Astronomers have discovered not one, not two, but seven Earth-sized planets orbiting a star called TRAPPIST-1.

NASA and astronomers around the world aren’t just hunting for any ordinary planets. They’re particularly focused on the search for planets in the “habitable zone” of star systems, meaning, in theory, that water could exist on the surface in a liquid form. Finding Earth-size, potentially habitable planets is believed to be the first step to finding other life in the universe.

Researchers have focussed on finding Earth-sized rocky planets with the right temperatures so that water, if any exists, would be liquid, a condition believed to be necessary for life.

The diameter of TRAPPIST-1 is about 8 percent of the sun's size. That makes its Earth-sized planets appear large as they parade past.

Because TRAPPIST-1 is so small and cool, its so-called "habitable zone" is very close to the star. Three planets are properly positioned for liquid water, said lead researcher Michael Gillon, with the University of Liege in Belgium.

"They form a very compact system," Gillon said on a conference call. "They could have some liquid water and maybe life."

Even if the planets do not have life now, it could evolve. TRAPPIST-1 is at least 500 million years old, but has an estimated lifespan of 10 trillion years. The sun, by comparison, is about halfway through its estimated 10-billion-year life.

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 an infant star, astronomer Ignas Snellen, with the Netherlands' Leiden Observatory, wrote in a related essay in Nature.

"It burns hydrogen so slowly that it will live for another 10 trillion years," he wrote, "which is arguably enough time for life to evolve."

The announcement will be streamed on NASA TV, which you can watch right here: