A team of professional and amateur Japanese astronomers found evidence for a thin atmosphere around a small body in the outer Solar System. The object is so small that it should not have a sustainable atmosphere, raising questions about when and how the atmosphere formed. Future observations to better characterize the atmosphere will help solve these mysteries.
In the cold reaches of the outer Solar System lie thousands of small objects known as trans-Neptunian objects (TNOs) because they lie outside the orbit of Neptune. A thin atmosphere has been observed around Pluto, the most famous TNO, but studies of other TNOs have yielded negative results. Most TNOs are so cold, and their surface gravity so weak, that they are not expected to retain atmospheres.
But astronomers like to expect the unexpected, so they took advantage of a lucky "natural experiment" to look for an atmosphere around a TNO known as (612533) 2002 XV93. This object, abbreviated as 2002 XV93, has a diameter of approximately 500 km. For reference, Pluto's diameter is 2,377 km. The orbit of 2002 XV93 is such that, as seen from Japan, it passed directly in front of a star on January 10, 2024. As the star disappears behind 2002 XV93, it might gradually fade, indicating that the light is being attenuated as it passes through a thin atmosphere; or it might suddenly wink out as it slips behind the solid surface of the TNO.
A team of professional and amateur astronomers, led by Ko Arimatsu at NAOJ Ishigakijima Astronomical Observatory, observed the star as 2002 XV93 passed in front of it from multiple sites in Japan. The obtained data are consistent with attenuation by an atmosphere.
Calculations show that the atmosphere found around 2002 XV93 is expected to last less than 1000 years unless it is replenished. So it must have been created or replenished recently. Observations by the James Webb Space Telescope show no signs of frozen gases on the surface of 2002 XV93 that might sublimate to form an atmosphere. One possibility is that some event brought frozen or liquid gases from deep inside the TNO to the surface. Another possibility is that a comet crashed into 2002 XV93, releasing gas that formed a temporary atmosphere. Further observations are needed to distinguish between these two scenarios.
