Temperature of a rocky exoplanet measured

Date:
March 27, 2023
Source:
NASA/Goddard Space Flight Center
Summary:
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.


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FULL STORY ==========================================================================
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." 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.

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."
* RELATED_TOPICS
o Space_&_Time
# Extrasolar_Planets # Stars # Astronomy # Pluto #
Kuiper_Belt # Eris_(Xena) # Mars # Sun
* RELATED_TERMS
o Spitzer_space_telescope o Space_observatory o
Compton_Gamma_Ray_Observatory o Eris_(dwarf_planet) o Uranus
o Titan_(moon) o Venus o Neptune

========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Thomas P. Greene, Taylor J. Bell, Elsa Ducrot, Achre`ne Dyrek,
Pierre-
Olivier Lagage, Jonathan J. Fortney. Thermal Emission from the
Earth- sized Exoplanet TRAPPIST-1 b using JWST. Nature, 2023; DOI:
10.1038/ s41586-023-05951-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/03/230327163227.htm

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