James Webb Telescope measures temperature of rocky exoplanet TRAPPIST-1b

An illustrated image depicts the possible appearance of the hot rocky exoplanet TRAPPIST-1 b. TRAPPIST-1 b, which is the closest to the seven known planets in the TRAPPIST-1 system, orbits its star at a distance of 0.011 AU and completes one orbit in just 1.51 Earth-days. Although slightly larger than Earth, its density is similar, indicating a rocky composition. Webb's detection of mid-infrared light given off by TRAPPIST-1 b suggests the planet lacks a significant atmosphere. The star, TRAPPIST-1, is an ultracool red dwarf (M dwarf) with a mass only 0.09 times that of the Sun and a temperature of 2566 K. This image is based on findings published in the journal Nature and is a credit to NASA, ESA, CSA, J. Olmsted (STScI), T. P. Greene (NASA Ames), T. Bell (BAERI), E. Ducrot (CEA), P. Lagage (CEA). .

Credits: NASA, ESA, CSA, J. Olmsted (STScI), T. P. Greene (NASA Ames), T. Bell (BAERI), E. Ducrot (CEA), P. Lagage (CEA) 

March 27, 2023

An international team of researchers has used the NASA/ESA/CSA James Webb Space Telescope to detect the temperature of TRAPPIST-1 b, a rocky exoplanet. The team used the planet's thermal emission, which is the heat energy given off in the form of infrared light. The telescope's Mid-Infrared Instrument (MIRI) detected the infrared light and indicated that the planet's dayside has a temperature of roughly 230°C (500 kelvins), and there is no significant atmosphere.

This discovery marks a significant step towards determining whether planets orbiting small active stars like TRAPPIST-1 can sustain life-sustaining atmospheres. It is the first detection of any form of light emitted by a rocky exoplanet as small and cool as the planets in our own solar system. These findings are published in the journal Nature and led by Thomas Greene, an astrophysicist at NASA’s Ames Research Center. The study highlights the importance of Webb's mid-infrared capability and its ability to characterize temperate, Earth-sized exoplanets using MIRI.

In 2017, astronomers discovered an ultra-cool red dwarf star, 40 light-years away from Earth, that had seven rocky planets orbiting it. What made this discovery remarkable was the similarity in size and mass between these newly found planets and the rocky planets that are located in the inner regions of our own solar system. Despite the planets orbiting much closer to their star than any of the planets in our solar system, they receive comparable amounts of energy from their small star.

The innermost planet, TRAPPIST-1 b, orbits the star at an orbital distance that is one hundredth of Earth's and receives around four times the amount of energy that Earth gets from the Sun. Although not in the habitable zone of the system, the planet's observations can provide valuable information about the other planets in the system and those of other M-dwarf systems.

Thomas Greene, an astrophysicist at NASA’s Ames Research Center and lead author of the study, explained that ultra-cool red dwarf stars, such as TRAPPIST-1, are twice as likely to have rocky planets as stars like the Sun. However, they are very active, very bright when young, and give off flares and X-rays that can destroy the atmosphere of the planets.

Co-author Elsa Ducrot, from CEA in France, who was on the team that conducted the initial studies of the TRAPPIST-1 system, added that it is easier to characterize terrestrial planets around smaller, cooler stars. The TRAPPIST-1 system serves as an excellent laboratory for understanding habitability around M stars and is the best target for studying the atmospheres of rocky planets.

This study from the James Webb Space Telescope has shed new light on the atmosphere of TRAPPIST-1 b, one of the Earth-sized exoplanets orbiting the ultracool dwarf star TRAPPIST-1. Previous observations with the Hubble and Spitzer Space Telescopes were inconclusive about whether the planet had an atmosphere. To address this uncertainty, the team used a technique called secondary eclipse photometry, in which the Mid-Infrared Instrument (MIRI) on the Webb Telescope measured the change in brightness from the system as the planet moved behind the star. By subtracting the brightness of the star on its own during the secondary eclipse from the brightness of the star and planet combined, the team was able to calculate how much infrared light is being given off by the planet.

The Webb Telescope's detection of the secondary eclipse is a major milestone, as the change in brightness is less than 0.1% and the planet is over 1,000 times fainter than its host star. The team was able to measure the planet's temperature, which revealed that its dayside is about 500 kelvins, or roughly 230°C. The most likely interpretation of these results is that the planet does not have an atmosphere. The team plans to conduct additional secondary eclipse observations of TRAPPIST-1 b to capture a full phase curve showing the change in brightness over the entire orbit, which 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.

Astronomers have been analyzing the light curve of the system, specifically looking at the changes in brightness as the innermost planet, TRAPPIST-1 b, moves behind the star. This phenomenon is known as a secondary eclipse.

Using MIRI, the astronomers were able to measure the brightness of mid-infrared light emitted by the star and the planet's dayside. They found that when the planet is beside the star, the combined light makes the system appear brighter. However, when the planet moves behind the star, the light emitted by the planet is blocked, and only the starlight reaches the telescope. This causes the apparent brightness to decrease.

By subtracting the brightness of the star from the combined brightness of the star and planet, the astronomers were able to calculate the amount of infrared light coming from the planet's dayside, which was then used to determine the dayside temperature. The observations were made using MIRI's F1500W filter, which only allows light with wavelengths ranging from 13.5-16.6 microns to pass through to the detectors.

The graph presented by the astronomers shows the combined data from five separate observations made using MIRI. The blue squares represent individual brightness measurements, while 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 was found to be less than 0.1%, and MIRI was able to detect changes as small as 0.027% (or 1 part in 3700).

What makes these observations particularly significant is that this is the first time that TRAPPIST-1 b, or any planet as small as Earth and as cool as the rocky planets in the Solar System, has been observed through thermal emission. To confirm the results and narrow down the interpretations, the observations are being repeated using a 12.8-micron filter.

It's worth noting that the Mid-Infrared Instrument was developed as a partnership between Europe and the USA, with the main partners being the European Space Agency (ESA), a consortium of nationally funded European institutes, the Jet Propulsion Laboratory (JPL), and the University of Arizona. The instrument was nationally funded by the European Consortium under the auspices of the European Space Agency. This just goes to show that global cooperation can yield groundbreaking results. Stay tuned for more updates on this exciting astronomical discovery!

A comparison of the dayside temperature of TRAPPIST-1 b, as measured by Webb's MIRI, to computer models and Mercury's temperature. The models take into account the planet's size, density, orbital distance, and star temperature. TRAPPIST-1 b's dayside temperature is about 500K, assuming no atmosphere, no heat redistribution, and a dark surface. If the heat were distributed evenly, the temperature would be 400K, and if there were a significant amount of carbon dioxide, the temperature would be even cooler. TRAPPIST-1 b is cooler than Mercury's dayside temperature, which has no atmosphere and receives 1.6 times more energy from the Sun. MIRI was developed in partnership between Europe and the USA, funded by the European Consortium under ESA. 

Source - NASA/ESA