Artist's impression of TRAPPIST-1b. (NASA, ESA, CSA, J. Olmsted/STScI, T. P. Greene/NASA Ames, T. Bell/BAERI, E. Ducrot & P. Lagage/CEA)
An Earth-sized exoplanet in perhaps the most promising alien system for signs of life is not likely to be habitable to life as we know it.
New observations from the James Webb Space Telescope reveal that the innermost world of the TRAPPIST-1 system, an exoplanet named TRAPPIST-1b with 1.4 times the mass and 1.1 times the radius of Earth, reaches a sizzling 230 degrees Celsius (446 degrees Fahrenheit), and is unlikely to have an atmosphere wrapped around its rocky body.
This is not a surprise; TRAPPIST-1b is so close to its host star that it whips around in just 1.5 days, receiving 4 times the stellar irradiation Earth does; but this is the first time that we've been able to make these measurements for a world so small and cool – the closest, in fact, to our own planet yet.
And, of course, the information will help scientists learn more about the other six exoplanets known to be orbiting red dwarf star TRAPPIST-1, a system whose potential habitability is unknown.
"It's easier to characterize terrestrial planets around smaller, cooler stars," explains astronomer Elsa Ducrot of the French Alternative Energies and Atomic Energy Commission (CEA) in France.
"If we want to understand habitability around M stars, the TRAPPIST-1 system is a great laboratory. These are the best targets we have for looking at the atmospheres of rocky planets."
The discovery of the TRAPPIST-1 system was reported in 2017, seven rocky exoplanets orbiting a red M-dwarf star 40 light-years away. Three of those exoplanets are within the star's so-called habitable zone – at a distance from the star that's not so close that all liquid water would boil off, nor so far that it would freeze.
It's a truly tantalizing target in the search for life outside the Solar System, but TRAPPIST-1 has some major differences from the Solar System that raise questions about whether or not life could emerge there.
The worlds around it are much closer to the star than our home system, with the most distant on an orbit of just 18.8 days. Since red dwarf stars are much smaller, dimmer, and cooler than the Sun, this means that the habitable zone sits much closer to the star… but red dwarf stars are much more violent than the Sun, lashing the space around them with powerful flares.
"There are ten times as many of these stars in the Milky Way as there are stars like the Sun, and they are twice as likely to have rocky planets as stars like the Sun," says astrophysicist Thomas Greene of NASA's Ames Research Center.
"But they are also very active – they are very bright when they're young and they give off flares and X-rays that can wipe out an atmosphere."
TRAPPIST-1b is the first step towards understanding how this activity might have affected the system. Previous studies using instruments like Hubble and Spitzer had ruled out a tenuous, puffy atmosphere around the exoplanet, but the possibility that TRAPPIST-1b still harbored a thick, dense atmosphere remained.
This is where JWST's infrared capabilities came to the fore. The researchers harnessed the space telescope's unique power to try to measure the temperature of TRAPPIST-1b – the infrared light emitted by the thermal radiation emanating from the exoplanet.
The key was in the light curve produced as the exoplanet orbits the star. When an orbiting exoplanet passes between us and the star, the exoplanet blocks some of the star's light, causing it to dim a little.
But when the exoplanet passes behind the star – an event known as a secondary eclipse – dimming can also be observed.
This is because, when the exoplanet is on either side of the star, it reflects some of the star's light, in addition to emitting any radiation of its own, increasing the overall light observable from the system. That means any light observed during the secondary eclipse is emitted by the star alone.
By extracting the additional light that can be detected when the exoplanet is off to either side, as well as the estimated reflected starlight, scientists can determine how much infrared radiation is emitted by the exoplanet itself, thus taking its temperature. And this, in turn, can reveal the presence or absence of an atmosphere.
"This planet is tidally locked, with one side facing the star at all times and the other in permanent darkness," says CEA astronomer Pierre-Olivier Lagage of CEA. "If it has an atmosphere to circulate and redistribute the heat, the dayside will be cooler than if there is no atmosphere."
The researchers were able to catch five secondary eclipses for TRAPPIST-1b, and extract from those events a dayside temperature of around 230 degrees Celsius. This temperature, although cooler than the day side of Mercury in the Solar System, does not fit with the presence of an atmosphere.
"We compared the results to computer models showing what the temperature should be in different scenarios," Ducrot says.
"The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didn't see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements."
Future work, the researchers say, could further characterize the global heat distribution of TRAPPIST-1b, in order to better understand rocky planets orbiting red dwarf stars, and how these systems differ from our own.
The research has been published in Nature.
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