A very small body, far from the sun in an icy outer realm of the solar system, has mysteriously grown an atmosphere — and scientists are stumped as to how and why this happened.
One possibility is that the atmosphere was produced through cryovolcanism, or basically ice volcanoes Another idea is the atmosphere could be related to an impact, but in that case, the impact must have happened relatively recently since such a thin atmosphere would leak into space and vanish within a thousand years.
Furthermore, (612533) 2002 XV93 is significantly smaller than Pluto, being about 310 miles (500 kilometers) across compared to Pluto’s diameter of 1,477 miles (2,377 kilometers).
Pluto is massive enough to retain a thin atmosphere, known as an exosphere, when it is near perihelion (the closest point in its elliptical orbit to the sun). It also retains some ices, such as molecular nitrogen, methane and carbon monoxide, that can sublimate into gases. As Pluto begins to move away from the sun over the course of its 248-year-long orbit, those gases gradually freeze out again back onto the surface.
However, no other object in the Kuiper Belt or beyond has been found to possess an exosphere, although methane outgassing has been detected on Pluto’s fellow dwarf planet, Makemake. The Japanese astronomers, both professional and amateur, who took part in a series of observations of (612533) 2002 XV93 as it occulted — or passed in front of — a star on January 10, 2024, didn’t expect to find an atmosphere either.
If an object such as (612533) 2002 XV93 were truly airless, then the star would instantly wink out as the object passed in front of it from our point of view. However, that’s not what happened.
From Earth, stellar occultations of this sort can be seen along very narrow strips on the surface of our planet. By positioning observers along the edges of that strip, it is possible to determine the size and shape of the occulting body.
To that end, a team made of professional and amateur astronomers, led by Ko Arimatsu of the Ishigakijima Astronomical Observatory of the National Astronomical Observatory of Japan (NAOJ), observed (612533) 2002 XV93‘s occultation of a 15 magnitude star on January 10, 2024 from four different observing sites in Japan. (The magnitude of a star explains how bright it is; the moon’s magnitude, for instance, is about -12 so a magnitude 15 star is quite faint.)
The telescopes the team used included the 3.4-foot (1.05-meter) professional telescope at Kiso Observatory run by the University of Tokyo, and amateur-grade 200mm and 250mm (approximately 8- and 10-inch) telescopes armed with CMOS cameras, which are sensitive enough to detect the the light of the star fading gradually before disappearing behind the limb of the occulting object. This would only happen if there were an atmosphere to attenuate or refract the starlight.
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The exosphere is pretty thin, amounting to a surface pressure of between 100 and 200 nanobars, which is 5 million to 10 million times thinner than Earth’s atmosphere. To place the exosphere on (612533) 2002 XV93 into greater context, Pluto’s thin atmosphere has an average surface pressure of 10 millibars.
What the atmosphere of (612533) 2002 XV93 is made from is uncertain. As is the case with Pluto, a nitrogen-rich atmosphere with traces of methane and carbon monoxide would be expected, but previous observations by the James Webb Space Telescope found no evidence of these particular ices on the surface that could sublimate to form an atmosphere. And at between 40 and 50 degrees above absolute zero, (612533) 2002 XV93 is far too cold for water-ice and carbon-dioxide ice to sublimate into vapor.
With none of these ices available, where has the atmosphere come from? Arimatsu and their team have two possible explanations, but neither is without their problems.
One scenario is that a cometary body has impacted (612533) 2002 XV93. The gases in the atmosphere could then have come from the impactor, but given the rate of loss to space in the low gravity, such an atmosphere would be temporary and would dissipate within a thousand years. If true then this would mean that we are exceptionally fortunate to have observed (612533) 2002 XV93 at around the same time that this presumably rare impact happened.
A huge coincidence, or just good luck?
An alternative idea is that the ices that have sublimated are below the surface, and some form of cryovolcanic activity has released them. However, what could be driving this activity is unknown.
Either way, the existence of the exosphere around (612533) 2002 XV93 changes what we thought we knew about which kinds of objects can support an atmosphere.
“This discovery suggests that the traditional idea that global dense atmospheres form only around larger planets must be revised,” said Arimatsu’s team in their research paper.
The next step is to try and figure out what the composition of the exosphere is, a task ideally suited to the James Webb Space Telescope. Monitoring the density of the exosphere will also offer clues. If the density decreases over the next few years, then the source of the exosphere is likely to be due to an impact as the gases will be leaking into space. If the density remains constant, then the atmosphere is probably being replenished via outgassing.
The findings were published on May 4 in Nature Astronomy.
