June 19, 2023
NASA's James Webb Space Telescope has delivered groundbreaking findings, ruling out the presence of a thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c. An international team of researchers has harnessed the remarkable capabilities of Webb to calculate the amount of heat energy emitted by the planet, suggesting that any existing atmosphere is remarkably thin, if present at all.
TRAPPIST-1 c, the second of the seven known planets orbiting the TRAPPIST-1 system, is a slightly larger rocky exoplanet with a composition similar to Earth. It completes one orbit around its star in just 2.42 Earth-days, at a distance of approximately 1.5 million miles (0.016 AU). Webb's measurement of 15-micron mid-infrared light emitted by TRAPPIST-1 c indicates that the planet likely possesses either a bare rocky surface or an extremely thin carbon dioxide atmosphere.
With a dayside temperature of approximately 225 degrees Fahrenheit (380 kelvins), TRAPPIST-1 c has now claimed the title of the coolest rocky exoplanet characterized based on thermal emission. This breakthrough demonstrates the precision and utility of Webb in characterizing rocky exoplanets that resemble those in our solar system, both in size and temperature.
The recent findings mark a significant advancement in determining whether planets orbiting red dwarf stars, such as TRAPPIST-1, can sustain atmospheres capable of supporting life as we know it. Sebastian Zieba, a graduate student at Germany's Max Planck Institute for Astronomy and the first author of the published results in Nature, stated, "We want to know if rocky planets have atmospheres or not. With Webb, we can finally start to search for atmospheres dominated by oxygen, nitrogen, and carbon dioxide."
Co-author Laura Kreidberg, also from Max Planck, noted the significance of TRAPPIST-1 c, saying, "TRAPPIST-1 c is interesting because it's basically a Venus twin: It's about the same size as Venus and receives a similar amount of radiation from its host star as Venus gets from the Sun. We thought it could have a thick carbon dioxide atmosphere like Venus."
TRAPPIST-1 c is part of the TRAPPIST-1 system, consisting of seven rocky planets orbiting an ultracool red dwarf star located 40 light-years away from Earth. While these planets share similarities in size and mass with the inner rocky planets of our solar system, it remains uncertain whether they possess similar atmospheres. The powerful X-ray and ultraviolet radiation emitted by M dwarfs during their first billion years can easily strip away a young planetary atmosphere. Additionally, the availability of water, carbon dioxide, and other volatiles during the planet's formation may influence the presence of substantial atmospheres.
To address these questions, the research team leveraged Webb's Mid-Infrared Instrument (MIRI) to observe the TRAPPIST-1 system during four separate instances when the planet moved behind its star, causing a secondary eclipse. By comparing the brightness during these events, the team calculated the amount of mid-infrared light emitted by the dayside of TRAPPIST-1 c, with wavelengths of 15 microns.
The emitted mid-infrared light from a planet is directly related to its temperature, which is influenced by its atmosphere. Carbon dioxide gas selectively absorbs 15-micron light, causing the planet to appear dimmer at that wavelength. However, the presence of clouds can reflect light, making the planet appear brighter and concealing the presence of carbon dioxide.
Furthermore, a substantial atmosphere redistributes heat from the dayside to the nightside, resulting in lower temperatures on the dayside. Given TRAPPIST-1 c's close proximity to its star, approximately 1/50th the distance between Venus and the Sun, the planet is believed to be tidally locked, with one side experiencing perpetual daylight and the other in eternal darkness.
While these initial measurements do not provide definitive information regarding TRAPPIST-1 c's nature, they contribute to narrowing down the possibilities. Zieba explained, "Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds. If the planet had a thick CO2 atmosphere, we would have observed a really shallow secondary eclipse, or none at all. This is because the CO2 would be absorbing all of the 15-micron light, so we wouldn't detect any coming from the planet."
The data also suggests that TRAPPIST-1 c is unlikely to resemble Venus, with a thick carbon dioxide atmosphere and sulfuric acid clouds.
The absence of a dense atmosphere indicates that TRAPPIST-1 c might have formed with relatively limited water. If the other temperate TRAPPIST-1 planets experienced similar conditions during their formation, they too may have begun with insufficient water and other vital components for planetary habitability.
The remarkable sensitivity of Webb's measurements enables scientists to differentiate between various atmospheric scenarios on such a distant and small planet. Webb detected a mere 0.04 percent decrease in brightness during the secondary eclipse, equivalent to observing a display of 10,000 tiny light bulbs and noticing only four have extinguished.
Kreidberg emphasized the significance of this research, stating, "It is extraordinary that we can measure this. There have been questions for decades now about whether rocky planets can keep atmospheres. Webb's ability really brings us into a regime where we can start to compare exoplanet systems to our solar system in a way that we never have before."
These groundbreaking findings were conducted as part of Webb's General Observers (GO) program 2304, one of the eight programs initiated during Webb's first year of scientific operations to provide comprehensive characterization of the TRAPPIST-1 system. In the upcoming year, researchers plan to conduct a follow-up investigation, observing the complete orbits of TRAPPIST-1 b and TRAPPIST-1 c. This endeavor aims to explore the temperature variations between the day and night sides of the two planets, providing further insights into their potential atmospheres.