A small, faint star relatively close by is home to seven Earth-size planets with conditions that could be right for liquid water and maybe even life.
The discovery sets a record for both the most Earth-size planets and the most potentially habitable planets ever discovered around a single star.
The strange planetary system is quite compact, with all of these worlds orbiting their star closer than Mercury orbits the sun, according to a newly published report in Nature.
"If you were on the surface of one of these planets, you would see the other ones as we see the moon, or a bit smaller," says Michaël Gillon, an astronomer at the University of Liège in Belgium. "The view would be very impressive."
The cool, reddish star is about 40 light-years away, in the constellation Aquarius. "It's a very tiny star," says Gillon, "10 times smaller than the sun."
Until recently, no one was even looking for planets around so-called ultracool dwarf stars.
About a year ago, however, scientists announced that they'd found three planets around this star. They launched a campaign to study the star more intensively, and were surprised to find four more. All of the planets were spotted by looking for a faint dimming that occurs when starlight is blocked by a planet passing in front of the star.
Julien de Wit of MIT, one of the astronomers on the research team, says these planets have a "winning combination" of being temperate, Earth-size and ideally suited for follow-up observations with telescopes to analyze their atmospheres.
Already, initial observations have been made with the Hubble Space Telescope, he says, and the data is being analyzed. More observations with Hubble are planned to search for signs of water or methane.
Astronomers could get an even better sense after 2018, when the next-generation James Webb Space Telescope is expected to launch. It could provide an in-depth look at the atmospheres of these planets.
Some theoreticians question whether this type of star could support life, since it is so different from the sun and the close-in planets would get hit by so much ultraviolet radiation. "We really cannot know," says Emmanuël Jehin of the University of Liège. "So it looks like everything is possible, at this point. It's very exciting."
Planet-hunters have recently been gathering more and more evidence that Earth-size planets are common in our galaxy, according to Ignas Snellen at the University of Leiden in the Netherlands, who wrote a commentary that accompanied the scientists' announcement.
This new discovery, he notes, suggests such worlds are "even more common than previously thought."
AUDIE CORNISH, HOST:
Today, scientists announced a big discovery around a star in the constellation Aquarius - seven planets about the size of Earth. And as NPR's Nell Greenfieldboyce reports, they're most likely rocky planets that could be cozy enough for life.
NELL GREENFIELDBOYCE, BYLINE: The star is about 40 light years away, right nearby in the cosmic sense. It's reddish, dim and tiny.
MICHAEL GILLON: Ten times smaller than the sun.
GREENFIELDBOYCE: Astronomer Michael Gillon is at the University of Liege in Belgium. He says the seven Earth-sized worlds are crowded close to their star, closer than the planet Mercury orbits our own sun. But because this star is so small and cool, its planets shouldn't be scorching hot. Temperatures are probably mild.
GILLON: And so they are potentially habitable. As we say, this doesn't mean habitable for a human being. We don't know. But they could have liquid water at least.
GREENFIELDBOYCE: And he says because they're close together, if you were able to stand on the surface of one of these planets and look up...
GILLON: You would see the other ones as we see the moon or a bit smaller. The view would be very impressive.
GREENFIELDBOYCE: This unusual planetary system is described in the journal "Nature," and it now holds the record for the most Earth-sized planets ever found around one star. That's why NASA held a press conference today to announce the find. Someone asked the researcher from Belgium if the planets had names yet.
GILLON: Well, we have plenty of possibilities, which are all related to Belgium beers, but we don't think they will become official, so.
GREENFIELDBOYCE: Right now the planets are just named after their host star. Now, some scientists think this star might not be so hospitable. It's not like our own sun. Early on in its life, it might have fried the planets with intense radiation. But Sara Seager, a planetary scientist at MIT, says we no longer have to just speculate. We can start looking for signs of life.
SARA SEAGER: Because nature usually is smarter than we are. And if there's any way for a life to get a foothold, we like to believe it will.
GREENFIELDBOYCE: What scientists will do next is probe the planet's atmospheres with the Hubble Space Telescope. Indeed, they've already started. Nikole Lewis is an astronomer at the Space Telescope Science Institute in Baltimore. She says these planets are ideal for this type of work.
NIKOLE LEWIS: And we can start to begin this journey in trying to understand what the air is like around rocky planets outside of our solar system.
GREENFIELDBOYCE: An even better sense of that alien air should come from a new space telescope which NASA will launch next year. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.