In early May, a team of international scientists announced the discovery of three Earth-like planets orbiting a dwarf star in the TRAPPIST-1 galaxy — a star system 40 light-years away from our planet.

Named after the Chilean telescope TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope), the star system's three exoplanets offer scientists the best possible chance of detecting signs of alien life outside our own solar system.

Now, two months later, the same group of researchers revealed in a new study that the two innermost planets in the TRAPPIST-1 star system are mostly rocky, unlike typical gas giants such as Jupiter.

Furthermore, both exoplanets' atmospheres are not large and diffuse like Jupiter, but compact, like that on Earth, Mars and Venus. All these findings strengthen the case that these two exoplanets may be habitable.

A Double Transit

The research team performed a preliminary screening of the atmospheres of the exoplanets just days after announcing the discovery.

On May 4, scientists used NASA's Hubble Space Telescope to point at the dwarf star TRAPPIST-1 and caught a rare event known as a double transit. This phenomenon occurs when two planets simultaneously pass in front of their host star.

Thanks to the refined calculations of the orbital configuration of the exoplanets, experts realized that the exoplanets would transit just two weeks before the event.

Julien de Wit, the study's lead author and a postdoc fellow at MIT, says they thought whether the team that had controlled Hubble would allow them to perform their own observation, and so they wrote a proposal in less than a day.

For the first time ever, scientists now have spectroscopic observations of a double transit. De Wit says this will allow researchers to get more insight on both planets' atmosphere at the same time.

De Wit and his colleagues recorded a combined transmission spectrum of exoplanets TRAPPIST-1b and TRAPPIST-1c. As the exoplanets crossed in front of the host star, scientists measured the changes in wavelength as the amount of starlight fell with each transit.

"The data turned out to be pristine, absolutely perfect," says de Wit. "[T]he observations were the best that we could have expected."

Rocky Exoplanets

If the dips in starlight had a significantly varied range, this would indicate that the atmospheres of both exoplanets are large, light and puffy like that of Jupiter, de Wit says.

But researchers discovered that the transiting planets possess more compact atmospheres, which can help scientists ascertain that the exoplanets are rocky.

De Wit says the exoplanets possibly have high, thick clouds and an atmosphere filled with carbon dioxide like that on Venus, or they could have an atmosphere dominated by oxygen and nitrogen, like that on Earth.

The scenario could also be similar to the depleted Martian atmosphere, he says. The team's next step is to examine all these possible scenarios and disentangle them to find the most accurate answer.

Meanwhile, astrophysicist Joanna Barstow, who was not involved in the new study, says a rocky terrain is a good beginning for a habitable planet, but any potential life forms on TRAPPIST-1 star system may likely have a harder time than life on Earth.

Because the planets' orbits are near their host star, the radiation off the star may strip the atmospheres away and make it difficult for organisms to thrive.

The team's new findings are published in the journal Nature.

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