A new era in the study of long-period comets

Long-period comets have long been considered fleeting visitors to the inner Solar System, but new discoveries are changing that view. Thanks to modern sky surveys and the powerful capabilities of the James Webb Space Telescope, astronomers are uncovering unexpected cometary activity at great distances from the Sun. The case of comet C/2024 E1 (Wierzchos), which displayed CO2 outgassing at 7.2 au but lacked CO, challenges our understanding of these icy bodies. Could this reveal clues about the origins and evolution of comets? In today’s Astrobito, we explore how JWST is transforming our view of long-period comets and what this means for the study of the early Solar System.

Paper details:

Title: First JWST spectrum of distant activity in Long Period Comet C/2024 E1 (Wierzchos)

Authors: Colin Snodgrass, Carrie E. Holt, Michael S. P. Kelley, et. al.

First-author institution: Institute for Astronomy, University of Edinburgh, UK

Status: Open access in arXiv and currently under review at MNRAS

Comets, those icy wanderers from the outer reaches of the Solar System, have long fascinated humanity with their dramatic appearances and mysterious origins. Traditionally, long-period comets (LPCs)—comets that take longer than 200 years to complete a single orbit around the Sun—have been thought of as ephemeral visitors, brightening only briefly when they approach the inner Solar System. However, as modern sky surveys continue to discover these icy objects further and further from the Sun, we're realising that the story of LPCs might be more complex—and more exciting—than we once thought.

Take C/2024 E1 (Wierzchos), a comet discovered in March 2024 by the Catalina Sky Survey. When it was first spotted, it was already at a distant 8 astronomical units (au) from the Sun—beyond the orbit of Jupiter. This is far beyond the typical "water ice sublimation" region, where many comets display their most spectacular outgassing behaviours. At this distance, temperatures are so cold that water ice doesn't sublimate efficiently. Instead, comets like E1 are thought to be driven by other, more volatile gases such as carbon monoxide (CO) and carbon dioxide (CO2).

Comet C/2024 E1 (Wierzchos) trajectory (in red) compared with the orbit of other bodies in the Solar System. The comet is expected to be closest to the Sun at ~0.7 au (perihelion) in January 2026. Figure adapted from Astro vanBuitenen.

E1, however, has given scientists a glimpse into the intriguing complexity of cometary activity, thanks to its observation with the James Webb Space Telescope (JWST). At the time of observation, E1 was 7.2 au from the Sun. The observations revealed something extraordinary: a strikingly clear signature of CO2 outgassing, but a surprising lack of CO—typically a dominant species in distant comets. This discovery challenges existing models and raises new questions about how volatile materials are distributed within a comet's nucleus and how they evolve over time.

The changing face of long-period comets

In the past, comets like E1 would have been dismissed as inactive or "dormant" based on their distant location and the inefficient sublimation of water ice. But as observations from new sky surveys and telescopes like JWST have shown, LPCs can display surprising activity even at large heliocentric distances. This growing discovery of distant, active comets marks a turning point in how we think about the nature of these icy bodies.

The presence of CO2 at a distance of 7 au is not entirely surprising, as CO2 transitions from solid to gas phase bypassing the liquid state (sublimation) at lower temperatures than water ice. However, the absence of CO, which is often found in greater amounts in comets further from the Sun, is a new twist. This could be due to the comet's history or its "evolutionary path"—the idea that comets formed closer to the Sun and have since traveled outward, undergoing thermal processing along the way. As they journey through the cold outer reaches of the Solar System, these icy bodies can undergo significant changes. For E1, the lack of CO suggests that its icy materials have been altered—perhaps depleted or hardened—by past solar radiation and cosmic rays during its long journey through space.

Infrared spectrum of comet C/2024 E1 (Wierzchos) highlighting the most prominent absorption features from H2O ice and CO2 and the absence of any CO features (which are expected in LPCs). Figure adapted from Figure 1 in the main paper.

How JWST is changing the game for cometary science

The JWST's ability to capture the detailed spectra of distant comets is revolutionising our understanding of these icy visitors. Its sensitivity to infrared wavelengths has allowed astronomers to detect emissions from molecules like CO2 that were previously challenging to observe from Earth-based telescopes. JWST’s ability to map the spatial distribution of these emissions is also helping scientists understand how the comet’s coma—the cloud of gas and dust surrounding the nucleus—evolves as the comet approaches the Sun.

For example, the team behind the study of E1 observed a clear, highly symmetrical distribution of CO2 emission in the coma, suggesting that the outgassing is more evenly distributed across the comet’s surface. This could point to a relatively young comet, one that hasn’t undergone extensive alteration yet. In contrast, older comets or those that have been more active in the past often show a more heterogeneous coma with localised outgassing features.

H2O-ice and CO2 spatial distribution around comet E1. The almost symmetric distribution of both species is characteristic of relatively young comets. Figure adapated from Figure 4 in the main paper.

By tracking comets like E1 over time, scientists can see how their activity changes as they approach perihelion (the closest point to the Sun in their orbit). For E1, which is expected to make its perihelion passage in January 2026 at a distance of just 0.7 au, this is a unique opportunity to study how distant comets transition into the more familiar, water-driven activity we typically observe in comets closer to the Sun.

New insights into the origins and evolution of comets

The study of LPCs like E1 offers a glimpse into the origins and evolution of these ancient objects. The data collected by JWST provides a rare opportunity to understand how volatile species like CO and CO2 evolve as comets travel through the outer Solar System. For example, understanding why CO is absent in E1 could help researchers model how comets lose certain ices over time, offering clues about the processes that shape their nuclei.

Future observations of E1—especially as it nears perihelion—will provide crucial information about the comet’s activity and outgassing patterns. The hope is that by comparing E1 to other distant comets observed with JWST, we can piece together the broader story of cometary evolution, which could, in turn, shed light on the conditions of the early Solar System. After all, these comets are thought to be some of the oldest remnants of the material that formed the planets—and the fact that we’re only now able to study them in such detail speaks to just how much we have yet to learn.

In the coming years, we will be watching comets like E1 more closely than ever before. With the launch of the Vera C. Rubin Observatory and the ongoing work of missions like ESA’s Comet Interceptor, we may soon be able to catch up with these distant travelers and observe them in real time as they make their way through the Solar System. Whether they’re active or dormant, young or old, these icy bodies have a story to tell—and thanks to JWST, we’re beginning to hear it.

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