An international team of researchers has used the NASA/ESA/CSA James Webb Space Telescope to measure the temperature of the rocky exoplanet TRAPPIST-1 b. The measurement is based on the planet’s thermal emission: heat energy given off in the form of infrared light detected by Webb’s Mid-Infrared Instrument (MIRI). The result indicates that the day side of the planet has a temperature of about 500 Kelvin (roughly 230°C), and suggests that there is no significant atmosphere. This is the first detection of any kind of light emitted from an exoplanet as small and cool as the rocky planets in our own solar system. The result marks an important step in determining whether planets orbiting small active stars like TRAPPIST-1 can maintain the atmospheres needed to support life. It also bodes well for Webb’s ability to characterize Earth-sized temperate exoplanets using MIRI.

“These observations really take advantage of Webb’s mid-infrared capabilities,” said Thomas Greene, an astrophysicist at NASA’s Ames Research Center and lead author of the study, published today in the journal Nature. “No previous telescopes were so sensitive to measure such faint mid-infrared light.”

Rocky planets orbit 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. The remarkable thing about the planets is their similarity in size and mass to the inner rocky planets of our own solar system. While they all orbit much closer to their star than our planets orbit the sun — they could all easily fit into Mercury’s orbit — they receive similar amounts of energy from their tiny star.

TRAPPIST-1 b, the innermost planet, has an orbital distance about one-hundredth that of Earth and receives about four times as much energy as Earth gets from the sun. Although it is not within the system’s habitable zone, observations of the planet could yield important information about its sister planets, as well as those of other M dwarf systems.

“There are ten times as many of these stars in the Milky Way as stars like the sun, and they are twice as likely to be rocky planets as stars like the sun,” Green explained. “But they’re also very active — they’re very bright when they’re young and they give off flares and X-rays that can wipe out an atmosphere.”

Co-author Elsa Ducrot of CEA in France, who was part of the team conducting the initial studies of the TRAPPIST-1 system, added: “It is easier to characterize terrestrial planets around smaller, cooler stars. If we want to understand the habitability around M stars, the TRAPPIST-1 system is a great laboratory. These are the best targets we have for looking at the atmospheres of rocky planets.”

Detect atmosphere (or not)

Previous observations of TRAPPIST-1 b with the NASA/ESA Hubble Space Telescope, as well as NASA’s Spitzer Space Telescope, found no evidence of a swollen atmosphere, but could not rule out a dense atmosphere.

One way to reduce the uncertainty is to measure the temperature of the planet. “This planet is tidally locked, with one side always facing the star and the other in permanent darkness,” said CEA’s Pierre-Olivier Lagage, a co-author of the paper. “If it has an atmosphere to circulate and redistribute the heat, the day side will be cooler than if there is no atmosphere.”

Rocky exoplanet TRAPPIST-1 b (secondary eclipse light curve)
Light curve showing the change in brightness of the TRAPPIST-1 system as the inner planet, TRAPPIST-1 b, 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 next to the star, the light emitted from both the star and the day side of the planet reaches 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, reducing its apparent brightness. Astronomers can subtract the brightness of the star from the combined brightness of the star and the planet to calculate how much infrared light is coming from the day side of the planet. This is then used to calculate the daytime temperature. The graph shows combined data from five separate observations made using MIRI’s F1500W filter, which allows only light with a wavelength of 13.5-16.6 microns to pass through to the detectors. The blue squares are individual brightness measurements. The red circles represent measurements that have been “rolled out” 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 could detect changes as small as 0.027% (or 1 part in 3700). This is the first observation of thermal emission from TRAPPIST-1 b, or another planet as small as Earth and as cool as the rocky planets in the solar system. The observations are repeated with a 12.8 micron filter to confirm the results and refine the interpretations. MIRI was developed as a partnership between Europe and the US: the main partners are ESA, a consortium of nationally funded European institutes, the Jet Propulsion Laboratory (JPL) and the University of Arizona. The instrument was funded nationally by the European Consortium under the auspices of the European Space Agency.

