Astronomers Detect a Thin Atmosphere on (612533) 2002 XV93, the First Trans-Neptunian Object Beyond Pluto Found With One

Overview

A Japanese-led team of astronomers has announced what it describes as the first detection of an atmosphere on a trans-Neptunian object other than Pluto. The discovery, made by watching the small icy world (612533) 2002 XV93 pass in front of a distant background star, was published on May 4 in Nature Astronomy and reported by Sci.News, Space.com, Science News, and CBS News.

The team, led by Ko Arimatsu of the Ishigakijima Astronomical Observatory at the National Astronomical Observatory of Japan, says the data point to an extremely thin gas envelope around an object only about 500 kilometers across — far smaller than any solar-system body previously thought capable of holding onto an atmosphere.

What the team observed

On January 10, 2024, telescopes at three Japanese locations recorded the starlight of a distant background star gradually dimming for roughly 1.5 seconds as 2002 XV93 passed in front of it, according to Science News. The observing setup combined a professional 1.05-meter telescope at the Kiso Observatory with amateur 200-millimeter and 250-millimeter telescopes equipped with CMOS cameras, Space.com reports.

In a stellar occultation, a sharp on-off light curve is the signature of a body with no atmosphere. A more gradual dip indicates that starlight is being refracted by a layer of gas. The team interprets the curve recorded that night as a refractive signature of a thin atmosphere.

How thin the atmosphere is

The paper places the surface pressure of 2002 XV93 between 100 and 200 nanobars, as reported by Space.com. For context, CBS News notes the envelope is between 5 million and 10 million times thinner than Earth’s atmosphere, and 50 to 100 times thinner than Pluto’s own thin atmosphere. CBS News also reports that the object was more than 3.4 billion miles from the Sun at the time of the study, farther out than Pluto.

The likely chemical makeup, per CBS News, is methane, nitrogen, or carbon monoxide — the same volatile species that drive Pluto’s atmosphere.

What the object is

(612533) 2002 XV93 is classified as a plutino, a minor planet locked in a 2:3 orbital resonance with Neptune that completes two solar orbits for every three Neptune orbits, as CBS News explains. Sci.News reports a diameter of approximately 500 kilometers; Science News gives a slightly lower estimate of about 470 kilometers. Either way, it sits in a size class that astronomers had assumed could not retain volatiles against escape.

Until now, in Arimatsu’s words to Science News, “Pluto was the only trans-Neptunian object with a confirmed atmosphere.” Earlier occultation searches at much larger bodies — including Eris, Haumea, Makemake, and Quaoar — have found no detectable atmospheres.

How the gas could be there at all

The team offers two possible explanations for how an object this small could host any atmosphere at all. One is ongoing cryovolcanism, with subsurface ices venting to the surface; the other is a recent impact, in which a comet-like body slammed into 2002 XV93 and released trapped gases, per Space.com and CBS News.

Neither mechanism is comfortable for the longer term. Sci.News reports that the team calculates the atmosphere would “dissipate in less than 1,000 years unless it is somehow replenished.” That implies either that 2002 XV93 is currently active in a way that surprises planetary scientists, or that astronomers caught it shortly after a one-off event.

Arimatsu told Space.com that “This discovery suggests that the traditional idea that global dense atmospheres form only around larger planets must be revised.”

Outside reaction

Not every planetary scientist is ready to call the case closed. Alan Stern of the Southwest Research Institute, who led NASA’s New Horizons mission to Pluto, told CBS News, “This is an amazing development, but it sorely needs independent verification.”

What we don’t know

• The team has not pinned down which gas dominates the atmosphere; methane, nitrogen, and carbon monoxide are listed as candidates by CBS News, but spectroscopic confirmation has not been reported.
• Whether the signal indicates a steady, currently active atmosphere or a transient post-impact cloud cannot be settled from a single occultation. The dissipation timescale of less than 1,000 years cited by Sci.News leaves both interpretations open.
• It is unclear whether other plutinos of similar size also host atmospheres that prior surveys missed, or whether 2002 XV93 is a special case. Independent occultation campaigns at additional plutinos would be needed to settle that question.