As an AI language model, I do not have access to real-time information or data. However, I can explain how scientists might measure the temperature of a rocky exoplanet. One way to measure the temperature of an exoplanet is through transit observations. When an exoplanet passes in front of its host star, it blocks a small fraction of the star’s light, causing a slight decrease in brightness. By measuring the amount of light blocked at different wavelengths, scientists can determine the temperature of the planet’s atmosphere.
The temperature of the rocky exoplanet TRAPPIST-1 b has been measured by an international team of researchers using NASA’s James Webb Space Telescope. The measurement is based on the planet’s thermal emission: heat energy emitted as infrared light, which is detected by Webb’s Mid-Infrared Instrument (MIRI). The result suggests that the planet’s dayside has a temperature of about 500 kelvins (roughly 450 degrees Fahrenheit) and has no significant atmosphere.
This is the first detection of any kind of light emitted by an exoplanet as small and cool as our own solar system’s rocky planets. The discovery is a significant step toward determining whether planets orbiting small active stars like TRAPPIST-1 can support life-sustaining atmospheres. It also bodes well for Webb’s ability to characterize temperate, Earth-sized exoplanets using MIRI.
“These observations really take advantage of Webb’s mid-infrared capability,” said Thomas Greene, an astrophysicist at NASA’s Ames Research Center and lead author on the study published today in the journal Nature. “No previous telescopes have had the sensitivity to measure such dim mid-infrared light.”
We compared the results to computer models that showed what the temperature should be in various scenarios. The results are nearly perfect for a blackbody made of bare rock with no atmosphere to circulate the heat.
Elsa Ducrot
Rocky Planets Orbiting Ultracool Red Dwarfs
Astronomers announced the discovery of seven rocky planets orbiting an ultracool red dwarf star (or M dwarf) 40 light-years from Earth in early 2017. The planets are remarkable because they are similar in size and mass to the inner, rocky planets of our own solar system. They all receive comparable amounts of energy from their tiny star, despite the fact that they all orbit much closer to their star than any of our planets orbit the Sun.
TRAPPIST-1 b, the innermost planet, has an orbital distance one hundredth that of Earth and receives roughly four times the amount of energy from the Sun that Earth does. Although it is not within the system’s 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 Milky Way as there are stars like the Sun, and they are twice as likely to have rocky planets as stars like the Sun,” explained Greene. “But they are also very active – they are very bright when they’re young, and they give off flares and X-rays that can wipe out an atmosphere.”
Co-author Elsa Ducrot from the French Alternative Energies and Atomic Energy Commission (CEA) in France, who was on the team that conducted earlier studies of the TRAPPIST-1 system, added, “It’s easier to characterize terrestrial planets around smaller, cooler stars. If we want to understand 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.”
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 planet’s 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 known as secondary eclipse photometry, 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 emit visible light, it does emit an infrared glow. They were able to successfully calculate how much infrared light the planet emits by subtracting the brightness of the star on its own (during the secondary eclipse) from the brightness of the star and planet combined.
Measuring Minuscule Changes in Brightness
Webb’s 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 we’d 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 researchers examined data from five different secondary eclipse observations. “We compared the results to computer models that showed what the temperature should be in various scenarios,” Ducrot explained. “The results are nearly perfect for a blackbody made of bare rock with no atmosphere to circulate the heat. We also saw no evidence of light being absorbed by carbon dioxide, which would be visible in these measurements.”
This study was carried out as part of Webb Guaranteed Time Observation (GTO) program 1177, one of eight programs from Webb’s first year of science that were designed to help fully characterize the TRAPPIST-1 system. Additional secondary eclipse observations of TRAPPIST-1 b are currently underway, and the team hopes to eventually capture a full phase curve showing the change in brightness over the entire orbit now that they know how good the data can be. This will allow them to see how the temperature changes from day to night and confirm whether or not the planet has an atmosphere.
