Researchers studying Trappist-1b using the James Webb Space Telescope found that it may have a volatile surface and may have an atmosphere, although it was previously thought to be a dark, rocky object without an atmosphere. Initial observations suggested it had no atmosphere, but further analysis suggested it might have a thick carbon dioxide atmosphere, subject to a hydrocarbon haze similar to that of Saturn's moon Titan. More extensive studies are planned to confirm these findings.
Trappist-1b is one of seven rocky planets orbiting the star Trappist-1, 40 light-years from Earth. This planetary system is notable because it provides astronomers with a rare opportunity to study multiple Earth-like planets up close. Three of the planets are in the "habitable zone," where liquid water could exist on their surfaces. So far, ten research projects have observed the system using the James Webb Space Telescope (JWST) for a total of 290 hours.
The latest study involved researchers from the Max Planck Institute for Astronomy (MPIA) in Heidelberg and was led by Elsa Ducrot from the Commission for Atomic Energy (CEA) in Paris, France. The team used JWST's Mid-Infrared Imager (MIRI) to measure Trappist-1b's thermal infrared radiation (essentially, its thermal signature). The findings, now published in the journal Nature Astronomy, build on last year's research that described Trappist-1b as a dark, rocky planet with no detectable atmosphere.
"However, the idea that a rocky planet has a heavily weathered surface and no atmosphere is inconsistent with current measurements," said MPIA astronomer Jeroen Bouwman, who co-led the observing project. "So we think the planet is covered by relatively unchanging material." Typically, the surface is weathered by radiation from the central star and meteorite impacts. However, the results show that the surface rocks are at most about 1,000 years old, much younger than the age of the planet itself, which is estimated to date back several billion years.
This could indicate that the planet's crust is undergoing drastic changes, which could be explained by extreme volcanic activity or plate tectonics. Even though this scenario remains hypothetical for now, it is still plausible. The planet is large enough that its interior may retain residual heat from its formation - just like Earth. Tidal effects from the central star and other planets may also deform Trappist-1b, creating internal friction that generates heat — similar to what we see in Jupiter's moon Io. In addition, heating induced by the magnetic fields of nearby stars is also conceivable.
"Data also allows for completely different solutions," said MPIA Director Emeritus Thomas Henning. He is one of the principal designers of the MIRI instrument. "Contrary to previous thinking, in some cases the planet may have a thick atmosphere rich in carbon dioxide (CO2)," he added. In this case, hydrocarbon-generated haze, the smoke in the upper atmosphere, plays a key role.
The two observing projects complement each other in the current study, which aims to measure the brightness of Trappist-1b at different wavelengths in the thermal infrared range (12.8 and 15 microns). The first observations were sensitive to the absorption of planetary infrared radiation by a layer of carbon dioxide. However, no dimming was measured, leading the researchers to conclude that the planet has no atmosphere.
The team conducted model calculations that showed haze could reverse the temperature stratification of the carbon dioxide-rich atmosphere. Typically, lower ground layers are warmer than upper layers due to higher air pressure. When haze absorbs starlight and warms it, it heats the upper atmosphere through the greenhouse effect. As a result, the carbon dioxide there itself emits infrared radiation.
We see something similar on Saturn's moon Titan. Its haze layer is likely formed under the influence of the sun's ultraviolet (UV) radiation, which comes from carbon-rich gases in the atmosphere. A similar process may be occurring on Trapster-1b, because its star emits large amounts of ultraviolet radiation.
Even if the data fit this hypothesis, astronomers still think it's less likely. For one thing, generating the smog-forming hydrocarbons from a carbon dioxide-rich atmosphere is more difficult, but not impossible. However, Titan's atmosphere is primarily composed of methane. On the other hand, the problem remains that the radiation and winds produced by active red dwarfs, including Trappist-1, could easily erode the atmospheres of nearby planets over billions of years.
Trappist-1b is a vivid example of how difficult it is currently to detect and determine the atmospheres of rocky planets—even for JWST. Compared to gas planets, they are thin and produce only weakly measurable features. Two studies of Trappist-1b provided brightness values at two wavelengths, lasting nearly 48 hours, which were not enough to determine without a doubt whether the planet has an atmosphere.
The observations took advantage of the slight tilt of the planet's plane relative to our line of sight to Trappist-1. This orientation causes seven planets to pass in front of the star and dim slightly during each orbit. So this can be used to understand the properties and atmosphere of a planet in a number of ways.
So-called transit spectroscopy has proven to be a reliable method. This involves measuring how well a star is dimmed by its planets in terms of wavelength. In addition to occultation by opaque planetary bodies, which astronomers use to determine a planet's size, atmospheric gases also absorb certain wavelengths of starlight. From this, they can infer whether the planet has an atmosphere and what its atmosphere is made of. Unfortunately, this approach has drawbacks, especially for planetary systems like Trappist-1. Cool red dwarfs often exhibit large star spots and powerful eruptions that seriously affect measurements.
Astronomers have largely circumvented this problem by looking at the side of an exoplanet as its star heats up in thermal infrared light, as in the current study of Trappist-1b. The bright dayside is especially easy to see before and after the planet disappears behind its star. The infrared radiation emitted by a planet contains information about its surface and atmosphere. However, this observation is more time-consuming than transit spectroscopy.
Given the potential of these so-called secondary eclipse measurements, NASA recently approved an extensive observing program to study the atmospheres of rocky planets around nearby low-mass stars. This extraordinary project, "Rocky World," includes 500 hours of observations using JWST.
The team hopes to get clear confirmation using another observation method. It records the complete orbit of a planet around a star, including all phases of illumination from the dark night side passing in front of the star to the bright day side shortly before and after being obscured by the star. This method will allow the team to create a so-called phase curve, which indicates the change in brightness of a planet along its orbit. As a result, astronomers can infer the temperature distribution on the planet's surface.
The team has made this measurement using Trappist-1b. By analyzing the distribution of heat on the planet, they can infer the presence of an atmosphere. This is because the atmosphere helps transfer heat from the day side to the night side. If the temperature changes suddenly as the two sides transition, it indicates the absence of an atmosphere.
Compiled from /scitechdaily