Two rocky planets in a solar system 40 light years away could have Earth-like atmospheres, raising the exciting possibility they might be suitable for life. The planets in the Trappist-1 system had both previously been found to have potentially habitable regions, but whether their atmospheres would also be supportive of life was not known.

Now, scientists have said their atmospheres are likely to be similar to those of Earth, Venus or Mars, rather than being large and diffuse, as is the case with Jupiter. The findings are published in the journal Nature.

The planets, Trappist-1b and c, both sit in the 'habitable zone' of their star, meaning that liquid water could be present. When their discovery was first announced, researchers said the planets represent the best chance of finding alien life to date, and that we could discover such bio-signatures within a generation.

Both planets orbit an ultra-cool dwarf star, which is far smaller, dimmer and cooler than our Sun. However, the two planets are far closer, with orbits of 1.5 and 2.4 days. A third planet was found orbiting the star, but it is believed to be too cold to host life.

Shortly after their initial findings were released, the team noticed the two planets were set to pass in front of their star at the same time – a double transit. This provided them with a rare opportunity to examine the composition and atmospheres of both planets at the same time.

By looking at how much light the planets blocked from the star, the team was able to measure the changes in wavelengths. A big change would indicate the planets had light and large atmospheres such as with Jupiter, while little to no change would suggest a denser atmosphere on a rocky planet. Findings showed very little variation.

james webb telescope
The James Webb Telescope is due to launch in 2018 Nasa

First author Julien de Wit, from MIT, said: "Now we can say that these planets are rocky. Now the question is, what kind of atmosphere do they have? 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."

Researchers now hope to examine the Trappist-1 system further using ground-based telescopes. The launch of the James Webb Space Telescope in 2018 will also allow scientists to carry out detailed observations of the system.

"Screening Trappist-1's Earth-sized planets now – to distinguish progressively between their plausible atmospheric regimes, and to determine their amenability for detailed atmospheric studies – will allow the optimisation of follow-up studies with the next generation of observatories," they wrote.

De Wit added: "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. And that's very exciting."