Exoplanet Trappist-1 b

Recent measurements with the James Webb Space Telescope (JWST) cast doubt on the current understanding of the exoplanet Trappist-1 b’s nature. Until now, it was assumed to be a dark rocky planet without an atmosphere, shaped by a billion-year-long cosmic impact of radiation and meteorites. The opposite appears to be true. The surface shows no signs of weathering, which could indicate geological activity such as volcanism and plate tectonics. Alternatively, a planet with a hazy atmosphere composed of carbon dioxide is also viable. The results demonstrate the challenges of determining the properties of exoplanets with thin atmospheres.

Trappist-1 b is one of seven rocky planets orbiting the star Trappist-1, located 40 light-years away. The planetary system is unique because it allows astronomers to study seven Earth-like planets from relatively close range, with three of them in the so-called habitable zone. This is the area in a planetary system where a planet could have liquid water on the surface. To date, ten research programmes have targeted this system with the James Webb Space Telescope (JWST) for 290 hours.

The current study, in which researchers from the Max Planck Institute for Astronomy (MPIA) in Heidelberg are significantly involved, was led by Elsa Ducrot from the Commissariat aux Énergies Atomiques (CEA) in Paris, France. This study uses measurements of the thermal infrared radiation – essentially heat radiation – of the planet Trappist-1 b with MIRI (Mid-Infrared Imager) at the JWST and has now been published in the journal Nature Astronomy. It includes the results from last year, on which the previous conclusions were based, which describe Trappist-1 b as a dark rocky planet without an atmosphere.

The crust of Trappist-1 b could be geologically active.

‘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 programme. ‘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.

Could Trappist-1 b possibly have an atmosphere after all?

‘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 programmes, 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 micrometres). 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.

Representation of the infrared brightness emitted by Trappist-1 b at 12.8 and 15 micrometres for different scenarios involving bare rock and atmosphere. The four cases indicate which ones agree with the current data and which do not. (a) Dark, bare rock produces infrared brightness greater than observed. (b) The observed infrared brightness is well compatible with a surface of magmatic rock that is only slightly or not weathered. (c) An atmosphere of carbon dioxide and a high veil of haze could also explain the measurement data, in that a large part of the infrared radiation comes from upper atmospheric layers. (d) Earth-like atmospheres absorb some of the infrared radiation generated by the surface, which would lead to intensities not observed in Trappist-1 b.

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.

It’s complicated.

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 as a tool

This illustration shows the observation of Trappist-1 b during a transit. Different regions of its surface are revealed along its orbit. The side facing the star is much hotter and emits bright thermal infrared light. The full signal is captured just before and after the planet’s occultation, with only the star’s brightness recorded during the event. In the lower panel, the graph shows the brightness measurements of the star alone and in combination with the day side of the planet, emphasizing the changes in brightness over time.

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.

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 programme to study the atmospheres of rocky planets around nearby, low-mass stars. This extraordinary programme, ' Rocky Worlds’, includes 500 hours of observation with the JWST.

Certainty about Trappist-1 b

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 analysing 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.