Astronomers have discovered seven new planets which may host liquid water. Their find shows that Earth-like planets are even more common in the universe than previously thought.
It is not the first time astronomers have found Earth-like planets outside our solar system.
But, it is the first time they have found so many of them at one time around one single star.
Michaël Gillon from Liege University in Belgium, and his colleagues, discovered seven new planets - their size all comparable to that of Earth.
"All of them could have liquid water and maybe life on their surface," said Gillon, co-author of the study published in the "Nature" magazine on Wednesday .
The newly discovered planets orbit the star TRAPPIST-1, a dwarf star only one tenth the size of our sun, and about as big as Jupiter. TRAPPIST-1 lies 39 light years away from Earth.
All seven planets - simply called: 1b, 1c, up to 1h - are quite close to TRAPPIST-1, much closer than Earth is to the sun.
This is because TRAPPIST-1 is much smaller and cooler than the sun. The habitable zone, where it is neither too cold nor too hot and thus allows for the existence of liquid water, moves closer to the star compared to our solar system.
Do we have neighbors out there?
TRAPPIST-1 is not very bright, so sunbathing on its planets could be a rather unsatisfying experience.
"We think the amount of light that you would receive in your eye would be something like 200 times less than you would from the sun. It is like at the end of sunset," co-author Amaury Triaud, an astronomer at the University of Cambridge, said.
But the star is still brighter than our moon and it will still feel quite warm on these planets, Triaud added. "You still receive as much energy from the star - and you will feel that with your skin."
Most of the star's light is in the infrared spectrum which we cannot see. Triaud speculated that the sky might be painted in a "salmony" color.
Nobody knows what the surface and climate of these planets might be like, if there is liquid water, or even life.
"We don't know how life emerges. If life emerges in an ocean and there is an ocean there, then there won't be a problem. But if life is born elsewhere, then maybe the conditions are different."
Then the chances of life might depend on the amount of detrimental radiation that those planets receive from the star.
'We will know more soon'
The next step will be to investigate the atmosphere on those planets. The results might give a hint as to possible life out there.
The researchers will use the James Webb Space Telescope, scheduled for launch next year, to detect atmospheric components from the new planets.
If they find traces of methane, oxygen and ozone simultaneously on one planet, then "that is a strong indication for life," Dillon said.
Studies will start as soon as possible.
"We can expect that within a few years we will know a lot more about these planets," Cambridge astronomer Triaud said, "and [we] hope within a decade [we will know] if there is life there."
Successful search for extrasolar planets
The first planet outside our solar system was discovered in 1992.
With better technology and telescopes, the search for exoplanets has become easier.
Researchers can detect new planets when those planets pass in front of their host star and block a small amount of the stellar light.
Thousands of planets have been discovered beyond our solar system during the past decade using this method.
"We know now that most stars have planets and that even Earth-like planets in the habitable zones are quite common," Barbara Ercolano, an astrophysicist at the University of Munich, said last November.
Ercolano estimated then that there were about one billion Earth-like planets in the universe which could have liquid water.
Researchers may now have to rethink that number.
"Gillon and [the other] collaborators' findings indicate that these planets are even more common than previously thought," astronomy professor Ignas Snellen from the Leiden Observatory wrote in "Nature."
It is the first time researchers have taken a closer look at a dwarf star like TRAPPIST-1. Other researchers have focused on bigger, sun-like stars when looking for new planets.
Earth might not be so special after all
Do new planets mean a new home for humans? Michael Gillon and his co-researchers announced last May that they had found three planets in the habitable zone around TRAPPIST-1.
Less than a year later, they found enough data to show there are seven Earth-like planets.
It might be time to get used to the thought that there are many more out there, writes Snellen.
"Of course, the authors could have been lucky, but finding seven [...] Earth-sized planets in such a small sample suggests that the [our] solar system [...] might be nothing out of the ordinary."
And, even if there is no life, there is one fact which might be comforting for earthlings.
"In a few billion years, when the sun has run out of fuel and the solar system has ceased to exist, TRAPPIST-1 will still be only an infant star. It burns hydrogen so slowly that it will live for another 10 trillion years."
Earth's seven sisters and its potential inhabitants will still be there when we are long gone.