Orcus was discovered on 17 February 2004 by Mike Brown, Chad Trujillo and David Rabinowitz, although it later turned up on photographs taken as early as 1951. It was named after a Roman god of the underworld, a fitting partner for Pluto, which carries the name of the Greek one.
The anti-Pluto
The link with Pluto goes deeper than the name. Both are plutinos: they circle the Sun exactly twice for every three orbits of Neptune, a resonance that keeps them safe from close encounters with the giant planet. Orcus’s orbit has almost the same size, tilt and stretch as Pluto’s, but it is turned the other way. When Pluto is at its closest to the Sun, Orcus is near its farthest, and the two always stay on roughly opposite sides of the Sun. Astronomers call it the anti-Pluto.
A giant moon
The resemblance continues with its moon. In 2005 Hubble images revealed Vanth, named after an Etruscan spirit who guides the dead to the underworld. Vanth is about 443 kilometres across, nearly half as wide as Orcus itself, much as Pluto’s moon Charon is half the size of Pluto. Measurements with the ALMA radio telescope show Vanth has about 16% of Orcus’s mass. The pair circle each other every 9.7 days.
No known planet or dwarf planet has a moon so heavy compared with itself. Michael Brown and Bryan Butler, who made the ALMA measurements, say the ratio fits Vanth being a largely intact impactor left over from a giant collision, and that the pair has probably settled into a double synchronous state, each keeping the same face turned towards the other. Vanth is also much darker than Orcus: it reflects about 8% of the light that falls on it, against roughly 23% for Orcus.
Vanth’s size was measured directly on 7 March 2017, when it passed in front of a star. The event had been predicted for Orcus, and observers watched from five sites in North and South America. Two of them saw the star dim, and further imaging showed that the shadow was Vanth’s. The timings give its diameter of about 443 kilometres, with an uncertainty of about 10 kilometres.
Weighing the pair gives Orcus a mass of about 5.5 × 10²⁰ kilograms. Its surface is pale and greyish, unlike the red of Sedna or Gonggong, and the only ice identified on it with certainty is water ice.
Crystalline ice
Orcus’s water ice is crystalline, a form that may point to a surface renewed at some point in its history. In 2008 Barucci and colleagues, using the SINFONI instrument on ESO’s Very Large Telescope, found crystalline water ice and possibly ammonia ice, and suggested geological activity. In 2010 Audrey Delsanti and colleagues confirmed the crystalline ice with the Keck and Gemini telescopes and found that ammonium and traces of ethane, mixed with the water ice, matched the spectrum better than water ice alone. Their thermal models suggest that radioactive heating alone could have melted Orcus’s core, and that a past cryovolcanic event probably brought the crystalline ice to the surface.
Orcus takes about 245 years to orbit the Sun, ranging between 30 and 48 AU.