The recent discovery of a strange atmosphere on a small icy world beyond Pluto has sparked excitement and intrigue among astronomers. This finding challenges our understanding of the outer Solar System and raises questions about the nature of these distant celestial bodies. The object in question, 2002 XV93, is a trans-Neptunian object with a size of approximately 500 kilometers, much smaller than Pluto. Its small size and weak gravity make it difficult to sustain an atmosphere, as the gases would escape too quickly. However, observations of a stellar occultation, where the object passed in front of a distant star, revealed a gradual dimming of the star's light, suggesting the presence of an atmosphere. This discovery is particularly fascinating because it contradicts the expectation that small icy bodies like 2002 XV93 should not have the capability to maintain an atmosphere.
The team of astronomers, led by Arimatsu, organized observations at four stations along the predicted shadow path, capturing positive occultation chords and light curves. The results showed a gradual dimming of the starlight at both ingress and egress, lasting about 1.5 seconds. This pattern cannot be explained by diffraction or the finite size of the occulted star, indicating the presence of an atmosphere. The researchers then turned to atmospheric modeling, using ray-tracing calculations to test simplified atmospheres dominated by methane, nitrogen, or carbon monoxide. Each model reproduced the light curve far better than an atmosphere-free case, with surface pressures ranging from 124 to 177 nanobars.
However, the sustainability of this atmosphere is questionable. Bodies in the temperature range of 40 to 50 kelvin typically require hypervolatile ices to maintain an atmosphere, but observations with the James Webb Space Telescope showed no prominent absorption features from hypervolatile surface ices on 2002 XV93. This suggests that most of the surface volatiles have already been lost. The analysis also indicates that methane, nitrogen, or carbon monoxide would leak away quickly from an object with a radius near 250 kilometers, making a permanent atmosphere highly unlikely.
The research points to two main possibilities for the origin of the atmosphere. One is outgassing tied to cryovolcanic activity, where material from inside the body reaches the surface. However, 2002 XV93 is too small for long-lived cryovolcanic activity, and its limited heat budget suggests a thick, cold outer shell. The other possibility is a recent impact, where a small Kuiper Belt or Oort Cloud object could release gas directly or excavate buried volatiles from below the surface. However, the chance of such an impact occurring over 100 years is relatively low.
This discovery has significant implications for our understanding of the outer Solar System. It suggests that some smaller icy bodies may briefly acquire atmospheres through interior activity or chance collisions, then lose them on timescales short enough to make them hard to catch. Repeat observations are crucial to identifying the origin of the atmosphere, with a steady decline in pressure over time favoring an impact origin, while persistent or seasonal changes would point more toward internal outgassing. James Webb spectroscopy could also help identify the molecules present.
The success of this research highlights the importance of coordinated campaigns involving both professionals and citizen astronomers using small telescopes and fast CMOS cameras. It demonstrates that future monitoring does not depend on a single giant facility, but rather on catching the next shadow at the right moment with enough eyes on the sky. The findings are available in the journal Nature Astronomy, adding to our understanding of the diverse and dynamic nature of the outer Solar System.
