Of all the planets we’ve discovered beyond our solar system, the majority are not Earth-like at all. They are most often gas giants like Jupiter that orbit very close to their stars, making them extremely hot. These are also generally the easiest type of exoplanet to detect. Very few of the thousands of discovered exoplanets could be potentially habitable — fewer than 50 in total. That’s why astronomers were so excited by the finding in 2017 of a system called TRAPPIST-1, which has seven rocky Earth-sized exoplanets, four of which are in the habitable zone.

But TRAPPIST-1 is a long way away, at 40 light-years’ distance, so it isn’t easy to see these planets in detail. Astronomers had some information about the planets’ sizes and orbits, but to really know if life could ever have emerged there, they needed information on the planets’ atmospheres. And the James Webb Space Telescope provided the means to investigate that.

“We want to know if rocky planets have atmospheres or not,” said Sebastian Zieba, lead author of Webb’s research into the TRAPPIST-1 system, in a statement. “In the past, we could only really study planets with thick, hydrogen-rich atmospheres. With Webb, we can finally start to search for atmospheres dominated by oxygen, nitrogen, and carbon dioxide.”

The recent study looked at TRAPPIST-1 c, which was thought to be a Venus-like planet that was tidally locked to its star — so one side, the dayside, would always be facing the star and would have a temperature of about 225 degrees Fahrenheit. “TRAPPIST-1 c is interesting because it’s basically a Venus twin: It’s about the same size as Venus and receives a similar amount of radiation from its host star as Venus gets from the sun,” explained co-author Laura Kreidberg. “We thought it could have a thick carbon dioxide atmosphere like Venus.”

However, when the researchers used Webb to observe TRAPPIST-1 c’s atmosphere, they found it was very different from Venus’s thick, carbon dioxide-rich atmosphere. Instead, the planet has an extremely thin atmosphere — if it has an atmosphere at all.

The results are something of a disappointment for those hoping to find habitability, as it means that the planet probably formed without much water. And it’s possible that the other planets in the system could have formed without water too, making it less likely that they could be habitable.

Even though the results are not what was expected, they are still an exciting demonstration of how technology has developed to allow us to see exoplanet atmospheres. “It is extraordinary that we can measure this,” said Kreidberg. “There have been questions for decades now about whether rocky planets can keep atmospheres. Webb’s ability really brings us into a regime where we can start to compare exoplanet systems to our solar system in a way that we never have before.”

The research is published in the journal Nature.