The James Webb Space Telescope has detected evidence suggesting that TRAPPIST-1 e, a rocky planet located in the habitable zone of its star, might possess an atmosphere. This discovery opens exciting prospects in the search for potentially habitable worlds beyond our Solar System.
The planet TRAPPIST-1 e orbits a red dwarf star, much smaller and cooler than our Sun. These particular stars, with temperatures around 2500 degrees Celsius compared to 5600 for our star, offer unique conditions for exoplanet research. Their habitable zone, that region where water could exist in liquid form, is located much closer to the star than in our own system. A year on these worlds lasts only a few days, which significantly facilitates their observation by astronomers.
Artist's representation of the TRAPPIST-1 system, the most studied stellar system outside our own.
Credit: NASA, ESA, CSA, STScI, Joseph Olmsted (STScI) The detection method used, called the transit method, involves measuring the slight decrease in a star's brightness when a planet passes in front of it. This technique not only allows for planet detection but also for analyzing their potential atmosphere. During this passage, starlight passes through atmospheric layers, and certain gases absorb specific wavelengths, creating an identifiable chemical signature. Red dwarfs, due to their small size, amplify this phenomenon, making atmospheric detection more accessible.
The data analysis represented considerable technical work. The scientific team had to deal with what's called stellar contamination, caused by active regions similar to sunspots on TRAPPIST-1's surface. This parasitic noise required over a year of meticulous processing to distinguish the signal truly coming from the planet. Current observations suggest that TRAPPIST-1 e might possess an atmosphere rich in heavy molecules.
Definitive confirmation is expected by 2025 thanks to new scheduled observations. Astronomers are using an ingenious strategy by successively observing transits of TRAPPIST-1 b, a planet with no confirmed atmosphere, and those of TRAPPIST-1 e. This comparative method will better characterize stellar variations and precisely isolate the atmospheric signature of the target planet. This research could revolutionize our understanding of rocky planets in our galaxy.
If the presence of an atmosphere is confirmed, the next steps will involve determining its exact composition, particularly the presence of greenhouse gases like carbon dioxide or methane. These elements are essential for maintaining temperatures compatible with liquid water on the surface. The scientific community eagerly awaits these results, which could mark a historic milestone in the search for conditions favorable to life elsewhere in the Universe.
The habitable zone and its conditions
The habitable zone, often nicknamed the 'Goldilocks zone', represents the region around a star where temperatures theoretically allow water to remain in liquid form on a planet's surface. This zone varies considerably depending on the star type: around red dwarfs like TRAPPIST-1, it's located much closer than in the case of Sun-like stars.
The exact distance depends on the star's luminosity and temperature. For red dwarfs, which are cooler and less luminous, the habitable zone begins at just a few million kilometers, unlike the 150 million kilometers that separate us from the Sun. This proximity has important consequences for potential living conditions.
Planets located in this zone around red dwarfs are often tidally locked, always presenting the same face to their star. This creates extreme thermal contrasts between the day and night hemispheres. However, a sufficiently dense atmosphere could redistribute heat and mitigate these differences.
The presence of liquid water doesn't depend solely on distance from the star, but also on many other factors like atmospheric pressure, air composition, and stellar activity. Red dwarfs are known for their violent flares that could erode planetary atmospheres, making the persistence of a gaseous envelope particularly significant.
Spectral analysis of atmospheres
Spectral analysis represents the most powerful method for studying exoplanet atmospheres. When a planet transits in front of its star, light passes through its potential atmosphere, and gaseous molecules absorb certain specific wavelengths. This chemical 'signature' allows scientists to determine atmospheric composition.
Each type of molecule absorbs light in a characteristic way. For example, carbon dioxide strongly absorbs in the infrared, while oxygen and ozone have distinct signatures in the ultraviolet. The James Webb Telescope, specialized in infrared, is particularly suited for detecting molecules like methane, carbon dioxide, and water vapor.
Measurement accuracy depends on several factors: the star's size, the system's distance, and the instrument's stability. For small stars like red dwarfs, the ratio between the planet's size and the star's size is more favorable, amplifying the atmospheric signal. This is why TRAPPIST-1 represents an ideal target for this type of study.
Interpreting spectra requires models that account for temperature, pressure, and chemical composition. Scientists compare observed data with computer simulations to determine which gas mixtures best match the measurements. This approach allows for distinguishing between different hypotheses, such as the presence of a primary or secondary atmosphere.