James Webb Space Telescope Discovery

Webb Rules Out Thick Carbon Dioxide Atmosphere for Rocky Exoplanet

Infrared measurements by the NASA/ESA/CSA James Webb Space Telescope of TRAPPIST-1 c indicate that it is probably not as Venus-like as once imagined. Webb has successfully measured the heat radiating from TRAPPIST-1 c, an exoplanet orbiting a red dwarf star 40 light-years from Earth. With a dayside temperature of about 107 degrees Celsius, it is the coolest rocky planet ever characterised using this method. This light curve shows the change in brightness of the TRAPPIST-1 system as the second planet, TRAPPIST-1 c, moves behind the star. This phenomenon is known as a secondary eclipse. Astronomers used Webb’s Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is beside the star, the light emitted by both the star and the dayside of the planet reach the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease. Astronomers can subtract the brightness of the star from the combined brightness of the star and planet to calculate how much infrared light is coming from the planet’s dayside. This is then used to calculate the dayside temperature and infer the presence and possible composition of the atmosphere. The graph shows combined data from four separate observations made using MIRI’s F1500W filter, which only allows light with wavelengths ranging from about 13.5 - 16.7 microns to pass through to the detectors. The blue squares are individual brightness measurements. The red circles show measurements that are “binned,” or averaged to make it easier to see the change over time. The white line is the best fit, or model light curve that matches the data most closely. The decrease in brightness during the secondary eclipse is less than 0.05%. The temperature calculated from this observation is 380 +/- 31 kelvins (between 76 and 138 degrees Celsius). TRAPPIST-1 c is the coolest rocky exoplanet ever observed using secondary eclipse photometry. This research was conducted as part of Webb’s General Observers (GO) program 2304, which is one of eight programs from Webb’s first year of science designed to help fully characterise the TRAPPIST-1 system. This coming year, researchers will conduct a follow-up investigation to observe the full orbits of TRAPPIST-1 b and TRAPPIST-1 c. This will make it possible to see how the temperatures change from the day to the night sides of the two planets and will provide further constraints on whether they have atmospheres or not. Credit: NASA, ESA, CSA, J. Olmsted (STScI), S. Zieba (MPI-A), L. Kreidberg (MPI-A) TRAPPIST-1 c light curve This graph compares the measured brightness of TRAPPIST-1 c to simulated brightness data for three different scenarios. The measurement (red diamond) is consistent with a bare rocky surface with no atmosphere (green line) or a very thin carbon dioxide atmosphere with no clouds (blue line). A thick carbon dioxide-rich atmosphere with sulphuric acid clouds, similar to that of Venus (yellow line), is unlikely. The y-axis of the graph shows brightness (also called intensity or flux) of light in terms of eclipse depth, which is the difference between the combined brightness of the star and planet (when the planet is beside the star) and the brightness of the star on its own (when the planet is behind the star). Brightness increases from bottom to top on the graph: The greater the eclipse depth, the brighter the light from the planet. The x-axis shows the wavelength (or colour) of light being measured. All of the wavelengths shown here are in the infrared, which is invisible to human eyes. The brightness of light emitted by the planet varies with wavelength: Some colours are brighter than others. The pattern of brightness (the spectrum) depends on factors such as the type of rock that makes up the surface, what the atmosphere is made of, and whether or not there are clouds. Different materials absorb and emit different amounts of different wavelengths of light. The red diamond shows the brightness of TRAPPIST-1 c as measured using the F1500W filter on MIRI (Webb’s Mid-Infrared Instrument). The vertical lines extending above and below the diamond are error bars. The width of the blue box covers the range of wavelengths that were measured using MIRI’s F1500W filter, which allows light with wavelengths ranging from about 13.5 - 16.7 microns to pass through to the detectors. The blue line shows what the emission spectrum of the planet’s dayside would look like assuming it has an oxygen atmosphere with 0.01% carbon dioxide, a surface pressure of 0.1 bars, and no clouds. (For reference, this is significantly thinner than Earth’s atmosphere, which is nitrogen- and oxygen-rich, with 0.04% carbon dioxide and a surface pressure of 1 bar.) The green line shows what the emission spectrum of the planet’s dayside would be if it has no atmosphere and a rocky surface made of ultramafic rock. (Ultramafic rock is a type of igneous rock that is somewhat richer in iron and magnesium and poorer in silica than basalt, which makes up the crust beneath Earth’s oceans.) The orange line shows the emission spectrum of the planet’s dayside if it had an atmosphere closer to that of Venus, with 96.5% carbon dioxide, a surface pressure of 10 bars, and sulphuric acid clouds. TRAPPIST-1 c emission spectra This artist' concept shows what the hot rocky exoplanet TRAPPIST-1 c could look like based on this work. TRAPPIST-1 c, the second of seven known planets in the TRAPPIST-1 system, orbits its star at a distance of 0.016 AU (about 1.5 million miles), completing one circuit in just 2.42 Earth-days. TRAPPIST-1 c is slightly larger than Earth, but has around the same density, which indicates that it must have a rocky composition. Webb’s measurement of 15-micron mid-infrared light emitted by TRAPPIST-1 c suggests that the planet has either a bare rocky surface or a very thin carbon dioxide atmosphere. Illustrated in the background is TRAPPIST-1 b, the innermost planet in the TRAPPIST-1 system. TRAPPIST-1 b is also rocky and appears to have no substantial atmosphere. The star, TRAPPIST-1, is an ultracool red dwarf (M dwarf) with a temperature of only 2,550 kelvins (about 4,150 degrees Fahrenheit) and a mass just 0.09 times the mass of the Sun. This illustration is based on new data gathered by Webb’s Mid-Infrared Instrument (MIRI) as well as previous observations from other ground- and space-based telescopes. Webb has not captured any images of the planet. Rocky Exoplanet TRAPPIST-1 c (Artist Concept)