The seven newly discovered Earth-sized planets in the ultra cool dwarf star system may only face one way towards their sun, similar to the Earth's moon, and orbit very close to it, scientists have found.

Astronomers, using NASA's Spitzer Space Telescope, announced the discovery of the first known system of seven Earth-sized planets around a single star – located a mere 40 light years away – that could have liquid water, and possibly host alien life.

Three of these planets are firmly located in the habitable zone, the area around the parent star where a rocky planet is most likely to have liquid water, NASA said.

In contrast to our Sun, the TRAPPIST-1 star – classified as an ultra-cool dwarf – is so cool that liquid water could survive on planets orbiting very close to it, closer than is possible on planets in our solar system.

All seven of the TRAPPIST-1 planetary orbits are closer to their host star than Mercury is to our sun. The planets also are very close to each other.

If a person was standing on one of the planet's surface, they could gaze up and potentially see geological features or clouds of neighbouring worlds, which would sometimes appear larger than the moon in Earth's sky, said NASA.

The planets may also be tidally locked to their star, which means the same side of the planet is always facing the star, therefore each side is either perpetual day or night.

This could mean they have weather patterns totally unlike those on the Earth, such as strong winds blowing from the day side to the night side, and extreme temperature changes.

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

All of these seven planets could have liquid water – key to life as we know it – under the right atmospheric conditions, but the chances are highest with the three in the habitable zone.

"This is the most exciting result I have seen in the 14 years of Spitzer operations," said Sean Carey, manager of NASA's Spitzer Science Centre at Caltech/IPAC in California.

"Spitzer will follow up in the fall to further refine our understanding of these planets so that the James Webb Space Telescope can follow up. More observations of the system are sure to reveal more secrets," said Carey.

Following up on the Spitzer discovery, NASA's Hubble Space Telescope has initiated the screening of four of the planets, including the three inside the habitable zone.

These observations aim at assessing the presence of puffy, hydrogen-dominated atmospheres, typical for gaseous worlds like Neptune, around these planets, researchers said.