The artist's concept shows what the hot rocky exoplanet TRAPPIST-1 c might look like

image: This artist’s concept shows what the hot rocky exoplanet TRAPPIST-1 c might 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 away), completing one orbit in just 2.42 Earth days. TRAPPIST-1 c is slightly larger than Earth, but has about the same density, indicating that it must be a rocky composition. Webb’s measurement of the 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.
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Credit: Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

An international team of scientists has used NASA’s James Webb Space Telescope to calculate the amount of thermal energy coming from the rocky exoplanet TRAPPIST-1 c. The result suggests that the planet’s atmosphere – if it exists – is extremely thin.

With daytime temperatures of approx. At 380 kelvin (about 225 degrees Fahrenheit), TRAPPIST-1 c is now the coolest rocky exoplanet ever to experience thermal outflow. The precision required for these measurements further demonstrates Webb’s utility in characterizing rocky exoplanets similar in size and temperature to 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 the atmospheres needed to support life as we know it.

“We want to know whether rocky planets have atmospheres or not,” said Sebastian Zieba, a graduate student at the Max Planck Institute for Astronomy in Germany and first author of the findings published today in The nature. “Previously, we could only study planets with thick, hydrogen-rich atmospheres.” With Webb, we can finally start looking for an atmosphere 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 similar radiation from its host star as Venus does from the Sun,” explained co-author Laura Kreidberg, also of Max. Planck. “We thought it might have a thick carbon dioxide atmosphere like Venus.”

TRAPPIST-1 c is one of seven rocky planets orbiting a supercool 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 actually have similar atmospheres. During the first billion years of their lives, M-dwarfs emit bright X-rays and ultraviolet radiation that can easily remove young planetary atmospheres. In addition, there may have been enough water, carbon dioxide, and other volatiles present to create a substantial atmosphere when the planets formed.

To answer these questions, the team used MIRI (Mid-Infrared Instrument Webb) to observe the TRAPPIST-1 system on four separate occasions when the planet moved behind the star, a phenomenon known as secondary eclipses. By comparing the brightness when the planet is behind the star (starlight only) with the brightness when the planet is next to the star (light from the star and the planet combined), the team was able to calculate the amount of mid-infrared light with wavelengths within 15 microns of the planet’s solar cycle.

This method is the same as another research team used to determine that TRAPPIST-1 b, the innermost planet in the system, is likely devoid of an atmosphere.

The amount of mid-infrared light emitted by a planet is directly related to its temperature, which in turn is influenced by its atmosphere. Carbon dioxide gas preferentially absorbs light at 15 microns, making the planet appear fainter at that wavelength. However, clouds can reflect light, making the planet appear brighter and masking the presence of carbon dioxide.

Additionally, a substantial atmosphere of any composition will redistribute heat from day to night, causing daytime temperatures to be lower than they would be without an atmosphere. (Because TRAPPIST-1 c orbits so close to its star – about 1/50i.e the distance between Venus and the Sun – believed to be tidally locked, with one side in perpetual daylight and the other in perpetual darkness.)

Although these initial measurements do not provide definitive information about the nature of TRAPPIST-1 c, they do help to narrow down the likely possibilities. “Our results are consistent with the planet being bare rock without an atmosphere or with a very thin carbon dioxide2 atmosphere (thinner than on Earth or even Mars) without clouds,” Zieba said. “If the planet had thick CO2 atmosphere, we would have observed very shallow secondary eclipses, or none at all. This is because CO2 would absorb all the 15 micron light, so we wouldn’t detect anything coming from the planet.”

The data also show that the planet is unlikely to be a true Venus counterpart with thick CO2 atmosphere and sulfuric acid clouds.

The lack of a thick atmosphere suggests that the planet may have formed with relatively little water. If the colder, more temperate TRAPPIST-1 planets formed under similar conditions, they too may have started out with little water and other components necessary to make a planet habitable.

The sensitivity required to distinguish between various scenarios in the atmosphere of such a small planet so far away is truly remarkable. The drop in brightness that Webb found during the secondary eclipse was just 0.04 percent: the equivalent of looking at 10,000 tiny light bulbs and noticing that only four have gone out.

“It’s amazing that we can measure this,” Kreidberg said. “There have been questions for decades now about whether rocky planets can retain atmospheres. Webb’s ability really moves us into a regime where we can begin to compare exoplanet systems to our own solar system in a way we’ve never done before.”

This study was conducted as part of Webb’s General Observers (GO) program 2304, one of eight programs from Webb’s first year in science designed to help fully characterize the TRAPPIST-1 system. In the coming year, scientists will conduct a follow-up study to monitor the entire trajectories of TRAPPIST-1 b and TRAPPIST-1 c. This will make it possible to see how temperatures change from day to night on the two planets and will put further constraints on whether or not they have atmospheres.

The James Webb Space Telescope is the world’s first space science observatory. Webb will solve the mysteries of our solar system, peer out to distant worlds around other stars, and investigate the mysterious structure and origins of our universe and our place in it. Webb is an international program led by NASA along with its partners, ESA (European Space Agency), and CSA (Canadian Space Agency). MIRI was proposed by NASA and ESA, with the instrument designed and built by a group of nationally funded European institutions (MIRI European Consortium) and NASA’s Jet Propulsion Laboratory, in collaboration with the University of Arizona.

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