Naukowcy wykorzystali[{” attribute=”” tabindex=”0″ role=”link”>James Webb Space Telescope to study Trappist-1 b, revealing it might have a volatile surface and potentially an atmosphere despite previous beliefs of it being a dark, rocky body without one.
Initial observations suggested no atmosphere, but further analysis raised possibilities of a thick CO2 atmosphere, influenced by hydrocarbon haze similar to that on Saturn’s moon Titan. Additional extensive studies are planned to confirm these findings.
Breakthrough Observations of the Trappist-1 System
Trappist-1 b is one of seven rocky planets orbiting the star Trappist-1, located 40 light-years from Earth. This planetary system stands out because it offers astronomers a rare chance to study multiple Earth-like planets up close. Three of these planets are situated in the “habitable zone,” where conditions could allow liquid water to exist on their surfaces. So far, ten research programs have used the James Webb Space Telescope (JWST) to observe the system for a total of 290 hours.
The latest study, involving researchers from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, was led by Elsa Ducrot of the Commissariat aux Énergies Atomiques (CEA) in Paris, France. The team measured Trappist-1 b’s thermal infrared radiation — essentially its heat signature — using the JWST’s Mid-Infrared Imager (MIRI). The findings, now published in Nature Astronomy, build on last year’s research, which described Trappist-1 b as a dark, rocky planet with no detectable atmosphere.
A Dynamic and Evolving Planetary Crust
“However, the idea of a rocky planet with a heavily weathered surface without an atmosphere is inconsistent with the current measurement,” says MPIA astronomer Jeroen Bouwman, who was jointly responsible for the observation program. “Therefore, we think the planet is covered with relatively unchanged material.” Usually, the surface is weathered by the radiation of the central star and impacts from meteorites. However, the results suggest that the rock on the surface is at most about 1000 years old, significantly less than the planet itself, which is estimated to date back several billion years.
This could indicate that the planet’s crust is subject to dramatic changes, which could be explained by extreme volcanism or plate tectonics. Even if such a scenario is currently still hypothetical, it is nevertheless plausible. The planet is large enough that its interior may have retained residual heat from its formation – as with Earth. The tidal effect of the central star and the other planets may also deform Trappist-1 b so that the resulting internal friction generates heat – similar to what we see in Jupiter’s moon Io. In addition, inductive heating by the magnetic field of the nearby star would be conceivable.
The Hypothetical Atmosphere of Trappist-1 b
“The data also allow for an entirely different solution,” says Thomas Henning, emeritus director of the MPIA. He was one of the principal architects of the MIRI instrument. “Contrary to previous ideas, there are conditions under which the planet could have a thick atmosphere rich in carbon dioxide (CO2),” he adds. A key role in this scenario is haze from hydrocarbon compounds, i.e. smog, in the upper atmosphere.
The two observational programs, which complement each other in the current study, were designed to measure the brightness of Trappist-1 b at different wavelengths in the thermal infrared range (12.8 and 15 micrometers). The first observation was sensitive to the absorption of the planet’s infrared radiation by a layer of CO2. However, no dimming was measured, leading the researchers to conclude that the planet has no atmosphere.
The research team performed model calculations that show that haze can reverse the temperature stratification of a CO2-rich atmosphere. Typically, the lower, ground-level layers are warmer than the upper ones because of the higher pressure. As the haze absorbs the starlight and warms up, it would instead heat the upper atmospheric layers, supported by a greenhouse effect. As a result, the carbon dioxide there emits infrared radiation itself.
We see something similar happening on Saturn’s moon Titan. Its haze layer most likely forms there under the influence of the sun’s ultraviolet (UV) radiation from the carbon-rich gases in the atmosphere. A similar process may occur on Trappist-1 b due to its star emitting substantial UV radiation.
Challenges in Atmosphere Detection
Even if the data fit this scenario, the astronomers still consider it less likely by comparison. On the one hand, it is more difficult, though not impossible, to produce hydrocarbon compounds that form a haze from an atmosphere rich in CO2. Titan’s atmosphere, however, consists mainly of methane. On the other hand, the problem remains that the active red dwarf stars, which include Trappist-1, produce radiation and winds that can easily erode the atmospheres of nearby planets over billions of years.
Trappist-1 b is a vivid example of how difficult it currently is to detect and determine the atmospheres of rocky planets – even for the JWST. They are thin compared to gas planets and produce only weak measurable signatures. The two observations to study Trappist-1 b, which provided brightness values at two wavelengths, lasted almost 48 hours, which was not enough to determine beyond doubt whether the planet has an atmosphere.
