UPDATED 7.46pm Astronomers have found seven Earth-sized planets, three of which circle their parent star at the right distance for water to be on the surface, raising the prospect of life.

LISTEN ABOVE: Honorary Astronomer at the Stardome, Dr Grant Christie, spoke to Larry Williams about the discovery

NASA and Belgian researchers have found the planets orbiting the Trappist-1 star, meaning they're all in the same solar system.

Auckland Stardome astronomer Dr Grant Christie said the planets are close to us, relative to the size of the galaxy.

But he told Larry Williams there's no chance of humans going to the planets and little chance of communicating with them.

"Even if you sent a light beam it would take 40 years to get there and another 40 years to get back again, assuming there was some sort of technological civilisation there capable of receiving it."

If we were able to travel to the planets, it would take about 700,000 years.

Researchers said the proximity of the system, combined with the proportionally large size of its planets compared to the small star, make it a good target for follow-up studies. They hope to scan the planets' atmospheres for possible chemical fingerprints of life.

"I think that we've made a crucial step towards finding if there is life out there," University of Cambridge astronomer Amaury Triaud told reporters on a conference call on Tuesday.

The discovery, published in this week's issue of the journal Nature, builds on previous research showing three planets circling TRAPPIST-1. They are among more than 3500 planets discovered beyond the solar system, or exoplanets.

Researchers have focused 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 per cent 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.

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.