If you were going to search for life around planets outside our Solar System, you'd look for it around rocky worlds like ours that orbit the most common type of star in the cosmos.
It just so happens there is a solar system like this just 40 light-years away.
The TRAPPIST-1 system has seven rocky worlds orbiting a much smaller and cooler star than our Sun.
Finding atmospheres around Earth-like planets is one of the primary goals of the James Webb Space Telescope (JWST).
The first results from JWST, published today in the journal Nature, indicate the system's inner-most planet is a scorching ball of rock with no detectable atmosphere.
Thomas Greene of NASA's AMES Research Center and colleagues studied infrared irradiation emanating from TRAPPIST-1b
"We found [the planet] to be very hot, in fact, so hot that it's only consistent with no atmosphere or a very, very thin atmosphere," Dr Greene said.
He said the finding suggested the TRAPPIST-1 system may not be a benign environment for habitability.
"It's a little sobering that we don't see a healthy atmosphere there, and makes you wonder about the habitability of these small planets around M dwarf stars," he said.
"We will find out more when we observe TRAPPIST-1 c and maybe other planets in the system."
Looking for atmospheres on alien planets
The seven planets around the TRAPPIST-1 system are named from b to h based on their distance to their Sun.
TRAPPIST-1 b, which is slightly larger then Earth, takes just 1.5 days to orbit its sun, and travels so close to its star that it is locked so one side is in permanent daytime.
In terms of radiation, it is analogous to a rocky world sitting between Mercury and Venus, and receives four times the amount of heat from its sun than Earth.
Although TRAPPIST-1 b isn't in its star's habitability zone, there have been ongoing debates about whether or not it has an atmosphere.
"We know that Venus isn't in the habitable zone in our Solar System, but has a very dense atmosphere [of carbon dioxide]," Dr Greene said.
"So modellers were betting even money whether this one would have an atmosphere or not."
While planets in the TRAPPIST-1 system have been observed by the Hubble and Spitzer Space Telescopes using transmission spectroscopy — where planets are analysed as they pass in front of their star — no atmospheres have been detected on any of them.
Instead, the team used JWST's Mid Infrared Instrument to measure thermal emissions coming directly from the planet as it passed behind its star on five occasions.
"These are the first detections of actual radiation coming from any planet roughly the size of rocky planets," Dr Greene said.
Their measurements revealed that daytime temperatures on the planet reached up to 257 degrees Celsius, suggesting heat is not redistributed around the planet by an atmosphere.
Nataliea Lowson of the University of Southern Queensland said using the dayside temperature to estimate what the atmosphere might look like was a great first step.
"Their observations match the model scenario of a rocky-like planet with no atmosphere, which helps astronomers decide if we want to pursue further and obtain direct atmosphere detections through spectral observations," Ms Lowson said.
However, she said, without using the spectroscopy transmission technique to detect chemicals in the light surrounding the planet as it passes in front of its star, we can't know for certain if an atmosphere is completely absent.
Small star, big punch
M dwarf stars — such as the star at the centre of the TRAPPIST-1 system — are smaller and cooler than our Sun, and their planets tend to have closer orbits.
But they still pack a punch, explained Benjamin Pope of the University at Queensland, who is looking for exoplanets using the JWST but was not involved in this study.
"Because the star is so faint, you have to be to be that close in order to have a comfortable temperature, but the ultraviolet output and solar flares tend to be either as high or higher than the Sun," Dr Pope said.
"So these can be very difficult stars to live around."
If TRAPPIST-1b turned out to have an atmosphere, that would have meant other planets in its solar system would definitely have atmospheres.
"If they'd found something, that would have been really quite shocking," Dr Pope said.
But, he said, these results are a taste of what's to come.
"I think the subsequent TRAPPIST-1 results are going to be much more exciting, because they could resolve much deeper controversies relevant to life in the universe."
Ms Lowson agreed.
"It will be interesting to see what future JWST observations will uncover about the other planets in the TRAPPIST-1 system, particularly the outmost planet TRAPPIST-1h, and the habitable zone planets TRAPPIST-1e, TRAPPIST-1f, and TRAPPIST-1g," she said.
Where to next in the hunt for life?
Next cab off the rank is TRAPPIST-1 c, which gets roughly the equivalent amount of radiation as Venus.
But Dr Greene said the most Earth-like is likely to be TRAPPIST-d.
TRAPPIST-1d gets around the same amount of heat as Earth, while TRAPPIST-1e is nudging out towards Mars, and TRAPPIST-1f is just beyond Mars in the habitable zone.
But, he said, it would be harder to use the emissions technique to look for hints of atmospheres around planets that are further from their sun.
"The planets get cooler as they go out, so it's just going to be harder to measure their heat," he said.
"I think there's a good shot of getting a good answer on c, but d might take a long time."
