NASA held a ground-breaking press conference to announce the discovery of at least three planets capable of harbouring alien life.

Astronomers have found no less than seven Earth-sized worlds orbiting a cool dwarf star, known as TRAPPIST-1, that is 39 light years from our own planet in the Aquarius constellation.

"This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life," said Thomas Zurbuchen, associate administrator of NASA's mission directorate in Washington.

"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."

Here, we take a look at some of the most fascinating facts about these newly discovered worlds.

Seven Earth-sized worlds orbiting a cool dwarf star known as TRAPPIST-1
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    1. How far away are they?

    In space terms, these planets are very close to Earth, with only 39 light years between us and them.

    But it would take a conventional Earth rocket around 44 million years to fly there, so the chance of a human visit any time soon is very slim.

    2. How long do their years last?

    An Earth year is 365 days, but the TRAPPIST-1 planets orbit their star much more quickly - 1.5 days for the nearest planet and 20 days for the farthest.

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    3. How the planets interact with their star

    The Earth rotates as it orbits the Sun, creating the days and nights. But this is not the case with the TRAPPIST-1 planets.

    One side is believed always to face the star while the other faces away, locked in an eternal, frozen night.

    4. Is there any water?

    Although water could exist on some of these newly discovered planets, none has been detected yet because they are so far away.

    Scientists say it will now take a huge amount of observation work to determine if any of them has an atmosphere that could support surface water and life.

    The ground-breaking discovery is announced at a NASA press conference

    5. Do they have any moons like ours?

    No moons have been spotted orbiting any of the planets.

    But they are close enough together that if any of them does have water on its surface, the other planets in the system would cause tides like we see in our own oceans.

    6. How hot are the planets on the surface?

    The surface temperatures of the six inner planets will range from nought to 100 degrees centigrade - and three of these six could support life.

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    7. British technology helped pave the way

    A British-run telescope played a major role in the discovery.

    The robotic device in the Canary Islands, operated by Liverpool John Moores University, was instrumental in the project.

    It was one of a number of ground-based instruments that supported observations made by NASA's orbiting Spitzer telescope.

    8. How old the solar system is compared to ours?

    The newly discovered system could be around half a BILLION years old - one-tenth the age of our 4.5-billion-year-old Sun.

    Liverpool John Moores University's robotic telescope, which played a role in the discovery

    9) Is there alien life out there?

    There is a chance an advanced civilisation lives in the TRAPPIST-1 - but at the moment it's guesswork.

    Sara Seager, a professor of planetary science and physics at Massachusetts Institute of Technology, said: "They may be having a press conference saying there are three habitable planets in our solar system - Earth, Venus and Mars."

    10. What happens next?

    The Hubble Space Telescope will be a key player in studying the atmospheres of the TRAPPIST-1 planets and has already begun preliminary work.