NASA has announced the discovery of a nearby alien solar system.

The space agency’s Spitzer Space Telescope has revealed the first known system of seven Earth-size planets around a single star.

Three of the planets are located in a habitable zone, close enough to the central star that a rocky planet would likely have liquid water.

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

“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 Science 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.”

The alien system is about 40 light-years (378 trillion km) from Earth in the constellation Aquarius.

This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile.

In May 2016, researchers using TRAPPIST announced the discovery of three planets in the system.

Assisted by several ground-based telescopes, including the European Southern Observatory's Very Large Telescope, Spitzer confirmed the existence of two of these planets and discovered five additional ones, increasing the number of known planets in the system to seven.

The new results have been published in the journal Nature.

The team was able to precisely measured the sizes of the seven planets and developed estimates of the masses of six of them, allowing their density to be inferred.

Based on these assumed densities, all of the TRAPPIST-1 planets are likely to be rocky.

Further observations will reveal whether they are rich in water, and potentially reveal whether any could have liquid water on their surfaces.

All seven of the TRAPPIST-1 planets orbit closer to their host star than Mercury is to our Sun, but because the TRAPPIST-1 star is cooler - classified as an ultra-cool dwarf – liquid water could still survive on planets orbiting very close to it.

If a person was standing on the surface of one of the planets, they could gaze up and see neighbouring planets in such detail that they could spot geological features or even clouds, as they would sometimes appear larger than the moon in Earth's sky.

“The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star,” said Michael Gillon, lead author of the paper and the principal investigator of the TRAPPIST exoplanet survey.

“It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds.”