The Pluto system
Discovered in 1930 by the American astronomer Clyde Tombaugh, Pluto is the first dwarf planet by volume, with a diameter of 2,372 kilometres. Located in the Kuiper belt, beyond Neptune, the last planet in our solar system, Pluto has five moons, the largest of which, Charon, measures 1,200 kilometres in diameter.
Observed for several decades by major telescopes on Earth and in space, the Plutonian system has now been closely explored by NASA’s New Horizons spacecraft. During its historic flyby on 14 July 2015, the probe passed just 11,095 kilometres from Pluton, revealing a world of surprising landscapes (see Figure 1).
Pluto has a geology that is still active, shaping a complex topography of basins, mountains, valleys and even glaciers of nitrogen (N2) and methane (CH4). Its atmosphere, although tenuous, is chemically rich and contains volatile compounds such as nitrogen, methane and carbon monoxide (CO), accompanied by an extensive haze. This haze is formed by the coupled photochemistry of nitrogen and methane, a process similar to that observed on Titan, Saturn’s largest moon.
In comparison, Charon, which has no atmosphere, has a more uniform surface, composed mainly of water ice mixed with ammonia-based compounds (see Figure 2). Its polar regions are covered with a layer of organic soot, giving them a darker, reddish appearance than the rest of the surface. This soot results from the capture and chemical transformation of methane (CH4) molecules escaping from Pluto’s atmosphere.
These discoveries have raised fundamental new questions about the evolution of Pluto’s climate, the chemistry and energy balance of its atmosphere, and the thermal and compositional properties of the surfaces of Pluto and Charon.
Observations of the Pluto-Charon system by the James Webb Space Telescope
In order to acquire new data on the gaseous composition and haze of Pluto’s atmosphere, researchers have directed the James Webb Space Telescope (JWST) towards the Pluto-Charon system. They used the MIRI instrument, which has the significant advantage of capturing spectra and carrying out photometric measurements in the mid-infrared, a spectral range hitherto unexplored for Pluto.
Photometric measurements, obtained using the MIRI imager (known as MIRIm), provide distinct thermal light curves for Pluto and Charon at four characteristic wavelengths : 15, 18, 21 and 25 micrometres. These data can be used to determine the thermal and energetic properties of their surfaces.
As for the spectral measurements, acquired with the Medium Resolution Spectrometer (MRS) in the 5 to 28 micrometre band, they provide new information on the composition of Pluto’s atmosphere.
Results of photometric measurements
Complex surface-atmosphere interactions within the Pluto-Charon pair
The light curves measured by MIRIm reveal variations in the heat flux emitted by Pluto and Charon during their rotation (see Figure 3). These variations are sensitive to the surface properties of the various terrains, such as methane ice, water ice and dark deposits. By comparing these observations with thermal models, the researchers were able to establish significant constraints on the thermal inertia, emissivity and temperature of the different regions of Pluto and Charon.
On Pluto, these properties play a crucial role in the diurnal and seasonal cycles of volatile ice redistribution. On Charon, the results reveal major differences between the regions covered in pure water ice and the dark polar regions (see Figure 4). These observations provide new insights into a phenomenon that is unique in the Solar System : the deposition of material from Pluto’s atmosphere on the surface of its satellite and its subsequent evolution.
An organic, icy mist that shapes Pluto’s climate
Analysis of the data collected reveals for the first time that Pluto’s atmospheric haze emits a significant thermal signature. Such an emission had been predicted by models, but had never before been observed directly. This discovery is of major importance : it indicates that the temperature, dynamics and, more generally, the climate system of Pluto are strongly influenced - or even controlled - by this haze, the effects of which vary with the seasons.
The temperature of Pluto’s upper atmosphere is -203 degrees Celsius due to the presence of the haze, whereas it would be -173 degrees Celsius without it. The haze particles absorb heat and then emit infrared radiation into space, cooling the atmosphere. Our observations thus confirm that the atmospheric temperature on Pluto, and therefore an important part of its climate system, is controlled by haze particles. This highlights the chemical richness of Pluto’s atmosphere, which has similarities with Titan’s upper atmosphere," explains Tanguy Bertrand, assistant astronomer at LIRA at Paris Observatory-PSL and lead author of the paper published in Nature Astronomy.
The researchers also highlighted the complex nature of this haze. It is made up of organic particles as well as hydrocarbon and nitrile ices, the spectral signatures of which were detected in this study.
The haze results from chemical reactions in the upper atmosphere, where the Sun’s ultraviolet radiation ionises nitrogen and methane, which react to form tiny hydrocarbon particles a few tens of nanometres in diameter. As these particles descend into the atmosphere, they agglomerate to form aggregates. As they fall, the aggregates grow in size and eventually settle on the surface," explains Tanguy Bertrand.
Spectral measurement results
Valuable clues about the chemistry, origin and evolution of Pluto
Analysis of the infrared spectrum (4.9-27 μm) of Pluto’s atmosphere has revealed new details about its composition (see Figure 5), providing new constraints for understanding the chemistry of the atmosphere and its origin.
The spectrum shows clear signatures of several gases produced by the photolysis of methane by the Sun’s UV rays, such as ethane (C2H6), acetylene (C2H2), propyne (CH3C2H) and diacetylene (C4H2). These results refine our understanding of the photochemical reactions involved and allow detailed comparisons with those observed on Titan.
Unexpectedly, the spectrum shows fluorescent (non-thermal) emissions of methane (CH4) and deuteromethane (CH3D). This indicates complex processes of non-collisional excitation of their vibrational levels by solar radiation, similar to those observed in cometary atmospheres.
Finally, the detection of the C2HD molecule made it possible to measure a deuterium/hydrogen (D/H) ratio around three times higher than on Earth. This ratio is a marker of the origin and evolution of Pluto’s ice and atmosphere, although its interpretation is uncertain for the moment.
A new milestone in the exploration of the outer solar system
This work represents a significant advance in our understanding of Pluto and Charon. It opens up new perspectives on the climatic and chemical evolution of Pluto, as well as on the exchange of matter within the Pluto-Charon system. This research highlights the richness of the chemical processes and the central role of the haze in the thermal balance of Pluto’s atmosphere.
The chemistry and radiative effects of the haze could also occur in other tenuous atmospheres rich in nitrogen (N₂) and methane (CH₄), such as those of Triton (Neptune’s largest natural satellite), the upper layers of Titan’s atmosphere, the primitive Earth, or even some exoplanets.
Other teams have carried out studies of Pluto and Charon using JWST’s near-infrared instruments, in particular the NIRCam imager and the NIRSpec spectrometer. The next step will be to carry out a coherent analysis of all the observations over a wide spectral range to extend our understanding of this distant system.
To go further
Articles
- Article published in Astronomy & Astrophysics in April 2025 - E. Lellouch, I. Wong, P. Lavvas, T. Bertrand, G. Villanueva, J. Stansberry, B. Holler, N. Pinilla-Alonso, F. Merlin, A. C. Souza-Feliciano, and K. Murray
- Article published in Nature Astronomy in June 2025 - Tanguy Bertrand, Emmanuel Lellouch, Bryan Holler, John Stansberry, Ian Wong, Xi Zhang, Panayotis Lavvas, Elodie Dufaux, Frederic Merlin, Geronimo Villanueva, Linfeng Wan, Noemí Pinilla-Alonso, Ana Carolina de Souza Feliciano, Katherine Murray
Scientific contacts
- Tanguy Bertrand, astronomer at the LIRA – Observatoire de Paris-PSL (tanguy.bertrand@obspm.fr)
- Emmanuel Lellouch, research professor at the LIRA - Observatoire de Paris-PSL (Emmanuel.Lellouch@obspm.fr)
Contact presse
- Fabien Fichet, Director of Communications, Observatoire de Paris-PSL (fabien.fichet@obspm.fr / 0141502155)
