A star 40 light years away that hosts three Earth-sized worlds was discovered earlier this year.

Now, evidence is building that two of these planets could be the perfect place for alien life.

By looking at the system through the Hubble Space Telescope, astronomers have deduced that the innermost planets are rocky, like our own, and are surrounded by compact atmospheres.

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Artist's view of planets transiting a red dwarf star in the TRAPPIST-1 system. By looking at the system through the Hubble space telescope, astronomers have deduced the innermost planets are rocky, like our own, and are surrounded by compact atmospheres

THE TRAPPIST SURVEY The Trappist (Transiting planets and planetesimals small telescope) project goes against the flow of other exoplanet surveys, which sample large numbers of bright sun-like stars to search for potential ET homes. The brightness of those stars means that any planets could be lost in the glare. Cold dwarf stars are too small and cool to be seen in optical telescopes and 'shine' in the infrared band. The astronomers at Liege argued this means these tiny red stars would not drown out a planetary signal, and so should be prime targets in the search for habitable worlds. Advertisement

After discovering the planetary system, scientists at the Massachusetts Institute of Technology (MIT) judged from the size and temperature of the three planets that they may be suitable for life.

The international team of astronomers that found the planets said they are orbiting an ultracool brown dwarf star about one-eighth the size of our sun called 2MASS J23062928-0502285.

Also known as Trappist-1, this dim and cool star fades slightly at regular intervals, indicating several objects are passing between the star and the Earth.

To study this further, the team used Nasa's Hubble telescope to record the planets as one crossed in front of the other, then in front of the star.

The two innermost planets orbit in 1.5 and 2.4 days, while the third is less certain and might be anything between four and 73 days.

Trappist-1 is much cooler and redder than the sun and barely larger than Jupiter. Such stars are common in the Milky Way but this is the first time planets have been found around one. The trio of exoplanets is up to 100 times closer to their star than the Earth is to the sun (illustrated) and all three could be habitable

WATCHING THE DOUBLE TRANSIT On 4th May, the team commandeered Nasa's Hubble Space Telescope and pointed it at the system's star, to catch a rare event: the moment when two planets almost simultaneously pass in front of their star. The researchers realized the planets would transit just two weeks before the event. 'Now for the first time we have spectroscopic observations of a double transit, which allows us to get insight on the atmosphere of both planets at the same time.' Using Hubble, the team recorded a combined transmission spectrum of the two planets, meaning that as first one planet then the other crossed in front of the star, they were able to measure the changes in wavelength as the amount of starlight dipped with each transit. 'The data turned out to be pristine, absolutely perfect, and the observations were the best that we could have expected,' Dr de Wit said. 'The force was certainly with us.' Advertisement

The star, called Trappist-1, is around 40 light-years away. By comparison Mars is, on average and based on its orbit, 12.5 light-minutes away.

While other exoplanet searches have concentrated on bright stars, Michaël Gillon of the University of Liège, Belgium and his team set up the Trappist survey specifically to scan the 60 nearest dwarf stars.

'Systems around these tiny stars are the only places where we could detect life on Earth-sized exoplanets with our current technology' he said.

'So this is where we should start to look.'

The team looked at how the starlight dipped as the planet passed in front of it, in a variety of wavelengths.

If the dips had varied significantly as wavelength varied, this would have demonstrated the planets have light, large and, puffy atmospheres like the gas giant Jupiter.

But instead the dips remained constant, which showed both planets have more compact atmospheres, similar to those of rocky planets such as Earth, Venus, and Mars.

'Now we can say that these planets are rocky,' Dr Julien de Wit, lead author of the paper, said.

'Now the question is, what kind of atmosphere do they have?'.

The international team of astronomers that found the planets said they are orbiting an ultracool brown dwarf star about one-eighth the size of our sun called 2MASS J23062928-0502285, also known as Trappist-1. This artist's impression shows how the star looks from the surface of one of the newly-found planets

THE SEARCH FOR LIFE Astronomers have discovered three planets orbiting a small cold brown dwarf star. The international team believes at least one of these 'three bears' planets may be 'just right' for life to get going. The two innermost planets orbit in 1.5 and 2.4 days, while the third is less certain and might be anything between four and 73 days. As the planets are tidally locked they have one half in light and half in darkness. The team believes that the furthest out of the three, the 'Daddy Bear' planet, is the most likely to be habitable out of the three. Advertisement

'The plausible scenarios include something like Venus, where the atmosphere is dominated by carbon dioxide, or an Earth-like atmosphere with heavy clouds, or even something like Mars with a depleted atmosphere.

'The next step is to try to disentangle all these possible scenarios that exist for these terrestrial planets.'

To study the system further, the group is hoping to point more telescopes on the ground in its direction, along with observations from more space telescopes.

'With more observations using Hubble, and further down the road with James Webb, we can know not only what kind of atmosphere planets like TRAPPIST-1 have, but also what is within these atmospheres,' Dr de Wit said.

'And that's very exciting.'