A group of international scientists have announced that a dwarf star only 40 light years from the Earth has at least seven apparently rocky planets with potential to harbour water and life.

The discovery of the planets, which are all about the size of Earth or smaller, was announced at the NASA headquarters in Washington, DC, and published in the journal Nature on Wednesday.

"Looking for life elsewhere, this system is probably our best bet as of today," said Brice-Olivier Demory, professor at the University of Bern's Centre for Space and Habitability and one of the authors of a paper on the planets around TRAPPIST-1, a small, ultracool star.

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The TRAPPIST-1's planets are similar to the inner solar system around Earth's sun, Demory's analysis of data from NASA's Spitzer Space Telescope showed.

The discovery sets a new record for the greatest number of habitable-zone planets found around a single star outside our solar system, NASA said.

Like previous exoplanets - or planets beyond Earth's solar system - TRAPPIST-1's satellites were spotted when astronomers saw the planets "transit" the star, blocking some of the light captured by telescopes.

"The discovery gives us a hint that finding a second Earth is not just a matter of if, but when," said Thomas Zurbuchen, of the Science Mission Directorate at NASA.

"This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life," Zurbuchen added.

"Answering the question 'Are we alone?' is a top science priority, and finding so many planets like these for the first time in the habitable zone is a remarkable step forward toward that goal."

'Habitable zone'

All seven planets are in orbits even closer than Mercury, the Sun's nearest planet, causing frequent transits and plentiful opportunities for observation and collection of data.

But because TRAPPIST-1 is so small and cool - about one-tenth the size and half the temperature of the sun - at least three of the planets could be cool enough for water and, possibly, life, despite their tight orbits.

They fall clearly within the so-called habitable zone.



Further data used in the Nature paper was collected from several ground telescopes around the world.

Demory said that future telescopes, including the James Webb Space Telescope, which is to succeed the orbiting Hubble Space Telescope, "will have the possibility to detect the signature of ozone if this molecule is present in the atmosphere of one of these planets".

He said that ozone "could be an indicator for biological activity on the planet", but cautioned that signs of life are difficult to assert from interstellar distances.