A team of astronomers led by Michael Gillon, of the Institut d'Astrophysique et Geophysique at the University of Liege in Belgium, have used the Belgian TRAPPIST telescope to observe the star 2MASS J23062928-0502285, now also known as TRAPPIST-1. They found that this dim and cool star faded slightly at regular intervals, indicating that several objects were passing between the star and the Earth. Detailed analysis showed that three planets with similar sizes to the Earth were present.

TRAPPIST-1 is an ultracool dwarf star - it is much cooler and redder than the Sun and barely larger than Jupiter. Such stars are both very common in the Milky Way and very long-lived, but this is the first time that planets have been found around one of them. Despite being so close to the Earth, this star is too dim and too red to be seen with the naked eye or even visually with a large amateur telescope. It lies in the constellation of Aquarius (The Water Carrier).

Emmanuel Jehin, a co-author of the new study, is excited: "This really is a paradigm shift with regards to the planet population and the path towards finding life in the Universe. So far, the existence of such 'red worlds' orbiting ultra-cool dwarf stars was purely theoretical, butnow we have not just one lonely planet around such a faint red star but a complete system of three planets!"

Michael Gillon, lead author of the paper presenting the discovery, explains the significance of the new findings: "Why are we trying to detect Earth-like planets around the smallest and coolest stars in the solar neighbourhood? The reason is simple: systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology. So if we want to find life elsewhere in the Universe, this is where we should start to look."

Astronomers will search for signs of life by studying the effect that the atmosphere of a transiting planet has on the light reaching Earth. For Earth-sized planets orbiting most stars this tiny effect is swamped by the brilliance of the starlight. Only for the case of faint red ultra-cool dwarf stars - like TRAPPIST-1 - is this effect big enough to be detected.

Follow-up observations with larger telescopes, including the HAWK-I instrument on ESO's 8-metre Very Large Telescope in Chile, have shown that the planets orbiting TRAPPIST-1 have sizes very similar to that of Earth. Two of the planets have orbital periods of about 1.5 days and 2.4 days respectively, and the third planet has a less well determined period in the range 4.5 to 73 days.

"With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun. The structure of this planetary system is much more similar in scale to the system of Jupiter's moons than to that of the Solar System," explains Michael Gillon.

Although they orbit very close to their host dwarf star, the inner two planets only receive four times and twice, respectively, the amount of radiation received by the Earth, because their star is much fainter than the Sun. That puts them closer to the star than the habitable zone for this system, although it is still possible that they possess habitable regions on their surfaces. The third, outer, planet's orbit is not yet well known, but it probably receives less radiation than the Earth does, but maybe still enough to lie within the habitable zone.

"Thanks to several giant telescopes currently under construction, including ESO's E-ELT and the NASA/ESA/CSA James Webb Space Telescope due to launch for 2018, 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. That's a giant step in the search for life in the Universe," concludes Julien de Wit, a co-author from the Massachusetts Institute of Technology (MIT) in the USA.

This work opens up a new direction for exoplanet hunting, as around 15% of the stars near to the Sun are ultra-cool dwarf stars, and it also serves to highlight that the search for exoplanets has now entered the realm of potentially habitable cousins of the Earth. The TRAPPIST survey is a prototype for a more ambitious project called SPECULOOS that will be installed at ESO's Paranal Observatory.

This research was presented in a paper entitled "Temperate Earth-sized planets transiting a nearby ultracool dwarf star", by M. Gillon et al., to appear in the journal Nature.