Three exoplanets, discovered 40 light years away from Earth, could be the most likely yet to hold life European Southern Observatory/Flickr

Three potentially habitable planets have been discovered just 40 light years from Earth. Scientists say the planets are currently our best chance of finding extraterrestrial life, and that we could do this within a generation.

Previous research shows there could be as many as 40 billion Earth-like planets in the galaxy, all orbiting in the habitable zones of their stars. The three newly-discovered planets orbit an ultra-cool dwarf star – a star that is much cooler and dimmer than our Sun, and only a bit bigger than Jupiter. Because of its size, the star – Trappist-1 – cannot be seen by most telescopes.

Using the Trappist telescope in Chile, an international team of astronomers observed the three planets over 62 nights last year. They noticed changes to the star's brightness, recording three occasional dips. These dips represent the three planets, blocking the light from the star.

Further analysis, published in Nature, showed the planets to be a similar size to Earth although they are much closer to their star than we are to the Sun. Two of the planets take 1.5 days and 2.4 days to orbit their star, respectively, while the third planet takes between 4.5 and 73 days.

The planets proximity to their star, and the star's low temperature and small size means that radiation isn't too excessive for life to exist. After estimating the habitable zone of the solar system, the scientists said two of the planets sit just outside the inner edge.

While not inside the zone, the researchers say the planets could still hold liquid water if the conditions were right. They also said these two planets are likely to be tidally locked. That means one half of the planet will always face their star, while the other half is constantly dark. The scientists say this could restrict circulation of any potential liquid water, or atmosphere.

The scientists are not sure whether the third planet lies in the habitable zone, because its orbit is still not understood. They say it probably receives less radiation than Earth, but may still receive enough to be in the habitable zone.

Because the planets are relatively nearby, the researchers say they are currently our best bet at finding alien life. Emmanuël Jehin, a co-author of the study, said: "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, but now we have not just one lonely planet around such a faint red star but a complete system of three planets."

Julien de Wit, another co-author, added: "Now we have to investigate if they're habitable. We will investigate what kind of atmosphere they have, and then will search for biomarkers and signs of life. We have facilities all over the globe and in space that are helping us, working from UV to radio, in all different wavelengths to tell us everything we want to know about this system."

Researchers said the launch of the James Webb Telescope in 2018 will allow them to investigate the atmospheric compositions of all three planets. This will allow them to look for water, then traces of biological activity. Adam Burgasser, a researcher working on the study, said: "This facility will allow us to search for biogenic gases – oxygen or methane for example – that would firmly indicate the presence of life, or search for other gas species that would tell us about the planets' compositions, geothermal activity and evolutionary history."

De Wit added: "These planets are so close, and their star so small, we can study their atmosphere and composition, and further down the road, which is within our generation, assess if they are actually inhabited. All of these things are achievable, and within reach now. This is a jackpot for the field."