Astronomers Watch an Icy Body Transform Into a Comet 3 Billion Miles Away

Astronomers have witnessed a small, icy body that orbits the sun out beyond Jupiter transforming into a comet, providing a missing link between comets and inert bodies called centaurs. The object, called 450P/LONEOS, is a centaur, a class of inactive bodies that orbit the sun between Jupiter and Neptune after wandering in from the Kuiper Belt beyond Pluto. Because a centaur's orbit crosses the path of at least one giant planet, its orbit is unstable over millions of years, and it has long been suspected that centaurs pushed closer to the sun turn into Jupiter-family comets, comets with orbital periods of less than 20 years. No one had ever directly observed that transition.

Astronomers led by planetary scientist Charles Schambeau of the University of Central Florida used the Gemini North telescope in Hawaii to watch a cloud of gas and dust, called a coma, form and brighten around 450P's solid nucleus between 2019 and 2024, as the object arrived at perihelion, its closest point to the sun. The object's current orbital period is 22 years, so it isn't a Jupiter-family comet yet, but one more gravitational nudge from a giant planet could push it there.

By tracking 450P's orbit back before its 2004 discovery, the team found it had passed within 2.9 million miles of Saturn in 1992. That encounter significantly shortened its orbit, moving its perihelion to 5.4 astronomical units, only slightly beyond Jupiter's orbit at 5.2 astronomical units, and exposing it to far more solar heating than before. "That increased solar heating can warm the surface and the subsurface layers of the nucleus," Schambeau said. "As those layers heat up, volatile ices or trapped gases can be released, which can drag dust away from the surface and produce a coma."

Using the James Webb Space Telescope, the team analyzed the coma and found carbon dioxide gas along with hints of crystalline water-ice particles, but no water vapor. "At 450P's distance from the sun, the nucleus is too cold for normal water-ice sublimation to be the main activity source, so detecting carbon dioxide gives us an important clue about what is powering the coma," Schambeau said. The crystalline ice suggests some of the material has been thermally altered from its pristine, amorphous state, which likely dates back to the solar system's formation 4.5 billion years ago.

Only a small fraction of centaurs have ever shown signs of activity, making 450P's recent perihelion a rare opportunity to study an early stage of the transition. "Centaurs are scientifically important because they are thought to be transitional objects that originated further out in the solar system and are slowly evolving toward becoming Jupiter-family comets," Schambeau said.