Institute of Astronomy

Newly discovered planets could have water on their surfaces

Published on 22/02/2017 

Representation of the planetary system from above. The star is at the centre and the planets are in orbit around it. The relative sizes of the planets are correct, but on a different scale to their distance to the star. More figures available below.

An international team of astronomers has found a system of seven potentially habitable planets orbiting a star 39 light years away three of which could have water on their surfaces increasing the possibility they could host life. Using ground and space telescopes, the team identified the planets as they passed in front of the ultracool dwarf star known as Trappist-1. The star is around eight per cent of the mass of the Sun and is no bigger than Jupiter.

The team has been using the TRAPPIST–South telescope at the European Space Observatory’s (ESO) La Silla Observatory, the Very Large Telescope (VLT) at Paranal, the NASA Spitzer Space Telescope as well as two other telescopes supported by the UK’s STFC, the William Herschel Telescope and the Liverpool Telescope. All the planets, labelled TRAPPIST-1b, c, d, e, f, g and h in order of increasing distance from their parent star, have sizes comparable to Earth.

The astronomers identified the planets thanks to periodic drops in the brightness of the central star. As the planets passed in front of the star, they cast a shadow, events known as transits, from which the team could measure the planet’s orbital periods and calculate their sizes and masses. They found that the inner six planets were comparable in size, mass and temperature to the Earth raising the possibility that they host liquid water on their surface.

Michaël Gillon of the STAR Institute, at the University of Liège in Belgium, the lead author, 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.”

With just 8% the mass of the Sun, TRAPPIST-1 is very small in stellar terms, only marginally bigger than the planet Jupiter — and though nearby in the constellation Aquarius, it is invisible visually with anything less than powerful telescopes. Astronomers expected that such dwarf stars might host many Earth-sized planets in tight orbits, making them promising targets in the hunt for extraterrestrial life. TRAPPIST-1 is the first such system to be discovered.

Co-author Dr Amaury Triaud, of the University of Cambridge’s Institute of Astronomy, explains: “Stars like TRAPPIST-1 belong to the most common type of stars that exist within our Galaxy. The planets that we found are likely representative of the most common sort of planets in the Universe”. He add: “That the planets are so similar to Earth bodes well for the search for life elsewhere. Planets orbiting ultra-cool dwarfs, like TRAPPIST-1, likely represent the largest habitable real estate in the Milky Way!”

The team determined that all the planets in the system were similar in size to Earth and Venus in our Solar System, or slightly smaller. The density measurements suggest that at least the innermost six are probably rocky in composition.

The planetary orbits are not much longer than that of Jupiter’s Galilean moon system, and much smaller than the orbit of Mercury in the Solar System. However, TRAPPIST-1’s small size and low temperature means that the energy input to its planets is similar to that received by the inner planets in our Solar System; TRAPPIST-1c, d and f receive similar energy inputs to Venus, Earth and Mars, respectively.

All seven planets discovered in the system could potentially have liquid water on their surfaces, though their orbital distances make some of them more likely candidates than others. Climate models suggest the innermost planets, TRAPPIST-1b, c and d, are probably too hot to support liquid water, except maybe on a small fraction of their surfaces. The orbital distance of the system’s outermost planet, TRAPPIST-1h, is unconfirmed, though it is likely to be too distant and cold to harbour liquid water — assuming no alternative heating processes are occurring. TRAPPIST-1e, f, and g, however, are of more interest for planet-hunting astronomers, as they orbit in the star’s habitable zone and could host oceans of surface water.

These new discoveries make the TRAPPIST-1 system an even more important target in the search for extra-terrestrial life. Team member Didier Queloz, from the University of Cambridge’s Cavendish Laboratory, is excited about the future possibilities: “Thanks to future facilities like ESO’ Extremely Large Telescope, or NASA/ESA’s soon-to-be-launched James Webb Space telescope, we will be capable to measure the structure of the planets’ atmospheres, as well as their chemical composition. We are about to start the remote exploration of terrestrial climates beyond our Solar system.”

