Wikipedia says: 90482 Orcus is a Kuiper belt object with a large moon, Vanth. It was discovered on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified. It is probably a dwarf planet.
Orcus is a plutino, locked in a 2:3 resonance with Neptune, making two revolutions around the Sun to every three of Neptune’s. This is much like Pluto, except that it is constrained to always be in the opposite phase of its orbit from Pluto: Orcus is at aphelion when Pluto is at perihelion and vice versa.
Moreover, the aphelion of Orcus’s orbit points in nearly the opposite direction from Pluto’s, although the eccentricities and inclinations are similar. Because of these similarities and contrasts, along with its large moon Vanth that recalls Pluto’s large moon Charon, Orcus has been regarded as the anti-Pluto. This was a major consideration in selecting its name, as the deity Orcus was the Etruscan equivalent of the Roman Pluto, and later became an alternate name for Pluto.
NASA’s ‘Astronomy Picture of the Day’ described it like this:
‘A newly discovered object in the outer Solar System moves like an anti-Pluto. 90482 Orcus was first discovered in 2004 and is slightly smaller than Pluto, although still one of the largest Kuiper belt objects known. Orcus may one day have the same IAU designation as Pluto: a dwarf planet. Orcus and Pluto have similar orbits: each achieves nearly the same maximum and minimum distances from the Sun, each orbits on a similarly shaped ellipse, and each orbital ellipse is tilted toward the other planet’s orbital ellipse by roughly the same angle. The great mass of Neptune causes each to circle the Sun twice for every three Neptune orbits. Orcus is like an anti-Pluto, however, because the two objects always remain across the Solar System from each other.’
It turns out that:
41 Orcus orbits = 61 Neptune orbits = 20 Orcus-Neptune conjunctions (61 – 41), and 40:60 = 2:3 i.e. one orbit less for each body would give the exact resonance – which never happens, for stability reasons [using Wikipedia data].
UPDATE: JPL Small-Body Database Browser gives Orcus an orbit period of 247.88 years. In that case:
119 Orcus orbits = 179 Neptune orbits = 60 Orcus-Neptune conjunctions (179 – 119), and 120:180 = 2:3 i.e. one orbit less for each body would give the exact resonance.
Note they say ‘Neptune causes each to circle the Sun twice for every three Neptune orbits.’ Planetary resonance is implied.
For a beginner’s guide to Pluto and the other main dwarf planets, try the manic but informative commentary and cartoons of Edgar on Youtube.