It might be the ultra-coolest new real estate opportunity in the universe – if it's not already occupied.

Scientists have discovered seven Earth-sized planets orbiting a single red dwarf star approximately 40 light-years away, in an "extraordinarily rich" planetary system that could yield evidence of alien life within the next decade.

The planets were found orbiting a star, dubbed TRAPPIST-1, that's much smaller and cooler than our own sun.

Three of the seven planets are in the star's "Goldilocks zone" where conditions are right for the emergence of life, and all seven could potentially have liquid water on their surfaces, according to the study published Wednesday in the journal Nature. Any one of the planets could also possess an atmosphere of greenhouse gases, which is necessary for a habitable environment.

"We've made a crucial step towards finding out if there is life out there," study co-author Amaury Triaud, of the University of Cambridge, said in a conference call.

It's the first time so many Earth-sized planets have been found orbiting the same star, and a rare discovery in that most planet-hunting efforts tend to focus on stars like our own yellow sun.

"This is an amazing planetary system, not only because we have found so many planets, but because they are all similar in size to Earth," lead study author Michael Gillon, of the STAR Institute at the University of Liege, said in a release.

The planets are composed of rock and are roughly the same mass as Earth or Venus, plus or minus 10-20 per cent.

Gillon says the discovery offers an immediate chance to start looking for indicators of life on another planet. It could also make it much easier to find Earth-like planets in the future, because it demonstrates that different kinds of stars could potentially host the conditions that are right for the emergence of life.

TRAPPIST size comparison

Red sun

The seven planets orbiting close to the ultracool red dwarf star TRAPPIST-1 were found using a combination of Earth-based telescopes, as well as the orbiting Hubble and Spitzer space telescopes. The exoplanets are far closer to their star than Mercury is to our sun, but TRAPPIST-1's heat is not nearly as intense, so there's a chance at least one of them might be just right for life.

"Compared to a (typical) red dwarf, TRAPPIST is a very quiet… ultracool dwarf star," Gillon said on a conference call.

The study authors compared the TRAPPIST-1 planetary system to Jupiter and its moons, with all seven planets orbiting in fairly close proximity and "interacting" with each other's orbits. The planets are likely tidally locked to the star, meaning one side is constantly facing the star. However, if greenhouses gases are present, the day and night sides would be fairly close to the same temperature, they say.

TRAPPIST-1 orbits

The star itself is only eight per cent the size of the sun, and would likely appear red or salmon-pink from the surface of one of the planets, due to the primarily infrared and X-ray radiation it emits.

The star is 200 times dimmer than our sun, but Triaud says it would still be possible to see with the naked eye from the surface of the most habitable planet, where it would be "like the light you receive at the end of sunset."

Red dwarfs like TRAPPIST-1 burn cooler but much longer than a star like the Earth's sun. In fact, TRAPPIST-1 is thought to be billions of years younger than our sun, and will probably still be burning at the same rate when our sun explodes.

But now, he said, TRAPPIST-1 would appear 10 times larger from the surface than our own sun in the sky, while each nearby planet would appear to be twice as large as our moon. "The spectacle would be beautiful," Triaud said.


Of the seven planets, three are thought to be too close to the sun to be habitable, while one is probably too far away. However, the planets dubbed TRAPPIST-1e, f and g are the "holy grail" for planet-hunting astronomers, the research team says.

One of the planets remains a bit of a mystery, as it was only spotted once during a 20-day observation period from the Spitzer telescope.

All seven planets were observed using a technique called transit photometry, which measures the degree to which the light of a star dims while one of its planets passes in front of it.

Three of the seven planets were previously announced as discoveries in a paper published in May, by the same team. That initial discovery prompted the researchers to point more telescopes at TRAPPIST-1.

The Hubble Space Telescope is being used to search for atmospheres on the distant planets, but two telescopes currently under construction are expected to offer even more insight into the TRAPPIST-1 planets.

Researcher Emmanuel Jehin says it will soon be possible to learn a lot more about them, once the James Webb Space Telescope and the European Extremely Large Telescope are in operation. The James Webb is slated for launch in late 2018, while the E-ELT is expected to come online at a facility in Chile in 2024.

"We will soon be able to search for water and perhaps even evidence of life on these worlds," he said in a release.


The planet potentially best-suited for life would be slightly cooler than Earth, but Triaud says the "temperature should be fine" if there are greenhouse gases present to make up an atmosphere. Temperatures also wouldn't vary too much from the day side to the night side, he said.

Gillon says further research will focus on analyzing the molecules present on each planet, in hopes of detecting the necessary elements for life. Those include methane, oxygen, carbon dioxide and ozone, as well as liquid water. Gillon says the presence of all those elements would provide evidence of life "with 99 per cent confidence," short of investigating the planet in person or receiving a message from an alien species.

In a separate paper published in the same issue of Nature, astronomer Brice-Oliver Demory also suggests it may be possible to detect ozone in the TRAPPIST-1 system. "This could be an indicator for biological activity on the planet," Demory, of the University of Bern, said in a statement. However, he also cautioned against inferring too strongly that life might be present.

Triaud says it's still unclear exactly how planetary life emerges, but it might be more likely to happen on a planet where there is a large ocean, which can filter out some of the potentially harmful rays from the nearby star.

The image below, released by NASA, shows each planet in its orbit. The potential for water on each planet is represented by the frost, water pools and steam surrounding the scene.


Historically, the search for Earth-like planets has been focused on finding star systems similar to our own, with a yellow star and a planet more or less like Earth. But with the discovery of TRAPPIST-1, Gillon says astronomers can now compare seven planets of the same kind, and see which ones offer the most favourable conditions for life.

Triaud echoed Gillon, saying that at last, astronomers have "the right target" in their search for alien life.