Solar system of seven Earth-sized planets 'could include worlds with life'
An extraordinary solar system of seven Earth-sized planets orbiting a dwarf star could include at least three worlds on which life may have evolved, scientists believe.
The cool dim star, known as TRAPPIST-1, can be considered a close neighbour since it is only 39 light years away.
Six of its "exoplanets" lie in a temperate zone where surface temperatures range from zero to 100C.
Of these, at least three are thought to be capable of having watery oceans, greatly increasing the likelihood of life.
No other star system known contains such a large number of Earth-sized and probably rocky planets.
All are about the same size as Earth or Venus, or slightly smaller. Because the parent star is so dim, the planets are warmed gently despite having orbits much smaller than that of Mercury, the planet closest to the sun.
As well as being in tight orbits, the TRAPPIST-1 planets are unusually close to one another, conjuring an image straight out of science fiction.
From the viewpoint of someone standing on the surface of one of the planets, some of the other worlds would appear larger than the moon in the Earth's sky.
Gazing up, it would be possible to see the geological features, oceans and clouds of your planetary neighbours.
The discovery, reported in the journal Nature, was made by astronomers using the American space agency Nasa's exoplanet-hunting Spitzer Space Telescope.
Dr Sean Carey, who heads Nasa's Spitzer Science Center in Pasadena, California, said: "This is the most exciting result I have seen in the 14 years of Spitzer operations."
The telescope operates at the infrared wavelengths which glow brightest from TRAPPIST-1, and can detect the tiny dimming that occurs when a passing or "transiting" planet blocks out light from its star.
Spitzer's data allowed the team to measure precisely the sizes of the seven planets and estimate the masses and densities of six of them.
Follow-up observations were made by a number of ground-based telescopes, including a robotic instrument operated by Liverpool John Moores University on La Palma in the Canary Islands.
Dr Chris Copperwheat, leading the British team, said: "The discovery of multiple rocky planets with surface temperatures which allow for liquid water make this amazing system an exciting future target in the search for life."
TRAPPIST-1 is a small star with 8% the mass of the sun and only slightly bigger than the planet Jupiter.
Astronomers expect such dim red dwarf stars, which are abundant in our galaxy, the Milky Way, to host many Earth-sized planets in close orbits, making them promising targets in the search for extraterrestrial life.
The seven-planet star takes its name from the TRAPPIST robotic telescope at the European Southern Observatory, La Silla, Chile which found the first evidence of three of its worlds, reported in May last year.
Dr Michael Gillon, from the STAR Institute at the University of Liege in Belgium, which operates the telescope and contributed to the latest observations, said: "The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star. It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds."
Nasa plans follow-up studies of the system using the Spitzer, Hubble and Kepler space telescopes.
Future telescopes, including the European Extremely Large Telescope and James Webb Space Telescope, may be powerful enough to detect markers of life such as oxygen in the planets' atmospheres.
Dr Emmanuel Jehin, another member of the Liege team, said: "We will soon be able to search for water and perhaps even evidence of life on these worlds."
Dr Thomas Zurbuchen, associate administrator of Nasa's Science Mission Directorate in Washington D.C. said: "This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life.
"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."
Three of the planets, classified as TRAPPIST-1 e, f and g, were found to be firmly placed in the "habitable" or "Goldilocks" orbital zone where temperatures are not too hot or cold to permit surface oceans of liquid water.
Because the planets are so close to their star, they may be tidally locked, meaning they always face the star from the same side.
As a result they may have weather patterns totally unlike those on Earth, such as strong winds blowing from the day side to the night side, and extreme temperature changes.