University of Liège

New solar system found to have 7 Earth-size planets

seven planets
seven planets

Astronomers have just identified a nearby solar system hosting seven Earth-sized planets. Most intriguing: Three planets that orbit its central star — known as TRAPPIST-1 — may even be within a habitable zone. That means they fall within a region that could support life as we know it. As such, these newfound worlds are good sites to focus a search for alien life.

TRAPPIST-1’s big planetary family also hints that many more cousins of Earth may exist than astronomers had thought.

“It’s rather stunning that the system has so many Earth-sized planets,” says Drake Deming. He’s an astronomer at the University of Maryland in College Park. It seems like every stable spot where a planet could be, there is an Earth-sized one. And that, he adds, “bodes well for finding habitable planets.”

Astrophysicist Michaël Gillon works at the University of Liège in Belgium. He was part of a team that last year announced they had found three Earth-sized planets around TRAPPIST-1. This dwarf star is only about the size of Jupiter. It’s also much cooler than the sun. And it’s a relative neighbor to Earth, a mere 39 light-years away in the constellation Aquarius.

Follow-up observations with the Spitzer Space Telescope and additional telescopes on the ground now show that what first had appeared to be a third planet is actually a quartet of Earth-sized ones. Three of these may be habitable.

If those planets have Earthlike atmospheres, their surfaces may even host oceans of liquid water. Or at least that’s what Gillon and his colleagues reported online February 22 in Nature. Their data also offer signs of a seventh, outermost planet.

How they spotted the new worlds

All seven planets were detected by watching how their star dims as each passes — or transits — in front of it. Scientists measured how much of the star’s light each transit blocked from Earth’s view. Knowing how big a planet would have to be to do that, the astronomer calculated that all seven must have roughly the same radius as Earth.

Those dips in starlight also showed how fast the planets orbit their star: The innermost one makes a round trip in 1.5 Earth days. The outermost one takes roughly 20 days.

The planets’ masses range from about half to 1.5 times that of Earth. To figure that out, the researchers looked at the way the six inner planets tug on each other. The mass and size data then allowed the team to calculate the planets’ densities. All of this suggested that the inner six are rocky, as Earth is.

The length of each planet’s day — how quickly it spins on its axis — may sync with its sun’s orbit. That would make the innermost planet’s day 1.5 Earth days long and the outermost one’s 20 Earth days long. That would be like Earth rotating once in 365 days…