This illustration shows what the hot rocky exoplanet TRAPPIST-1 b could look like based on this work. 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 2,566 kelvins and a mass just 0.09 times the mass of the Sun. This illustration is based on new data gathered by Webb’s Mid-Infrared Instrument (MIRI) as well as previous observations from other ground- and space-based telescopes. Webb has not captured any images of the planet. Credits: NASA, ESA, CSA, J. Olmsted (STScI)
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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
degrees Fahrenheit) 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.”
This graphic compares the dayside temperature of TRAPPIST-1 b as measured using Webb’s Mid-Infrared Instrument (MIRI) to computer models of what the temperature would be under various conditions. The models take into account known properties of the system, including the temperature of the star and the planet’s orbital distance. The temperature of the dayside of Mercury is also shown for reference. The dayside brightness of TRAPPIST-1 b at 15 microns corresponds to a temperature of about 500 kelvins (roughly 450 degrees Fahrenheit). This is consistent with the 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 dayside to the nightside. If the heat energy from the star were distributed evenly around the planet (for example, by a circulating carbon dioxide-free atmosphere), the temperature at 15 microns would be 400 kelvins (260 degrees Fahrenheit). If the atmosphere had a substantial amount of carbon dioxide, it would emit even less 15-micron light and would appear to be even cooler. Although TRAPPIST-1 b is hot by Earth standards, it is cooler than the dayside of Mercury, which consists of bare rock and no significant atmosphere. Mercury receives about 1.6 times more energy from the Sun than TRAPPIST-1 b does from its star. Credits: Illustration: NASA, ESA, CSA, J. Olmsted (STScI); Science: Thomas Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)
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 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.
“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
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.
This light curve shows 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. Astronomers used Webb’s Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is beside the star, the light emitted by both the star and the dayside of the planet reach 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, causing the apparent brightness to decrease. Astronomers can subtract the brightness of the star from the combined brightness of the star and planet to calculate how much infrared light is coming from the planet’s dayside. This is then used to calculate the dayside temperature. The graph shows combined data from five separate observations 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 blue squares are individual brightness measurements. 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 is less than 0.1%. MIRI was able to detect changes as small as 0.027% (or 1 part in 3,700). This is the first thermal emission observation of TRAPPIST-1 b, or any planet as small as Earth and as cool as the rocky planets in our solar system. The observations are being repeated using a 12.8-micron filter in order to confirm the results and narrow down the interpretations. Credits: Illustration: NASA, ESA, CSA, J. Olmsted (STScI); Science: Thomas Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)
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 team analyzed data from five
separate secondary eclipse observations. “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 Webb Guaranteed Time Observation (GTO) program 1177,
which is one of eight programs from Webb’s first year of science 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.”
The James Webb Space Telescope is the world's premier space science observatory. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency), and CSA (Canadian Space Agency). MIRI was contributed by NASA and ESA, with the instrument designed and built by a consortium of nationally funded European Institutes (the MIRI European Consortium) and NASA’s Jet Propulsion Laboratory, in partnership with the University of Arizona.
Source: NASA’s Webb Measures the Temperature of a Rocky Exoplanet | NASA
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