Seven Earth-sized planets found orbiting nearby star

By Ellen Rykers | February 22, 2017

Dubbed ‘Earth’s seven sisters’, the rocky exoplanets could potentially harbour liquid water.

ASTRONOMERS HAVE DISCOVERED a system hosting seven exoplanets, all similar in size and mass to our own planet Earth. This ‘sister’ solar system orbits an ultracool dwarf star, known as TRAPPIST-1, located 40 light years away in the Aquarius constellation.

This extraordinary planetary system contains the largest number of Earth-sized planets found yet – and many of these could support liquid water, thought to be an important prerequisite for life.

Three of the exoplanets are located at the right distance from the star for liquid water to exist at their surface – the so-called ‘habitable zone’. They could host oceans, which would greatly increase the possibility of finding extra-terrestrial life on these distant worlds.

The configuration of the exoplanets – six of which are rocky – is similar to Jupiter and its moons, but scaled up by a factor of 80. If this arrangement is common, our universe may play host to more Earth-like planets – and potentially more life – than initially thought.

exoplanet discovery

Artist’s impression showing TRAPPIST-1 and its planets reflected in a surface. The potential for water on each of the worlds is represented by the frost, water pools, and steam surrounding the scene. The image appears on the 22 February 2017 Nature cover.

“Surprisingly similar to Earth”

The discovery, reported today in Nature, has scientists excited. “This is an amazing planetary system – not only because we have found so many planets, but because they are all surprisingly similar to Earth,” said lead author Michaël Gillon, from the University of Liège in Belgium.

Professor Chris Tinney, an expert on exoplanets at the University of NSW in Sydney, says this is an exciting discovery. “This is an example of a new class of planetary system with closely-packed terrestrial bodies,” he said. “A few of these systems were found unexpectedly by the Kepler mission, but they orbited bright, sun-like stars and were basically irradiated cinders. Now, we know these systems can form around other types of stars.”

The host star in this case is very small and dim, with only 8 per cent of the mass of our own sun and one thousandth of its brightness. “The star is so faint that surface temperatures on the planets mean they can potentially sustain liquid water – thought to be a touchstone for potentially habitability,” explained Chris.

Next: the search for water and life

The team initially announced the discovery of three exoplanets orbiting TRAPPIST-1 in early 2016. Since then, observations have revealed four more planets in the system. Researchers use transits to detect these exoplanets: when a planet passes in front of its host star, a tiny portion of the star’s light is blocked. Detecting this dip in the brightness of the star reveals the planet’s presence and size.

“In this case, researchers could also estimate the mass of the planets, because they are so close together that they interact with each other,” said Chris. “They can then figure out the planetary densities which tells us about their make-up.”

Co-author Emmanuël Jehin, also at the University of Liège, is excited about future research opened up by this discovery. “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,” he said.

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