English: 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 170 and 280 degrees Fahrenheit). TRAPPIST-1 c is the coolest rocky exoplanet ever observed using secondary eclipse photometry.