An international team of researchers has used NASA’s 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 planet’s dayside has a temperature of about 500 kelvins (roughly 450°F) and suggests that it has no significant atmosphere.
This is the first detection of any form of light emitted by an exoplanet as small and as 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 sustain atmospheres needed to support life.
It also bodes well for NASA’s Webb’s ability to characterise temperate, Earth-sized exoplanets using MIRI.
Thomas Greene, an astrophysicist at NASA’s Ames Research Center and lead author on the study, said: “These observations really take advantage of Webb’s mid-infrared capability. No previous telescopes have had the sensitivity to measure such dim mid-infrared light.”
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 1/100th that of Earth’s and receives about four times the amount of energy that Earth gets from the sun.
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.
Greene explained: “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, 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 characterise 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 of 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.
CEA’s Pierre-Olivier Lagage, a co-author on the paper, said: “This planet is tidally locked, with one side facing the star at all times and the other in permanent darkness. 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, 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
NASA’s 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%.
Taylor Bell, the post-doctoral researcher at the Bay Area Environmental Research Institute who analysed the data, stated: “There was also some fear that we’d miss the eclipse. The planets all tug on each other, so the orbits are not perfect.
“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 analysed data from five separate secondary eclipse observations.
Ducrot explained: “We compared the results to computer models showing what the temperature should be in different scenarios. The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat.
“We also didn’t see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements.”
Lagage, who worked on the development of the MIRI instrument for more than two decades, concluded: “There was one target that I dreamed of having, and it was this one. This is the first time we can detect the emission from a rocky, temperate planet.
“It’s a really important step in the story of discovering exoplanets.”
The study is published in the journal Nature.
Image 1: This illustration shows what the hot rocky exoplanet TRAPPIST-1 b could look like based on this work. © NASA/ ESA/ CSA/ J Olmsted (STScI). Public domain.