An artist's conception of the view from the fifth planet of the TRAPPIST-1 system. Credit:NASA/JPL Since the 1990s astronomers have detected hundreds of planets orbiting stars in our galaxy, including around the nearest star to the sun, Proxima Centauri. But this discovery of so many terrestrial planets in our galactic neighbourhood is making people take notice. Charley Lineweaver from the Australian National University said: "Detections of Earth-like planets in habitable zones are still quite rare, so this is significant." TRAPPIST-1, as the star is called, is an "ultracool dwarf" with a temperature of 2300 degrees (less than half that of the sun), a radius slightly larger than Jupiter's and a mass just 8 per cent that of the sun.

An artist's impression of the TRAPPIST-1 planetary system based on available data about their diameters. Credit:NASA/JPL "By choosing such a small star means we will be able to study the climate and chemical composition of any planetary atmospheres," said one of the authors, Amaury Triaud from Cambridge University. Astronomers cannot directly observe exoplanets but must deduce their existence from the movement and light from their parent stars. However, they believe direct observation of planetary atmospheres will be possible within a decade. Top row: TRAPPIST-1 planetary orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars. Credit:NASA/JPL Chemical analysis of the atmospheres, if they exist, will help us determine if these planets harbour life or are habitable.

Dr Gillon said the fourth, fifth and sixth planets "could have liquid water – and maybe life – on their surfaces" assuming Earth-like atmospheres. The TRAPPIST-1 star, an ultra-cool dwarf, has seven Earth-sized planets orbiting it. This is the front cover image for this week's edition of Nature. Credit:Nature The paper, published on Thursday in Nature, said the three inner planets, if they have atmospheres, will likely have "runaway greenhouse scenarios", like Venus. Dwarf stars, while cool, are also very slow-burning and TRAPPIST-1 is just a baby at 500 million years old. Ignas Snellen, an astronomer at Leiden University in the Netherlands, writing in Nature said: "In a few billion years, when the sun has run out of fuel and the solar system has ceased to exist, TRAPPIST-1 will still be only an infant star.

"It burns hydrogen so slowly that it will live for another 10 trillion years, which is arguably enough time for life to evolve." And possibly enough time for us to work out how to travel the 39 light years across interstellar space. Graphic showing relative size of TRAPPIST-1 to the sun and of its planets relative to Earth. Credit:Nature Professor Chris Tinney from the University of NSW said the discovery was an exciting result. "This is a new class of planetary system we didn't know about five years ago," he said. "A lot of our models about how planetary systems are formed are heavily biased towards explaining our solar system. This discovery could mean these compact systems around smaller stars are actually more common."

Dr Lineweaver said: "With TRAPPIST-1 having so many rocky planets in and around the goldilocks zone, no matter how sceptical you are, one or more of the planets will lie in the goldilocks zone." The goldilocks zone is a planet's distance from a star where it's "not too hot, not too cold" for liquid water to form. However, he said there were some important caveats when considering the discovery. "Low-mass dwarfs have very long pre-main sequence lifetimes during which the planets that are now in the goldilocks zone could have lost all their water. "What's the sense of saying that a planet is in the goldilocks zone if the planet lost all of its water billions of years earlier?"

The planets are also likely tidally locked, which means the same face of the planet is always facing the star. The moon is tidally locked to the Earth. Professor Tinney said tidally locked planets could have extremes of temperature between the light and dark sides. "Unless they have oceans and atmospheres to allow heat transference, the temperature differences could be extreme," he said. The orbital periods of the planets are very short, ranging from 1.5 to 12.4 days, with masses ranging from 0.4 to 1.4 times that of Earth. Radiuses of the planets range from 0.8 to 1.1 Earths with planetary densities between 0.6 and 1.2 Earth densities.

TRAPPIST-1 is named after the robotic telescope in Chile used to first discover the system. The Transiting Planets and Planetesimals Small Telescope in the Atacama desert is operated by the University of Liege. As well as being the name of an order of monks, trappist is the name of a popular type of Belgian beer. m. Credit:Nature