At this point, we've discovered lots of exoplanets that fall under the general label "Earth-like." They're rocky, and many orbit at distances from their host stars to potentially have moderate temperatures. But "like" is doing a lot of work there. In many cases, we have no idea whether they even have an atmosphere, and the greenhouse effect means that the atmosphere can have a huge impact on the planet's temperature. So the Earth-like category can include dry, baking hellscapes like Venus with its massive atmosphere, as well as dry, frozen tundras with sparse atmospheres like Mars.
But we're slowly getting the chance to image the atmospheres of rocky exoplanets. And today, researchers are releasing the results of turning the Webb Space Telescope on a rocky planet orbiting a nearby star, showing that the new hardware is so sensitive that it can detect the star blocking out light originating from the planet. The results suggest that the planet has very little atmosphere and is mostly radiating away heat from being baked by its nearby star.
The ultra-cool dwarf and its seven planets
TRAPPIST-1 is a small, reddish star—in astronomical terminology, it's an "ultra-cool dwarf"—that's about 40 light-years from Earth. While the star itself is pretty nondescript, it's notable for having lots of planets, with seven in total having been identified so far. All of these are small, rocky bodies, much like the ones that occupy the inner portion of our Solar System. While the star emits very little light, the planets are all packed in closer to it than Mercury is to the Sun.
That leaves a number of them in what's called the habitable zone, the area at which the heat delivered by the star could allow liquid water to exist on the planet's surface. But that again depends on the properties of the planet's atmosphere, should one exist. And there are reasons to think planets so close to a dwarf star might lack atmospheres. For the first billion years or so of a dwarf star's existence, it's prone to violent outbursts that could cook off any atmospheres that are not protected by strong magnetic fields.
There's still a chance that geological processes could create a secondary atmosphere after the star settles down. But these atmospheres are likely to be rich in oxygen or carbon dioxide, with little in the way of hydrogen-containing molecules.
So, TRAPPIST-1 provides a fantastic opportunity—really, seven opportunities—to test some of our ideas about exoplanet atmospheres. And both the Hubble and Spitzer space telescopes have imaged some starlight that passes close to some of the planets as they pass between Earth and TRAPPIST-1. These observations didn't provide any indications of an atmosphere, setting limits on how thick any gases above these planets could be.
But there's a lot of uncertainty in those measurements. And the Webb Telescope, with its huge mirror and advanced imaging hardware, offers a new opportunity to take a second look at some of the TRAPPIST planets.
One-sided
The Webb is so sensitive that it enabled an entirely different sort of observation. Most attempts at imaging exoplanet atmospheres rely on light from the host star that grazes the planets, and thus would pass through any atmosphere that's there. This relies on the planet passing in front of the host star.
This new work relies on the planet passing behind the host star—having the star eclipse the planet, in other words. Shortly before and after that happens, the telescope will receive light from both the star itself and any light that's emitted or reflected by the planet. This sort of "secondary eclipse" is difficult to detect, given that the star is so much brighter. In addition, the Webb's detectors are sensitive to wavelengths that would allow it to detect any carbon dioxide.
This initial work focused on the innermost planet, TRAPPIST-1b, where the star would be roughly 1,000 times brighter than any light we should see from the planet. Fortunately, it completes an orbit every day and a half, allowing plenty of opportunities to image its secondary eclipse at a time when no other planets should be eclipsing anything.
The work used the drop in light caused by the secondary eclipse to infer what portion of the light outside the eclipse was coming from the planet. This light would be a mix of reflected light and heat given off the planet, which is baked by roughly four times the radiation that Earth receives from the Sun. But, by imaging in the infrared, most of what's being detected is primarily the heat radiated off the planet. By assuming this is approximately a black body radiation, it's possible to estimate the temperature of the planet needed to produce that radiation.
This produced a result of about 500 K, or 230° C, which tells us there's probably no atmosphere.
TRAPPIST-1b is close enough to its host star to be tidally locked, with one side perpetually facing the star, absorbing light and radiating away heat. And, based on the geometry of things, that's the side we're going to be imaging shortly before and after the secondary eclipse.
If there's an atmosphere, it will absorb some of the heat radiated by the side facing the star and carry it away to the other side of the planet. That will lower the apparent temperature of the side facing the star. But the apparent temperature of that side, based on this imaging, is roughly what you'd expect if it were releasing all the energy it received right back out into space.
Not nothing
There was no indication of any carbon dioxide, either, which should have shown up at the wavelengths imaged. So, as far as this data goes, there's no indication of an atmosphere at all. But there's still some uncertainty in the data, so the researchers estimate that an atmosphere thick enough to produce 0.1 bar of pressure could be present. That's roughly a tenth the thickness of Earth's atmosphere and far more dense than Mars'. So, while it wouldn't be very hospitable, there's still potential for something more substantial than two of the planets we have in our Solar System.
Still, it's in keeping with the idea that the violence in the early history of dwarf stars is capable of stripping away the initial atmosphere that formed with the planet. But it's far too soon to consider that the rule. After all, there are six other planets to examine in the TRAPPIST-1 system alone, and we'd want to look at additional stars before drawing too many conclusions.
Nature, 2023. DOI: 10.1038/s41586-023-05951-7 (About DOIs).
