Does the exoplanet Trappist-1 b have an atmosphere after all?

Trappist-1 b is one of seven rocky planets orbiting the star Trappist-1, 40 light-years away. This planetary system is unique because it allows astronomers to study seven Earth-like planets from relatively close distances, three of which are in the so-called habitable zone. This is the region in a planetary system where liquid water may exist on a planet’s surface. To date, 10 research projects have used the James Webb Space Telescope (JWST) to study the system for 290 hours.

The current study was led by Elsa Ducrot from the Commission for Atomic Energy (CEA) in Paris, France, with the active participation of researchers from the Max Planck Institute for Astronomy (MPIA) in Heidelberg. The study, which used JWST’s MIRI (Mid-Infrared Imager) to measure the thermal infrared radiation (essentially thermal radiation) of the planet Trappist-1 b, has now been published in the journal natural astronomy. It includes results from last year, on which previous conclusions were based, describing Trappist-1 b as a dark, rocky planet without an atmosphere.

Trappist-1 b’s crust may be geologically active

“However, the idea of ​​a rocky planet with a heavily weathered surface and no atmosphere is inconsistent with current measurements,” said MPIA astronomer Jeroen Bouwman, who co-led the observing project. “So we think the Earth is covered with relatively unchanging material.” Normally, the surface is weathered by radiation from the central star and impacts by meteorites. However, the results show that surface rocks are only about 1,000 years old at most, far younger than the age of the Earth itself, which is estimated to date back billions of years.

This could indicate that Earth’s crust is undergoing dramatic changes, which could be explained by extreme volcanic activity or plate tectonics. Even though this scenario remains hypothetical for now, it is still plausible. The planet is large enough that its interior may retain residual heat from its formation – just like Earth. Tidal effects from the central star and other planets may also be deforming 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 fields of nearby stars is also conceivable.

Is it possible that Trappist-1 b has an atmosphere?

“These data also provide a completely different solution,” said MPIA Director Emeritus Thomas Henning. He is one of the principal designers of the MIRI instrument. “Contrary to previous thinking, under certain conditions Earth may have a thick atmosphere rich in carbon dioxide (CO₂),” he added. A key role in this case is the smoke from hydrocarbons in the upper atmosphere, known as smog.

The two observing programs complement each other in the current study, which aims to measure the brightness of Trappist-1 b at different wavelengths (12.8 and 15 microns) in the thermal infrared range. First observation sensitive to absorption of planetary infrared radiation by carbon dioxide layers₂. However, no dimming was measured, leading the researchers to conclude that the planet has no atmosphere.

Model calculations conducted by the research team show that haze can reverse the temperature stratification of carbon dioxide₂– Intense atmosphere. Typically, lower ground layers are warmer than the layers above due to higher pressure. When haze absorbs starlight and warms it, it instead heats the upper atmosphere, supported by the greenhouse effect. As a result, the carbon dioxide there itself emits infrared radiation.

We see something similar happening on Saturn’s moon Titan. Its haze layer likely formed under the influence of solar ultraviolet (UV) radiation emitted by carbon-rich gases in the atmosphere. Because Trappist-1 b’s stars emit large amounts of ultraviolet radiation, a similar process may occur in Trappist-1 b.

it’s complicated.

Even if the data are consistent with this scenario, astronomers still think it’s less likely. For one thing, producing smog-forming hydrocarbons from a carbon dioxide-rich atmosphere is more difficult, though not impossible.₂. However, Titan’s atmosphere is primarily composed of methane. On the other hand, the problem remains that the radiation and winds produced by active red dwarfs, including Trappist-1, could easily erode the atmospheres of nearby planets over billions of years.

Trappist-1 b is a vivid example of how difficult it is currently to detect and determine the atmospheres of rocky planets—even for JWST. Compared to gas planets, they are thin and produce only weak measurable features. Two observations studying Trappist-1 b, which lasted nearly 48 hours, provided brightness values ​​at two wavelengths that were not sufficient to determine without a doubt whether the planet has an atmosphere.

Eclipses and occultations as tools

These observations take advantage of the slight tilt of the planet’s plane to our line of sight to Trappist-1. This orientation causes seven planets to pass in front of the star and dim the star slightly with each orbit. So there are many ways to learn about the planet’s properties and atmosphere.

So-called transmission spectroscopy has proven to be a reliable method. This involves measuring the star’s planetary dimming based on wavelength. In addition to occultations by opaque planetary bodies, from which astronomers can determine a planet’s size, gases in the atmosphere absorb certain wavelengths of starlight. From this, they can deduce whether the planet has an atmosphere and what its atmosphere is composed of. Unfortunately, this approach has drawbacks, especially for planetary systems like Trappist-1. Cold red dwarfs often exhibit large star spots and intense outbursts that significantly affect measurements.

Astronomers have largely circumvented this problem by looking at the sides of exoplanets heated by their stars in thermal infrared light, as is the case with the current study of Trappist-1 b. It’s especially easy to see in bright daylight, before and after the planet disappears behind its star. The infrared radiation emitted by a planet contains information about its surface and atmosphere. However, this observation is more time-consuming than transmitting the spectrum.

Given the potential of these so-called subsolar eclipse measurements, NASA recently approved a broad observing program to study the atmospheres of rocky planets around nearby low-mass stars. This extraordinary project, “Rocky World,” includes 500 hours of observation by JWST.

Certainty about Trappist-1 b

The team hopes to obtain clear confirmation using another observed variant. It records the planet’s complete orbit around the star, including all phases of illumination from the dark night side as it passes in front of the star to the bright day side just before and shortly after being overshadowed by the star. This approach will allow the team to create so-called phase curves, which indicate changes in a planet’s brightness along its orbit. From this, astronomers can deduce the temperature distribution on the planet’s surface.

The team has made this measurement using Trappist-1 b. By analyzing how heat is distributed across the Earth, they can infer the existence of an atmosphere. This is because the atmosphere helps transport heat from day to night. If the temperature changes suddenly at the transition between the two sides, it means there is no atmosphere.