NASA’s Webb House Telescope reveals the scorching secrets and techniques of a distant rocky exoplanet

NASAs Webb Measures the Temperature of a Rocky Exoplanet

An international team of researchers has used NASAs James Webb Space Telescope to measure the temperature of the rocky

Rocky Planets Orbiting Ultracool Red Dwarfs

In early 2017, astronomers reported the discovery of seven rocky planets orbiting an ultracool red dwarf star (or M dwarf) 40 light-years from Earth. What is remarkable about the planets is their similarity in size and mass to the inner, rocky planets of our own solar system. Although they all orbit much closer to their star than any of our planets orbit the Sun all could fit comfortably within the orbit of Mercury they receive comparable amounts of energy from their tiny star.

TRAPPIST-1 b, the innermost planet, has an orbital distance about one hundredth that of Earths and receives about four times the amount of energy that Earth gets from the Sun. Although it is not within the systems habitable zone, observations of the planet can provide important information about its sibling planets, as well as those of other M-dwarf systems.

There are ten times as many of these stars in the

This light curve shows the change in brightness of the TRAPPIST-1 system as the innermost planet, TRAPPIST-1 b, moves behind the star. This phenomenon is known as a secondary eclipse.
Astronomers used Webbs 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 planets dayside. This is then used to calculate the dayside temperature.
The graph shows combined data from five separate observations made using MIRIs F1500W filter, which only allows light with wavelengths ranging from 13.5-16.6 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 decrease in brightness during the secondary eclipse is less than 0.1%. MIRI was able to detect changes as small as 0.027% (or 1 part in 3,700).
This is the first thermal emission observation of TRAPPIST-1 b, or any planet as small as Earth and as cool as the rocky planets in our solar system.
Credit: Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI), Science: Thomas P. Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)

Detecting an Atmosphere (or Not)

Previous observations of TRAPPIST-1 b with the Hubble and Spitzer space telescopes found no evidence for a puffy atmosphere, but were not able to rule out a dense one.

One way to reduce the uncertainty is to measure the planets temperature. This planet is tidally locked, with one side facing the star at all times and the other in permanent darkness, said Pierre-Olivier Lagage from CEA, a co-author on the paper. If it has an atmosphere to circulate and redistribute the heat, the dayside will be cooler than if there is no atmosphere.

The team used a technique called secondary eclipse photometry (see Secondary Eclipse Light Curve image above), in which MIRI measured the change in brightness from the system as the planet moved behind the star. Although TRAPPIST-1 b is not hot enough to give off its own visible light, it does have an infrared glow. By subtracting the brightness of the star on its own (during the secondary eclipse) from the brightness of the star and planet combined, they were able to successfully calculate how much infrared light is being given off by the planet.

Measuring Minuscule Changes in Brightness

Webbs detection of a secondary eclipse is itself a major milestone. With the star more than 1,000 times brighter than the planet, the change in brightness is less than 0.1%.

There was also some fear that wed miss the eclipse. The planets all tug on each other, so the orbits are not perfect, said Taylor Bell, the post-doctoral researcher at the Bay Area Environmental Research Institute who analyzed the data. But it was just amazing: The time of the eclipse that we saw in the data matched the predicted time within a couple of minutes.

The team analyzed data from five separate secondary eclipse observations. We compared the results to computer models (see Dayside Temperature Comparison image above) showing what the temperature should be in different scenarios, explained Ducrot. The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didnt see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements.

This research was conducted as part of Webb Guaranteed Time Observation (GTO) program 1177, which is one of eight programs from Webbs first year of science designed to help fully characterize the TRAPPIST-1 system. Additional secondary eclipse observations of TRAPPIST-1 b are currently in progress, and now that they know how good the data can be, the team hopes to eventually capture a full phase curve showing the change in brightness over the entire orbit. This will allow them to see how the temperature changes from the day to the nightside and confirm if the planet has an atmosphere or not.

There was one target that I dreamed of having, said Lagage, who worked on the development of the MIRI instrument for more than two decades. And it was this one. This is the first time we can detect the emission from a rocky, temperate planet. Its a really important step in the story of discovering exoplanets.

Reference: Thermal Emission from the Earth-sized Exoplanet TRAPPIST-1 b using JWST by Thomas P. Greene, Taylor J. Bell, Elsa Ducrot, Achrne Dyrek, Pierre-Olivier Lagage and Jonathan J. Fortney, 27 March 2023, Nature.
DOI: 10.1038/s41586-023-05951-7

The James Webb Space Telescope is the worlds premier space science observatory. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (