Citation: Smith, J. et al. (2025). “Characterising Exoplanetary Atmospheres: Insights from Recent Observations.” Astronomy & Astrophysics, 625, A1.
In the ever-evolving field of astronomy, the study of exoplanets, planets orbiting stars beyond our Solar System, has captivated scientists, enthusiasts and the general public alike. A recent publication in Astronomy & Astrophysics sheds light on the complex atmospheres of these distant worlds, offering a glimpse into their compositions, climates, and potential habitability, since the first exoplanet discovery in the early 1990s, strangely orbiting a pulsar, astronomers have confirmed some 7399 of these misterious worlds we are desperate to know more about, especially about their ability to host life. While primary efforts focus on detection, the desire is to move the frontier of our knowledge forward about as many of these worlds as possible so this involves understanding their atmospheres, by using spectroscopy, which is the analysing of the light from the parent star that passes through or reflects off an exoplanet’s atmosphere, researchers can infer the presence of various gases, clouds, and even weather patterns in some cases. As a minimum we are able to calculate the black body temperature of the planet, this would be the temperature that planet would have on its daytime side if it had no atmosphere, taking into account its distance form the star.
How astronomers calculate the Black Body Temperature
Astronomers calculate the black body temperature of a planet by considering the balance between the energy it receives from its star and the energy it radiates back into space. This is done using the planet’s distance from its star, the star’s luminosity, and an assumption that the planet absorbs and emits energy like a perfect black body. The formula involves:
- Star’s Luminosity and Distance – The amount of stellar energy reaching the planet depends on the star’s luminosity and the planet’s distance (using the inverse-square law).
- Albedo (Reflectivity) – The fraction of energy reflected by the planet’s surface or clouds is subtracted to find the absorbed energy.
- Black Body Radiation – The absorbed energy is assumed to be radiated uniformly as thermal infrared radiation. This gives the planet’s equilibrium temperature.
If the planet has no atmosphere, the measured dayside temperature matches the calculated black body temperature. However, if the actual temperature is higher or shows significant differences between the day and night sides, it suggests the presence of an atmosphere that traps heat (via the greenhouse effect) or redistributes it across the surface.
The study revisited data from HARPS and ESPRESSO observations.
Here is a quote from the abstract.
“Aims. We present an analysis of the RV data of the star HD 20794, a target whose planetary system has been extensively debated in the literature. The broad time span of the observations makes it possible to find planets with signal semi-amplitudes below 1 m s−1 in the habitable zone.Methods. We analyzed RV datasets spanning more than 20 years. We monitored the system with ESPRESSO. We joined ESPRESSO data with the HARPS data, including archival data and new measurements from a recent program. We applied the post-processing pipeline YARARA to HARPS data to correct systematics, improve the quality of RV measurements, and mitigate the impact of stellar activity.
Results. We confirm the presence of three planets, with periods of 18.3142 ± 0.0022 d, 89.68 ± 0.10 d, and 647.6−2.7+2.5 d, along with masses of 2.15 ± 0.17 M⊕, 2.98 ± 0.29 M⊕, and 5.82 ± 0.57 M⊕ respectively. For the outer planet, we find an eccentricity of 0.45−0.11+0.10, whereas the inner planets are compatible with circular orbits. The latter is likely to be a rocky planet in the habitable zone of HD 20794. From the analysis of activity indicators, we find evidence of a magnetic cycle with a period of ~3000 d, along with evidence pointing to a rotation period of ~39 d.”
The research highlights the incredible diversity among exoplanetary atmospheres that we have been able to detrmine so far – this knowledge is increasing as JWST observations add to this broad dataset., but in summary.
- Hot Jupiters: These gas giants, orbiting very close to their stars, exhibit temperatures exceeding 1,000°C. The study detected signatures of water vapour, sodium, and potassium in their atmospheres. Some also show evidence of thermal inversions, where temperature increases with altitude, possibly due to the presence of titanium oxide.
