An international team of researchers has used NASA’s James
Webb Space Telescope to calculate the amount of heat energy coming from the
rocky exoplanet TRAPPIST-1 c. The result suggests that the planet’s atmosphere
– if it exists at all – is extremely thin.
This artist' concept shows what the hot rocky exoplanet TRAPPIST-1 c could look like based on this work. TRAPPIST-1 c, the second of seven known planets in the TRAPPIST-1 system, orbits its star at a distance of 0.016 AU (about 1.5 million miles), completing one circuit in just 2.42 Earth-days. TRAPPIST-1 c is slightly larger than Earth, but has around the same density, which indicates that it must have a rocky composition. Webb’s measurement of 15-micron mid-infrared light emitted by TRAPPIST-1 c suggests that the planet has either a bare rocky surface or a very thin carbon dioxide atmosphere. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)
With a dayside temperature of
roughly 380 kelvins (about 225 degrees Fahrenheit), TRAPPIST-1 c is now the
coolest rocky exoplanet ever characterized based on thermal emission. The precision necessary for these
measurements further demonstrates Webb’s utility in characterizing rocky
exoplanets similar in size and temperature to those in our own solar system.
The result marks another step in
determining whether planets orbiting small red dwarfs like TRAPPIST-1 – the most
common type of star in the galaxy – can sustain atmospheres needed to support
life as we know it.
“We want to know if rocky planets
have atmospheres or not,” said Sebastian Zieba, a graduate student at the Max
Planck Institute for Astronomy in Germany and first author on results being
published today in Nature.
“In the past, we could only really study planets with thick, hydrogen-rich atmospheres.
With Webb we can finally start to search for atmospheres dominated by oxygen,
nitrogen, and carbon dioxide.”
“TRAPPIST-1 c is interesting
because it’s basically a Venus twin: It’s about the same size as Venus and
receives a similar amount of radiation from its host star as Venus gets from
the Sun,” explained co-author Laura Kreidberg, also from Max Planck. “We
thought it could have a thick carbon dioxide atmosphere like Venus.”
TRAPPIST-1 c is one of seven rocky planets orbiting an ultracool red
dwarf star (or M dwarf) 40 light-years from Earth. Although the planets are
similar in size and mass to the inner, rocky planets in our own solar system,
it is not clear whether they do in fact have similar atmospheres. During the
first billion years of their lives, M dwarfs emit bright X-ray and ultraviolet
radiation that can easily strip away a young planetary atmosphere. In addition,
there may or may not have been enough water, carbon dioxide, and other
volatiles available to make substantial atmospheres when the planets formed.
To address these questions, the
team used MIRI (Webb’s Mid-Infrared Instrument) to observe the TRAPPIST-1
system on four separate occasions as the planet moved behind the star, a
phenomenon known as a secondary eclipse. By comparing the brightness when
the planet is behind the star (starlight only) to the brightness when the
planet is beside the star (light from the star and planet combined) the team
was able to calculate the amount of mid-infrared light with wavelengths of 15
microns given off by the dayside of the planet.
This light curve shows the change
in brightness of the TRAPPIST-1 system as the second planet, TRAPPIST-1 c,
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. Credits: NASA, ESA, CSA,
Joseph Olmsted (STScI)
This method is the same as that
used by another research team to determine that TRAPPIST-1 b, the innermost planet in the
system, is probably devoid of any atmosphere.
The amount of mid-infrared light
emitted by a planet is directly related to its temperature, which is in turn
influenced by atmosphere. Carbon dioxide gas preferentially absorbs 15-micron
light, making the planet appear dimmer at that wavelength. However, clouds can
reflect light, making the planet appear brighter and masking the presence of
carbon dioxide.
In addition, a substantial
atmosphere of any composition will redistribute heat from the dayside to the
nightside, causing the dayside temperature to be lower than it would be without
an atmosphere. (Because TRAPPIST-1 c orbits so close to its star – about 1/50th the distance between Venus and the Sun – it is
thought to be tidally locked, with one side in perpetual daylight and the other
in endless darkness.)
Although these initial measurements
do not provide definitive information about the nature of TRAPPIST-1 c, they do
help narrow down the likely possibilities. “Our results are consistent
with the
planet being a bare rock with no atmosphere, or the planet having a really thin
CO2 atmosphere (thinner than on Earth or even Mars)
with no clouds,” said Zieba. “If the planet had a thick CO2 atmosphere, we would have observed a really
shallow secondary eclipse, or none at all. This is because the CO2 would be absorbing all of the 15-micron light,
so we wouldn’t detect any coming from the planet.”
The data also shows that it is
unlikely the planet is a true Venus analog with a thick CO2 atmosphere and sulfuric acid clouds.
The absence of a thick atmosphere
suggests that the planet may have formed with relatively little water. If the
cooler, more temperate TRAPPIST-1 planets formed under similar conditions, they
too may have started with little of the water and other components necessary to
make a planet habitable.
The sensitivity required to
distinguish between various atmospheric scenarios on such a small planet so far
away is truly remarkable. The decrease in brightness that Webb detected during
the secondary eclipse was just 0.04 percent: equivalent to looking at a display
of 10,000 tiny light bulbs and noticing that just four have gone out.
This graph compares the measured
brightness of TRAPPIST-1 c to simulated brightness data for three different
scenarios. The measurement (red diamond) is consistent with a bare rocky
surface with no atmosphere (green line) or a very thin carbon dioxide
atmosphere with no clouds (blue line). A thick carbon dioxide-rich atmosphere
with sulfuric acid clouds, similar to that of Venus (yellow line), is unlikely.
Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)
“It is extraordinary that we can
measure this,” said Kreidberg. “There have been questions for decades now about
whether rocky planets can keep atmospheres. Webb’s ability really brings us
into a regime where we can start to compare exoplanet systems to our solar
system in a way that we never have before.”
This research was conducted as part
of Webb’s General Observers (GO) program 2304, which is one of eight programs
from Webb’s first year of science designed to help fully characterize the
TRAPPIST-1 system. This coming year, researchers will conduct a follow-up investigation to observe the full orbits of TRAPPIST-1 b and
TRAPPIST-1 c. This will make it possible to see how the temperatures change
from the day to the nightsides of the two planets and will provide further
constraints on whether they have atmospheres or not.
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: Webb Rules Out Thick Carbon Dioxide Atmosphere for Rocky Exoplanet | NASA
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