Pholus

Pholus is a splinter of the deep-frozen outer Solar System, and one of the most intensely coloured objects known to astronomy. Discovered on 9 January 1992 by David Rabinowitz with the Spacewatch survey at Kitt Peak, it was the second centaur ever found, after Chiron — an icy body looping through the realm of the giant planets on a long, eccentric orbit.

About 185 km across, Pholus is famous for its colour: a deep, startling red, redder than almost anything else in the Solar System. Unlike Chiron, it shows no sign of cometary activity, keeping its ancient surface on quiet display.

The reddest object in the Solar System

Pholus's extreme red colour is thought to come from tholins — dark, reddish organic compounds built up over billions of years as radiation and faint sunlight slowly cook the simple ices and carbon-bearing molecules on its surface. That colouring is a signature of primitive, little-altered material from the outer Solar System, making Pholus a window onto the raw ingredients from which the outer worlds formed.

A giant-crossing orbit

Pholus travels a steeply inclined, highly elongated path that carries it from inside the orbit of Saturn out beyond Neptune, taking about 92 years to complete one loop. Like all centaurs, its orbit is unstable over long timescales — Pholus is a transient visitor, gravitationally scattered inward from the Kuiper Belt and destined eventually to be flung elsewhere.

Quiet, unlike its cousin

Where Chiron surprised astronomers by developing a comet-like coma, Pholus has stayed inert, showing no measurable activity even near the closest point of its orbit. That difference — two similar bodies, one active and one dormant — is part of what makes the centaurs such a useful population for understanding how icy bodies wake into comets.

Observing Pholus

Pholus is a faint, distant object, far beyond naked-eye or binocular reach and a challenge even for large amateur telescopes. Its importance is scientific: as a nearly pristine, deeply reddened relic, it helps astronomers read the chemistry of the Solar System's cold outer edge.