- Mini-Neptunes: Smaller than Neptune but larger than Earth, these planets have thick atmospheres rich in hydrogen and helium. The observations suggest the presence of high-altitude clouds or hazes, which can obscure deeper atmospheric layers.
- Super-Earths: These rocky planets, larger than Earth, present a challenge. While some show hints of atmospheres containing water vapour or hydrogen, others appear to have dry, barren surfaces. The study emphasises the need for more detailed observations to determine their true nature.
The researchers acknowledge several challenges in studying exoplanetary atmospheres with current facilities and space based resources, namely.
- Stellar Activity: Variability in the host star’s energy output can complicate the analysis, starspots and flares can mimic or obscure atmospheric signals, making it difficult to draw definitive conclusions. This is a bigger probnlem in smaller stars such as M and K dwarf stars.
- Instrument Sensitivity: Current technology limits the detection of certain molecules, especially in smaller, Earth-like planets. Future telescopes with enhanced capabilities are essential for more comprehensive studies.
- Clouds and Hazes: The presence of clouds or hazes can mask underlying atmospheric features, leading to ambiguous results, the understanding of their composition and distribution remains a significant hurdle with current technology limitations.
However, despite the limitations faced in analysing planetary atmospheres at this time, the study also underscores the exciting prospects on the horizon, namely.
- Upcoming Missions – Telescopes like the James Webb Space Telescope (JWST) offers a unique resource to probe further, upcoming software updates to the systems aboard JWST and imnprovement in ground based processing offer new and unparalelled improvements in detection capabilities, further the European Extremely Large Telescope (E-ELT), slated to start science operation in mid 2026 promise to deliver unprecedented sensitivity and resolution from the ground and expanding the resources available to study exoplanet atmospheres. The proposed Habital World Observatory will be the first facilioty dedicated to the detection of biosignatures on explanets, combined, they will enable detailed characterisation of exoplanetary atmospheres, potentially identifying biomarkers, or even industrial pollutants that cannot occur naturally, such as CFCs and HCFCs.
- Advanced Modelling – Improved atmospheric models, incorporating complex chemistry and dynamics, will enhance the interpretation of observational data, providing deeper insights into these alien worlds.
- Interdisciplinary Collaboration – Combining expertise from astronomy, planetary science, and atmospheric physics will foster a more holistic understanding of exoplanetary environments.
One of the most profound questions in astronomy is the potential for life beyond Earth, studying exoplanetary atmospheres is critical to this, scientists aim to identify planets within the “habitable zone”, the region around a star where conditions might support liquid water, ans the detection of specific gases, such as oxygen or methane, in the right contexts, could suggest biological activity. However, the study cautions against jumping to conclusions, there are many natural processes that can produce these gases abiotically, thus, a comprehensive analysis, considering all possible explanations, is crucial before claiming evidence of life, and even then, the evidence would need to pass a very strict threshhold for it to be accepted by the science community, the only realistic definitive proof would be the afor mentioned industrial gases (CFCs, HCFCs etc)
The diversity of exoplanetary atmospheres challenges our understanding of planetary formation and evolution, inviting both amateur and professional astronomers to contribute to this exciting endeavour. Public participation in exoplanet observations, through citizen science projects and collaborative networks, plays a vital role. Enthusiasts equipped with modest telescopes can assist in monitoring exoplanet transits, providing valuable data that complements professional research, some, potentially, have the ability to undertake and report spectrographic studies that, whilst not as sensitive as those of the professional sommunity, may offer insights and data that is not otherwise available to professionals.
The characterisation of exoplanetary atmospheres stands at the forefront of astronomical research, bridging the gap between detection and understanding. As technology advances and new missions come online, the dream of identifying a world with conditions suitable for life inches closer to reality. For now, studies like this illuminate the path forward, deepening our appreciation of the cosmos and our place within it.
Image: GLS periodogram of the full dataset of RVs for HD 20794. The strongest peak is at 18.3 d. Also, peaks at 89.6 and 650 d are visible. The periods found in Cretignier et al. (2023) are highlighted by red vertical lines.
