Back in May, astronomers working with the Transiting Planets and Planetesimals Small Telescope — TRAPPIST — stumbled upon three Earth-sized exoplanets orbiting an ultra-cool dwarf star. From just the preliminary data, scientists knew the potential for these planets to exhibit habitable qualities was rivaled by only a few other exoplanet discoveries. And to boot, the star system was just a mere 40 light-years away — barely a stone’s throw away in the scale of the universe.

Now, the same research group has bolstered hopes of finding E.T. in the region by determining the two innermost planets in the system possess rocky surfaces as well as atmospheres that are compact (astronomer-speak for dense clouds laying close to the surface) like Earth, Venus, and Mars, in a study published Wednesday in Nature.

Just days after the initial discovery of the star system and its triplets was announced in May, the research team, led by MIT astronomer Julien de Wit, used NASA’s Hubble Space Telescope to study a double transit — where the two planets’ orbits cross in front of the sun relative to the observer.

An artist’s depiction of planets transiting a red dwarf star in the TRAPPIST-1 System

“We obtained transmission spectra of the planets’ atmospheres between 1.1 and 1.7 microns with the instrument [Wide Field Camera 3] onboard of Hubble,” de Wit tells Inverse. The data basically suggested that the planets did not possess clear helium and hydrogen atmosphere. Hydrogen-dominated atmospheres are pretty big signatures of gaseous planets like Jupiter and Saturn.

“Therefore,” says de Wit, “we can fully reject the scenario of large and puffy hydrogen-dominated atmospheres for these planets. They are ‘terrestrial’, meaning like Earth, Venus, Mercury, and Mars.”

Of course, there are still many kinds of questions to resolve. For one, we don’t yet know what kind of geology the planets possess, nor do we know what kinds of gaseous elements and compounds do permeate throughout the pair’s atmospheres. De Wit, however, thinks there are a lot of possible scenarios.

For example, he says one possibility could be that these planets had water-rich atmospheres which got progressively depleted in hydrogen due to the irradiation of TRAPPIST-1, which is the host star of the planetary system. “If that is the case, it would be very interesting to check how much water still remains in the atmosphere, and where.”

“But once again,” he says, “until recently we had not detected planets around such stars, so we have no idea what these are like … its pure exploration!”

Follow-up observations are sure to follow, and will hopefully answer many of those questions. One thing worth emphasizing here is how strongly these findings are helping to validate the TRAPPIST project. The telescope, just 60 cm and located in Chile, is really a prototype that wasn’t supposed to actually find anything special. It was built as a proof-of-concept for demonstrating how cost-efficient technology could study stars that give off faster, cooler glows (like ultra-cool dwarf stars, and red stars).

“These are very faint so you can only monitor those that are not too far from our system,” says de Wit. TRAPPIST-like instruments would be an inexpensive way to identify candidate planets or star systems that might be habitable. “About a thousand of such stars would be excellent hosts of planets in the context of follow-up atmospheric studies using more powerful instruments, like the upcoming James Webb Space Telescope.”

Already, TRAPPIST has gone above and beyond its initial goals, and may have inadvertently found the first worlds outside of Earth that are home to life in the universe.