7 Earth-size worlds found orbiting star, could hold life

"But finding seven transiting Earth-sized planets in such a small sample suggests that the solar system with its four (sub-) Earth-sized planets might be nothing out of the ordinary," Snellen wrote.

Altogether, astronomers have confirmed close to 3,600 planets outside our solar system since the 1990s, but barely four dozen are in the potential habitable zone of their stars, and of those, just 18 are approximately the size of Earth.

Gillon and his team used both ground and space telescopes to identify and track the seven Trappist-1 planets, which they label simply by lowercase letters, "b'' through "h." As is typical in these cases, the letter "A'' - in upper case - is reserved for the star. Planets cast shadows on their star as they pass in front of it; that's how the scientists spotted them.

Tiny, cold stars like Trappist-1 were long shunned by exoplanet-hunters (exoplanets are those outside our solar system). But the Belgian astronomers decided to seek them out, building a telescope in Chile to observe 60 of the closest ultracool dwarf stars. Their Trappist telescope lent its name to this star.

While faint, the Trappist-1 star is close by cosmic standards, allowing astronomers to study the atmospheres of its seven temperate planets. All seven look to be solid like Earth - mostly rocky and possibly icy, too.

They all appear to be tidally locked, which means the same side continually faces the star, just like the same side of our moon always faces us. Life could still exist at these places, the researchers explained.

"Here, if life managed to thrive and releases gases similar to that that we have on Earth, then we will know," Triaud said.

Chemical analyses should indicate life with perhaps 99 percent confidence, Gillon noted. But he added: "We will never be completely sure" without going there.