🔭 Detection of an atmosphere around a small trans-Neptunian object

Far out in the solar system, well beyond Neptune, a small icy body has just surprised astronomers: it has developed a tenuous atmosphere. This is a true enigma for scientists, who struggle to explain the origin of this gaseous envelope around (612533) 2002 XV93.

This object is a small body in the Kuiper Belt, a plutino in a 2:3 resonance with Neptune. About 310 miles (500 km) in diameter, it is much smaller than Pluto (2,377 km / about 1,477 miles). Yet, like Pluto, it possesses a thin atmosphere, called an exosphere, which appears when it is closest to the Sun in its elliptical orbit.


This exosphere was detected on January 10, 2024, during a stellar occultation. A team of Japanese astronomers, both professional and amateur, observed a magnitude 15 star gradually dimming behind the object. This phenomenon indicates the presence of an atmosphere that refracts and attenuates starlight before the full occultation.

The detected atmosphere is extremely tenuous, with a surface pressure between 100 and 200 nanobars. This is 5 to 10 million times less than Earth's atmospheric pressure. To give an idea, Pluto's average atmospheric pressure is 10 millibars, or 100,000 times higher.

As for its composition, it remains a mystery. On Pluto, nitrogen, methane, and carbon monoxide ices sublimate to form the atmosphere. However, observations from the James Webb Space Telescope have not detected these ices on the surface of (612533) 2002 XV93. Moreover, the temperature, between 40 and 50 kelvins, is far too low to sublimate water ice or carbon dioxide.

Faced with this puzzle, two hypotheses are being considered by Ko Arimatsu's team. The first is a recent impact with a cometary body, whose gases would form a temporary atmosphere that could last up to a thousand years. The second would be cryovolcanic activity releasing subsurface ices, but the mechanism that could trigger it remains unknown.


Whatever the explanation, this discovery challenges preconceived ideas about atmosphere formation. Until now, it was thought that only large planets could retain a dense gaseous envelope. This small object proves that our understanding of atmospheric processes in the solar system needs revision.

The next steps will be to determine the exact composition of the exosphere using the James Webb Space Telescope. By monitoring its density over time, it will be possible to decide between the two scenarios: a decrease would indicate an impact, while a stable density would suggest a continuous supply from outgassing. The results were published in Nature Astronomy.

The exosphere

The exosphere is the outermost layer of a celestial body's atmosphere. In this region, gas particles are so sparse that they can escape into space if their speed is sufficient. On Earth, the exosphere begins at about 310 miles (500 km) altitude, but extends up to several thousand miles (kilometers). Molecules are very rare there, with collisions almost nonexistent.

In the solar system, exospheres exist around bodies like Pluto, Mercury, or the Moon. They generally form by sublimation of surface ices, by micrometeorite impacts, or by internal outgassing. Each exosphere is unique, depending on the body's composition and its environment.