By Ashley Strickland, CNN
(CNN) — Astronomers have detected a thin atmosphere around a tiny celestial body in the outer solar system for the first time — an object previously thought to be too small to support the presence of an atmosphere.
Thousands of frozen, rocky bodies called trans-Neptunian objects, or TNOs, exist in the Kuiper Belt on the edge of our solar system, remnants from its formation 4.5 billion years ago.
The dwarf planet Pluto is the largest of these TNOs, so named because they’re found beyond the orbit of Neptune.
The frigid temperatures and weak surface gravity of the small bodies has long caused astronomers to believe they aren’t capable of retaining atmospheres — with the exception of Pluto, which has a thin one. Atmospheres, especially dense ones, typically form around large planets or moons, including Saturn’s biggest satellite, Titan.
Meanwhile, dwarf planets Eris, Haumea, Makemake and dwarf planet candidate Quaoar, the largest TNOs after Pluto, don’t appear to have atmospheres.
During a rare observation opportunity, astronomers in Japan spotted the thin shell of an atmosphere around a TNO known as (612533) 2002 XV93, according to a study published Monday in the journal Nature Astronomy.
While Pluto has a diameter of 1,477 miles (2,377 kilometers), 2002 XV93 only spans about 311 miles (500 kilometers) across.
The unexpected discovery — made by Dr. Ko Arimatsu, associate professor and senior lecturer at the National Astronomical Observatory of Japan, and his colleagues — could offer an unprecedented glimpse into how an atmosphere forms and remains around a small object, and change how astronomers think about objects in the Kuiper Belt.
Seizing the observation opportunity
As January 2024 neared, Arimatsu and his colleagues prepared for the unique chance to observe a TNO as it passed in front of a bright star, as seen from Japan.
2002 XV93 has a standard orbit for a Kuiper Belt object and is smaller than a dwarf planet, so it wasn’t considered to be different from other TNOs.
But such moments when a TNO is illuminated by a star in the cosmic background, called stellar occultations, are rare opportunities to study the size, shape and features of a small, distant object, Arimatsu said. The researchers set up at three different locations across Japan, using observatories in Kyoto and the Nagano Prefecture, as well as a citizen scientist-run telescope in Fukushima.
The star’s light gradually faded as the TNO moved in front of it, suggesting the presence of of an atmosphere. If an object has no atmosphere, a star disappears and reappears much more sharply.
“The observation data showed a smooth change of the star’s brightness near the edge of the shadow, lasting about 1.5 seconds,” Arimatsu wrote in an email. “This kind of smooth brightness change is naturally explained if the starlight was bent by a very thin atmosphere around the object.”
The researchers calculated that 2002 XV93 has an atmosphere about 5 million to 10 million times thinner than Earth’s — and suspect two possibilities as to what created it.
The atmosphere could be the product of cryovolcanoes on the small, icy body, which release internal gas such as methane, nitrogen or carbon monoxide from beneath its surface. Or, another Kuiper Belt object such as a comet might have struck 2002 XV93, also releasing gases from the subsurface.
If the atmosphere was created due to impact, it may only last for several hundred years, Arimatsu said. But if regular cryovolcanic activity routinely replenishes the atmosphere with the release of gas, it could last much longer, he added.