The discovery is described in today’s edition of Nature, which also includes a science fiction short story, written by Laurence Suhner. Amaury Triaud comments: “We were thrilled at the idea of having artists be inspired by our discoveries right away. We hope this helps convey the sense of awe and excitement that we all have within the team about the TRAPPIST-1 system.”

The star draws its name from the TRAPPIST-South telescope, which made the initial discovery. TRAPPIST is the forerunner of a more ambitious facility called “SPECULOOS” that includes Cambridge as core partner, conducted by researchers of the “Cambridge Centre for Exoplanet Research” in the broad research context related to  “Universal Life”. SPECULOOS is currently under construction at ESO’ Observatory of Cerro Paranal. SPECULOOS will survey 10 times more stars for planets, than TRAPPIST could do. We expect to detect dozens of additional terrestrial planets.

Additional Links:

Link to the scientific paper:

Link to the science-fiction short story:

ESO Press release:

NASA Press release:

For additional information, images, videos, a graphic novel and short stories, visit

IoA contact details

Amaury Triaud, Institute of Astronomy, University of Cambridge

Tel: +44 (0)1223 766 690 or Mob: +44 (0)747 0087 217


Posters and Infographics from IoA (click on each to download higher-res version):

Instead of photo-realist views of the planets, we instead turned to material that is more clearly an illustration. The public sometimes confuses photo-realist renditions of planets, for actual photographs of those planets. All graphics should be credited to IoA/Amanda Smith

Artistic impression of the TRAPPIST-1 system:

Artistic impression of the TRAPPIST-1 system:

View of the planetary system from above. The star is at the centre and the planets are in orbit around it. Their relative position corresponds to what the system would have looked like when we saw the first planet pass in front of the star. The relative sizes of the planets are correct, but on a different scale to their distance to the star.

Comparing the TRAPPIST-1 system with the Solar System. Here the vertical axis represents the amount of energy received from the star, for Earth = 1. Numbers indicate how much flux (2=twice more, ½ = half) a certain distance corresponds.

Sizes of the planets are all relative to each other.

All the planets in the TRAPPIST-1 system in comparison to the size of the Earth.

Comparing the sizes of the main objects of the Solar system, and of the planets of TRAPPIST-1, but keeping the aspect ratio the same.

The shadow that Jupiter would cast by passing –transiting- in front of the Sun is similar to the shadow that the TRAPPIST-1 planets cast as they transit the TRAPPIST-1A star. Measuring the brightness of small stars, like TRAPPIST-1A allows us to detect planets that are much smaller since they cast a bigger shadow.

Comparing the sizes of the main objects of the Solar system, and of the planets of TRAPPIST-1.

The TRAPPIST-1 star is small, barely bigger than Jupiter. The planets of TRAPPIST-1 are comparable to Earth.


Comparison of the sizes of the objects of the TRAPPIST-1 system, with the objects within the system of satellite around Jupiter. The horizontal axis represents the orbital period of the planets/satellites. All the sizes are made relative to Jupiter.

TRAPPIST-1 is remarkably similar in architecture to the Jupiter system.


Representation to scale of the distances and sizes of two of the planets in the TRAPPIST-1 system. The star is located on the left hand side, out of the image.

To ease comparison, we show how large the Earth, the Moon and the orbit the Moon takes, also to scale.

TRAPPIST-1f and TRAPPIST-1g are a little over three times the Earth-Moon distance, which is remarkably close.


Representation of the orbits of the Moon around the Earth, of the satellites of Jupiter, around Jupiter and of the planets of TRAPPIST-1, to scale.

Then, below, how the size of the TRAPPIST-1 system would look compared to the inner parts of the Solar system. The TRAPPIST-1 system is very compact.

Sizes of the planets are relative to each other, on scale different to the one used for distances.


Page last updated: 22 February 2017 at 18:49