Chiron

Chiron was the object that created a whole new category. Discovered on 1 November 1977 by Charles Kowal at Palomar Observatory, it orbits far out between Saturn and Uranus and refused to fit the tidy line astronomers had drawn between rocky asteroids and icy comets. In time it became the first known centaur — a family of icy bodies, named for the half-horse, half-human creatures of myth, that share the hybrid nature of their namesakes.

Chiron behaves like both an asteroid and a comet, and it is officially catalogued as both: minor planet 2060 Chiron and periodic comet 95P/Chiron. At roughly 200 km across, it is large enough to be considered a possible dwarf planet.

Neither asteroid nor comet

When it was found, Chiron looked like an ordinary, if distant, asteroid. Then in 1988-89, as it neared the closest point of its orbit, astronomers Karen Meech and Michael Belton watched it brighten and sprout a coma — the fuzzy halo of gas and dust that defines a comet, produced as surface ices warm and sublimate. An object that large showing cometary activity so far from the Sun was unprecedented, and it earned Chiron its second, cometary designation.

A comet with rings

In the 2010s and early 2020s, astronomers timing Chiron as it passed in front of background stars found something stranger still: evidence of a ring system around it. That makes Chiron one of only a handful of small bodies known to have rings — and the only comet among them. How so small and active an object holds onto a ring remains an open question.

A wandering, unstable orbit

Chiron circles the Sun once every roughly 50 years on an eccentric path that carries it from near Saturn out toward Uranus. Centaur orbits like this are chaotic and short-lived by cosmic standards — gravitational nudges from the giant planets will, over a few million years, eventually fling Chiron inward to blaze as a bright comet or outward again. It is best understood as an object in transition, escaped from the Kuiper Belt and on its way to somewhere else.

Observing Chiron

Chiron is far too faint for the naked eye or binoculars; at roughly 15th to 18th magnitude it is a target for larger telescopes and astrophotography. It matters less as a sight than as an idea — the body that first showed the outer Solar System is full of worlds that blur the old categories.