The Webb Telescope confirmed that an exoplanet in a nearby star system does not have a significant atmosphere
Today, many exoplanets that fall under the general name “Earth-like” have been discovered. They are rocky, and many of them orbit at such distances from their stars that they can potentially have moderate temperatures. But in many cases, we don’t even know if they have an atmosphere. The category similar to Earth can include both dry, hellish planets like Venus with its massive atmosphere, and dry, frozen tundras with a rarefied atmosphere like Mars.
But we can gradually get images of the atmospheres of rocky exoplanets. Researchers have released the results of pointing the Webb Space Telescope at a rocky planet orbiting a star, showing that it has a very thin atmosphere and mainly radiates heat, reports ArsTechnica.
TRAPPIST-1 is a small reddish star, in astronomical terminology it is an “ultra-cold dwarf”, located at a distance of about 40 light years from Earth. Although the star itself is rather inconspicuous, it is notable for having many planets (seven have been discovered to date). All of them are small rocky bodies, very similar to those that occupy the inner part of our solar system. Although the star emits very little light, the planets are closer to it than Mercury is to the Sun.
This leaves some of them in the so-called habitable zone, in which heat from the star can allow liquid water to exist on the planet’s surface. But this again depends on the properties of the planet’s atmosphere, if it exists. And there are reasons to believe that planets located so close to a dwarf star may be devoid of atmosphere. During the first billion years of the dwarf star’s existence, it is prone to powerful flares that can destroy any atmosphere not protected by strong magnetic fields.
So TRAPPIST-1 provides a fantastic opportunity to test some of our ideas about exoplanet atmospheres. The Hubble and Spitzer space telescopes have captured starlight passing close to some planets as they pass between Earth and TRAPPIST-1. These observations gave no indication of the presence of an atmosphere.
But there is a lot of uncertainty in these measurements. And the Webb telescope offers a new opportunity to look at some of the TRAPPIST planets in a new way.
Webb is so sensitive that it enabled a completely different type of observation. Most attempts to image the atmospheres of exoplanets rely on light from the host star passing through whatever atmosphere is there.
This new work is about a planet passing behind the host star — in other words, about the star eclipsing the planet. Shortly before and after this happens, the telescope receives light both from the star itself and from light emitted or reflected by the planet.
Initial work focused on the most distant planet, TRAPPIST-1b, where the star would be about 1,000 times brighter than any light we would see from the planet. Fortunately, it completes its orbit every day and a half, giving plenty of opportunities to photograph its secondary eclipse at a time when other planets should not be obscuring anything.
The images obtained by Webb showed no sign of TRAPPIST-1b’s atmosphere. But there is still some uncertainty in the data, so the researchers speculate that the atmosphere may be thick enough to create a pressure of 0.1 bar. It is about one-tenth the thickness of the Earth’s atmosphere and much denser than the Martian atmosphere. So while the planet’s surface isn’t very hospitable, it still has the potential to reveal something more substantial than, say, the two planets we have in our solar system.