earth, the researchers think they may be rocky like our own world. And three orbit within the star’s habitable zone, where temperatures are just right that there could be whole oceans on the planets’ surfaces. Given that liquid water is such an essential ingredient for life here on our planet, astronomers are eager to find it on other worlds outside our Solar System. The presence of liquid water on an exoplanet could mean that life has thrived there as well, so that makes these seven planets now top candidates in the search for alien life.

The astronomers say there’s a good chance they’ll get some answers, since they’ll be able to study these exoplanets and their atmospheres in greater detail. In the grand scheme of the Universe, 40 light-years is a relatively short distance, which makes observing this system a bit easier with our telescopes. Plus, peering into the planets’ atmospheres is less challenging since these planets orbit around a star that’s much smaller and fainter than our yellow Sun. If they orbited a star the size of ours, the intense starlight would make the worlds and their atmospheres difficult to see. “Of course it’s super exciting, but what makes the system so special is that all these seven planets are suited for detailed atmospheric characterization,” says Gillon.

This is why small, super-cool stars — known as red dwarfs — have become popular targets for exoplanet hunters; it’s easier to study the planets around them. Over the past couple of years, Gillon and his team have been focused on looking for worlds around red dwarfs using the TRAPPIST telescope at the La Silla Observatory in Chile. Less than a couple years ago, their search led them to TRAPPIST-1, a star just a little bigger than Jupiter.

They found three worlds orbiting TRAPPIST-1 by watching the planets as they passed in front of the star — a process known as transiting. Whenever a planet transits in front of its host star, it slightly dims the star’s light. That dimming is incredibly small, but with the right instruments, astronomers can sometimes pick up these minute light changes from Earth. Through this process, astronomers can use the dimming to calculate the size, mass, and orbit of a passing planet.

The astronomers decided to keep observing the system and have spent more than 1,000 hours spying on the star and its planets with other telescopes. The new data has helped bring the rest of the planets into view, with NASA’s Spitzer Telescope revealing two planets that could not have been seen from telescopes on the ground. (The telescope’s location in space allows it to bypass Earth’s noisy atmosphere and gather more precise data.) The follow-up observations also revealed that some of the scientists’ original findings had been misinterpreted. One of the original three planets the team had identified turned out to be multiple planets.

The planets, which have been named alphabetically from b to h, all orbit closer to TRAPPIST-1 than Mercury orbits the Sun. The closest planet takes just 1.5 Earth days to complete one orbit, while the farthest planet takes around 20 days to circle the star. Because of this, they’re all a super tight bunch. When TRAPPIST-1f and TRAPPIST-1g are at their closest to one another, they’re just at three times the distance between the Earth and the Moon. So if you were to stand on TRAPPIST-1f, sometimes TRAPPIST-1g would look twice as big as the Moon in the sky. “It’s remarkable that you could see another world right there,” Amaury Triaud, an exoplanet fellow at the Kavli Institute at the University of Cambridge and a study author, tells The Verge.

The properties of the TRAPPIST-1 planets compared to the four innermost planets in our Solar System.
Photo: NASA / JPL-Caltech

And in a beautiful coincidence, the inner six planets seem to orbit somewhat in sync — a phenomenon known as orbital resonance. For instance, when TRAPPIST-1g completes four orbits around the star, TRAPPIST-1f completes three orbits — a ratio of 4:3. It’s an interesting trait that will help astronomers better understand how the system came to be. “They’re all synchronized, and it’s telling us about how they formed,” says Triaud. “It’s a clue about past formation, so it’s both beautiful and handy.”

But the biggest draw of this system is its promise for habitability. Sure, the star’s close proximity to its planets means these worlds receive substantially more radiation than Earth does. But Triaud maintains TRAPPIST-1 is a fairly quiet red dwarf, meaning it doesn’t send out solar flares very often. Plus, six of the planets could have surface temperatures that range between 32 and 212 degrees Fahrenheit, which may allow liquid water to exist; some could have water on a fraction of their surfaces, while the most enticing three — e, f, and g — may be covered in the stuff.

To figure this out, astronomers will try to detect the components of each planet’s atmosphere. One way to do that is to look at the star’s light as it passes through the gas circulating around the planet. Different gases will filter the light in different ways, giving astronomers a good idea of what compounds surround each world. Additionally, astronomers can measure the drop in brightness of the system when a planet passes behind the star. It’s a negative measurement that can be used to figure out the light that’s coming off of each planet, which can also be used to deduce what’s in the worlds’ atmospheres.

An artist rendering of what it might look like from the surface of TRAPPIST-1f.
Photo: NASA / JPL-Caltech

If there is an abundant amount of H20 in an exoplanet’s atmosphere, then chances are there is liquid water on the surface below. “If we see water vapor in the atmosphere, we’re going to be very excited,” Sara Seager, an exoplanet expert at MIT, tells The Verge. “We couldn’t prove 100 percent, but we’d be pretty sure that there’s liquid water on the planet.” So far, liquid water has never been found on an exoplanet. The next step after that is to look for other gases that don’t belong in the atmosphere — especially ones that could be coming from biological life. “Our favorite one is oxygen,” says Seager. “Without life on Earth, we wouldn’t have oxygen at all. So we’ll go down our list of things we’re looking for, kind of like a triage.”

To get these tantalizing answers, the astronomers hope to use NASA’s upcoming James Webb Space Telescope (JWST). When it launches, JWST will sit at more than 1 million miles from Earth and will observe the Universe in the infrared. It’s a type of light that can’t be seen but can be felt as heat. Observing star systems in the infrared makes it much easier to pinpoint when light is coming from a star and when it’s being reflected off of a planet. Plus, JWST will be the most powerful space telescope ever built, seeing cosmic objects with more precision than ever before.

However, JWST doesn’t launch until 2018. In the meantime, Gillon and his team will be using NASA’s Hubble Space Telescope to keep looking at their seven-planet system. “Maybe it will tell us something about the frequency of life and habitable conditions in the Universe,” says Gillon. “This system is really a cornerstone in exoplanetology.”

A closer look at the James Webb Telescope