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| Illustration showing what the hot rocky exoplanet TRAPPIST-1 b could look like. TRAPPIST-1 b, the innermost of seven known planets in the TRAPPIST-1 system, orbits its star at a distance of 0.011 AU, completing one circuit in just 1.51 Earth-days. TRAPPIST-1 b is slightly larger than Earth, but has around the same density, which indicates that it must have a rocky composition. Webb’s measurement of mid-infrared light given off by TRAPPIST-1 b suggests that the planet does not have any substantial atmosphere. The star, TRAPPIST-1, is an ultracool red dwarf (M dwarf) with a temperature of only 2566 K and a mass just 0.09 times the mass of the Sun. Credit: European Space Agency |
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 planet's dayside has a temperature of about
500 Kelvin (roughly 230°C), 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 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."
Rocky planets orbiting ultra cool 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 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.
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| Light curve showing 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. Credit: European Space Agency |
"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 CEA in France, who was on the team that conducted
the initial 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 NASA/ESA Hubble Space
Telescope, as well as NASA's Spitzer Space Telescope, 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 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.
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| Comparison of the dayside temperature of TRAPPIST-1 b as measured using Webb’s Mid-Infrared Instrument (MIRI) to computer models showing what the temperature would be under various conditions. |
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."
Analysis of data from five separate secondary eclipse observations indicates
that TRAPPIST-1 b has a dayside temperature of about 500 Kelvin, or roughly
230°C. The team thinks the most likely interpretation is that the planet
does not have an atmosphere.
"We compared the results to computer models 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 didn't see any signs of light being absorbed by
carbon dioxide, which would be apparent in these measurements."
This research was conducted as part of Guaranteed Time Observation (GTO)
program 1177, which is one of eight approved GTO and General Observer (GO)
programs 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. It's a really important step in the story of
discovering exoplanets."
Reference:
Thomas Greene, Thermal Emission from the Earth-sized Exoplanet TRAPPIST-1 b
using JWST, Nature (2023).
DOI: 10.1038/s41586-023-05951-7
Tags:
Space & Astrophysics