Eclipses and Occultations for Atmospheric Study
The observations took advantage of the slight inclination of the planet’s plane to our line of sight to Trappist-1. This orientation causes the seven planets to pass before the star and dim it slightly during each orbit. Consequently, this results in learning about the planets’ nature and atmospheres in several ways.
So-called transit spectroscopy has proven to be a reliable method. This involves measuring the dimming of a star by its planet, depending on the wavelength. In addition to the occultation by the opaque planetary body, from which astronomers determine the planet’s size, the atmospheric gases absorb the starlight at specific wavelengths. From this, they can deduce whether a planet has an atmosphere and what it consists of. Unfortunately, this method has disadvantages, especially for planetary systems like Trappist-1. Cool, red dwarf stars often exhibit large starspots and strong eruptions, significantly affecting the measurement.
The Promise of Secondary Eclipse Measurements
Astronomers largely circumvent this problem by instead observing the side of an exoplanet heated by the star in the thermal infrared light, as in the current study with Trappist-1 b. The bright dayside is particularly easy to see just before and after the planet vanishes behind the star. The infrared radiation the planet releases contains information about its surface and atmosphere. However, such observations are more time-consuming than transit spectroscopy.
Given the potential of these so-called secondary eclipse measurements, NASA has recently approved an extensive observation program to study the atmospheres of rocky planets around nearby, low-mass stars. This extraordinary program, ‘Rocky Worlds’, includes 500 hours of observation with the JWST.
Future Observations for Certainty
The research team expects to be able to obtain definitive confirmation using another observation variant. It records the planet’s complete orbit around the star, including all illumination phases from the dark night side when passing in front of the star to the bright dayside shortly before and after being covered by the star. This approach will allow the team to create a so-called phase curve indicating the planet’s brightness variation along its orbit. As a result, the astronomers can deduce the planet’s surface temperature distribution.
The team has already carried out this measurement with Trappist-1 b. By analyzing how the heat is distributed on the planet, they can deduce the presence of an atmosphere. This is because an atmosphere helps to transport heat from the day side to the night side. If the temperature changes abruptly at the transition between the two sides, this indicates the absence of an atmosphere.
For more on this discovery, see TRAPPIST-1b: Bare Rock or Hazy Alien World?
Reference: “Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b” by Elsa Ducrot, Pierre-Olivier Lagage, Michiel Min, Michaël Gillon, Taylor J. Bell, Pascal Tremblin, Thomas Greene, Achrène Dyrek, Jeroen Bouwman, Rens Waters, Manuel Güdel, Thomas Henning, Bart Vandenbussche, Olivier Absil, David Barrado, Anthony Boccaletti, Alain Coulais, Leen Decin, Billy Edwards, René Gastaud, Alistair Glasse, Sarah Kendrew, Goran Olofsson, Polychronis Patapis, John Pye, Daniel Rouan, Niall Whiteford, Ioannis Argyriou, Christophe Cossou, Adrian M. Glauser, Oliver Krause, Fred Lahuis, Pierre Royer, Silvia Scheithauer, Luis Colina, Ewine F. van Dishoeck, Göran Ostlin, Tom P. Ray and Gillian Wright, 16 December 2024, Nature Astronomy.
DOI: 10.1038/s41550-024-02428-z
The MPIA team involved in this study comprised Jeroen Bouwman, Thomas Henning, Oliver Krause, and Silvia Scheithauer.
Other researchers included Elsa Ducrot (LESIA, Observatoire de Paris, CNRS, Université Paris Diderot, Université Pierre et Marie Curie, Meudon, France and Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, Gif-sur-Yvette, France [CEA]), Pierre-Olivier Lagage (CEA), Michiel Min (Holenderski Instytut Badań Kosmicznych SRON, Leiden, Holandia) i Michaël Gillon (Jednostka Badań nad Astrobiologią, Uniwersytet w Liege, Liêge, Belgia)
Konsorcjum MIRI składa się z państw członkowskich ESA: Belgii, Danii, Francji, Niemiec, Irlandii, Holandii, Hiszpanii, Szwecji, Szwajcarii i Wielkiej Brytanii. Prace konsorcjum finansują krajowe organizacje naukowe – w Niemczech Towarzystwo Maxa Plancka (MPG) i Niemieckie Centrum Lotnictwa i Kosmonautyki (DLR). Niemieckie instytucje uczestniczące obejmują Instytut Astronomii Maxa Plancka w Heidelbergu, Uniwersytet w Kolonii i Hensoldt AG w Oberkochen, dawniej Carl Zeiss Optronics.
JWST jest wiodącym na świecie obserwatorium badań kosmicznych. Jest to międzynarodowy program prowadzony przez NASA i jej partnerów, ESA (Europejska Agencja Kosmiczna) i CSA (Kanadyjska Agencja Kosmiczna).