[Image description: At the top of the infographic is a diagram showing a planet moving behind its star (a secondary eclipse). Below the diagram is a graph showing the change in brightness of 15-micron light emitted by the star-planet system over the course of 3.5 hours. The infographic shows that the brightness of the system decreases markedly as the planet moves behind the star.]

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

The team used a technique called secondary eclipse photometry, in which MIRI measures the system’s change in brightness as the planet moved behind the star. Although TRAPPIST-1 b is not hot enough to emit its own visible light, it does have an infrared glow. By subtracting the brightness of the star alone (during the secondary eclipse) from the brightness of the star and the planet together, they were able to successfully calculate how much infrared light is being emitted by the planet.

Measuring tiny changes in brightness

Webb’s detection of a secondary eclipse is itself an important 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 we might miss the eclipse. The planets are all pulling together, so the orbits aren’t perfect. said Taylor Bell, the postdoctoral researcher at the Bay Area Environmental Research Institute who analyzed the data. “But it was just amazing. The eclipse time we saw in the data matched the predicted time within a few minutes.”

Analysis of data from five separate secondary eclipse observations indicates that TRAPPIST-1 b has a daytime temperature of about 500 Kelvin, or about 230°C. The team thinks the most likely interpretation is that the planet has no atmosphere.

Rocky exoplanet TRAPPIST-1 b (temperature equation)
Comparison of the daytime temperature of TRAPPIST-1 b as measured by Webb’s Mid-Infrared Instrument (MIRI) with computer models showing what the temperature would be under different conditions. The models take into account the known properties of the system, including the size and density of the planet, the temperature of the star and the orbital distance of the planet. The temperature of the day side of Mercury is also shown for reference. The daytime brightness of TRAPPIST-1 b at 15 microns corresponds to a temperature of about 500 K (about 230°C). This is consistent with temperature, assuming the planet is tidally locked (one side facing the star at all times), with a dark-colored surface, no atmosphere, and no redistribution of heat from the day side to the night side. If the star’s heat energy were evenly distributed across the planet (for example, by a circulating carbon dioxide-free atmosphere), the temperature at 15 microns would be 400 K (125°C). If the atmosphere contained a significant amount of carbon dioxide, it would emit even less light of 15 microns and would appear even cooler. Although TRAPPIST-1 b is hot by Earth standards, it is cooler than Mercury’s day side, which consists of bare rock and no significant atmosphere. Mercury receives about 1.6 times more energy from the sun than TRAPPIST-1b does from its star. MIRI was developed as a partnership between Europe and the US: the main partners are ESA, a consortium of nationally funded European institutes, the Jet Propulsion Laboratory (JPL) and the University of Arizona. The instrument was funded nationally by the European Consortium under the auspices of the European Space Agency.

[Image description: Infographic titled, “Rocky Exoplanet TRAPPIST-1 b Dayside Temperature Comparison, MIRI F1500W” showing five planets plotted along a horizontal temperature scale: Earth, TRAPPIST-1 b, Mercury, and two different models of TRAPPIST-1 b.]

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

“We compared the results with computer models that show what the temperature should be in different scenarios,” Ducrot explained. “The results match almost perfectly with a black body made of bare rock and with no atmosphere to circulate the heat. We also saw no signs of light being absorbed by carbon dioxide, which would be evident in these measurements.”

This study was conducted as part of Guaranteed Time Observation (GTO) Program 1177, one of eight approved GTO and General Observer (GO) programs designed to fully characterize the TRAPPIST-1 system. Additional secondary eclipse observations of TRAPPIST-1 b are currently underway, 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 across the entire orbit. This allows them to see how the temperature changes from day to night and confirm whether the planet has an atmosphere or not.

“There was one target I dreamed I had”, said Lagage, who spent more than two decades developing the MIRI instrument. “And it was this one. This is the first time we can detect the emission from a rocky temperate planet. It is a very important step in the story of exoplanet discovery.”