In a major boost to the search for alien life outside our solar system, astronomers have spotted the first known system of seven Earth-size planets around a single star, NASA said.

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 three of the plants which are located in the habitable zone, according to the researchers.

"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.

At about 40 light-years (235 trillion miles) from Earth, the system of planets is relatively close to us, in the constellation Aquarius.

Because they are located outside of our solar system, these planets are scientifically known as exoplanets.

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 they had discovered three planets in the system.

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

Using Spitzer data, the team precisely measured the sizes of the seven planets and developed first estimates of the masses of six of them, allowing their density to be estimated.

Based on their densities, all of the TRAPPIST-1 planets are likely to be rocky, according to the new results published on Wednesday in the journal Nature

Further observations will not only help determine whether they are rich in water, but also possibly reveal whether any could have liquid water on their surfaces.

The mass of the seventh and farthest exoplanet has not yet been estimated. But scientists believe it could be an icy, "snowball-like" world, but further observations are needed.

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

"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 at the University of Liege, Belgium.

"It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds," Gillon 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 researchers said.