Seven Earth-sized planets have been discovered orbiting a star just 40 lightyears away, three of which could host oceans of liquid water on their surfaces. The exoplanets were spotted orbiting a dwarf star known as TRAPPIST-1 and make up the largest number of Earth-sized planets found, as well as the largest number of worlds discovered that could support liquid water.


Astronomers made the discovery by looking for dips in the light emitted by TRAPPIST-1, as this can indicate the presence of an orbiting planet.

These dips are known as ‘transits’ and studying them also enables astronomers to learn much about the planets’ composition, sizes and orbits.

TRAPPIST-1 is just 8 per cent the mass of our Sun and appears very dim when viewed from Earth.

The orbits of the seven planets are thought to be smaller than the orbits of Mercury, which is the closest planet to the Sun in our Solar System.

Because TRAPPIST-1 is much smaller and cooler than our own Sun, the close orbit of the planets puts them in the ‘habitable’ zone, meaning they are just the right temperature to potentially support life.

Density measurements also revealed the six innermost planets could be rocky in composition.

Climate models suggest the three innermost planets, known as TRAPPIST-1b, c and d, may be too hot to support liquid water, except possibly on a small fraction of their surfaces.

The outermost planet, TRAPPIST-1h, may be too far from the star and cold to harbour liquid water.

Planets e, f and g, however, are thought to orbit in the 'Goldilocks zone' and could host water, meaning there is the potential for life to exist.

Lead author Michaël Gillon of the STAR Institute at the University of Liège in Belgium says:

“This is an amazing planetary system — not only because we have found so many planets, but because they are all surprisingly similar in size to the Earth.”

Study team member Emmanuël Jehin says:


“With the upcoming generation of telescopes, such as ESO’s European Extremely Large Telescope and the NASA/ESA/CSA James Webb Space Telescope, we will soon be able to search for water and perhaps even evidence of life on these worlds.”


Iain Todd BBC Sky at Night Magazine
Iain ToddScience journalist

Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.