Never before had astronomers observed a planet orbiting an ultra-cool dwarf star. Then, like cosmic London buses, three came along at once. The discovery has led to hope that the Earth-sized worlds are at least habitable, and at best, harbour life.
The planets were discovered orbiting Trappist-1, a small star in the Aquarius constellation a relatively short distance – 40 light years – from Earth. They will be scoured for signs of life using giant telescopes.
“We will soon be able to study the atmospheric composition of these planets and to explore them first for water, then for traces of biological activity,” said Julien de Wit, a post-doctoral assistant at the Massachusetts Institute of Technology and co-author of a paper on the findings. “That’s a giant step in the search for life in the Universe.”
The astronomers found that the light from Trappist-1 faded at regular intervals, suggesting that several objects were passing in front of it. Further analysis revealed three planets, all a similar size to Earth.
They will soon be able to examine the way that the orbiting planets distort the star’s light, providing clues about their atmospheres, if they have them. The presence of gases such as oxygen and methane would indicate the presence of life.
Advertisement
Michael Gillon, lead author of the paper, published in the journal Nature, said that Earth-like planets around small, cool “brown dwarf” stars such as Trappist-1 were the only places where life could be detected using existing technology.
“If we want to find life elsewhere in the Universe, this is where we should start to look,” Mr Gillon, from the Université de Liège in Belgium, said.
Trappist-1 is, in stellar terms, cold and dim. It is only a little larger than Jupiter and much cooler than the Sun, which means that it gives off a dim red glow rather than a bright, yellow light. This allows astronomers to examine the planets that orbit it. The same method is impossible with brighter stars, whose light overwhelms equipment.
Two of the planets orbit Trappist-1 in between 1.5 days and 2.4 days and the third, somewhere between 4.5 and 73 days. The scientists believe that the planets are between 20 and 100 times closer to Trappist-1 than the Earth is to the Sun, meaning that the system is closer in structure to Jupiter and its moons than to the solar system.
However, because Trappist-1 is far cooler than the Sun, the inner two planets receive only two to four times as much radiation as the Earth, and the third, probably less. This means that there may be habitable zones, containing areas of liquid water, on each planet.
Advertisement
Emmanuël Jehin, a co-author of the paper, said: “So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical. But now we have not just one lonely planet around such a faint red star but a complete system of three planets.”
Adam Burgasser, professor of physics at the University of California, San Diego, said: “It [is] likely that we will be able to detect the atmospheric gases of these planets in the next decade with the launch of the James Webb Space Telescope.” The telescope, a successor to the Hubble space telescope, will be launched in 2018.