WATCH: NASA discovers seven earth-like planets

All seven roughly match the size and mass of our own planet and are almost certainly rocky, and three are perfectly perched to harbour life-nurturing oceans of water, they reported in the journal Nature.

Most critically, their proximity to Earth and the dimness of their red dwarf star, called Trappist-1, will allow astronomers to parse each one's atmosphere in search of chemical signatures of biological activity.

"We have made a crucial step towards finding life out there," said co-author Amaury Triaud, a scientist at the University of Cambridge.

"Up to now, I don't think we have had the right planets to find out. Now we have the right target," he said in a press briefing. 

The Trappist-1 system, a mere 39 light years distant, has the largest number of Earth-sized planets known to orbit a single star. 

It also has the most within the so-called "temperate zone" - not so hot that water evaporates, nor so cold that it freezes rock-solid.

The discovery adds to growing evidence that our home galaxy, the Milky Way, may be populated with tens of billions of worlds not unlike our own - far more than previously suspected.

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Remarkably, professional stargazers may simply have been looking in the wrong place.

"The great idea of this approach was to study planets around the smallest stars of the galaxy, and close to us," said lead author Michael Gillon, a professor at the University of Liege in Belgium.

'Ultracool' dwarf star 

"That is something nobody did before us - most astronomers were focused on stars like our Sun," he told journalists ahead of publication.

Gillon and his team began to track Trappist-1 - a so-called "ultracool" dwarf star with less than 10% the mass of the Sun - with a dedicated telescope in 2010, and reported last year on three planets in its orbit.

They detected the invisible exoplanets using the so-called "transit" method: when an orbiting world passes between a star and an astronomer peering through a telescope, it dims the starlight by a tiny but measurable amount.

But when subsequent calculations didn't quite tally, Gillon realised that there might be other stars that had escaped Earth-bound observation. 

"So we requested time with NASA's Spitzer Space Telescope," said co-author Emmanuel Jehin, also at the University of Liege. This allowed us to get 20 consecutive 24-hour periods of observation, which was crucial to discovering that we had seven transiting planets."

Looking from Earth, the astronomers could only track activity around the star at night.

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"From space, we observed continually and matched all the transits - 34 in all." 

Compared to the distance between our Sun and its planets, the Trappist-1 family is very tightly bunched.

Indeed, the dwarf star and its seven satellites - with orbits ranging from 1.5 to 12 days - would all fit comfortably in the distance between the Sun and its closest planet, Mercury.