Orcus – the anti-Pluto

Posted: July 16, 2015 by oldbrew in Celestial Mechanics, solar system dynamics
Tags: ,

Orcus in blue, Pluto in red, Neptune in grey [credit: Eurocommuter / Wikipedia]

Orcus in blue, Pluto in red, Neptune in grey
[credit: Eurocommuter / Wikipedia]

The ‘anti-Pluto’ label arose from the fact that the orbit of probable dwarf planet Orcus looks like a mirror-image of that of Pluto (as shown above), and is less than three years weeks shorter than Pluto’s 248 years. It also has its own relatively large moon – or binary neighbour – just like Pluto. [More details about the graphic here]

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.

Comments
  1. To me the fact that Orcus is classified as a KBO and Pluto as a dwarf planet underscores the silliness of the current classification system. We always have to categorise, it seems, but in the face of the many new discoveries about the solar system that we have made in the last generation I can’t help thinking that this might lead us to miss other patterns in the way it all works.

  2. oldbrew says:

    It’s well up the list of ‘possible dwarf planets’…
    http://en.wikipedia.org/wiki/List_of_possible_dwarf_planets

    Expert Mike Brown reckons anything with a diameter of 400 km. or more is a very strong dwarf planet candidate. Orcus is about twice that, if not more.

  3. gymnosperm says:

    It seems a bit insensitive to kick “dwarf” planets out of the planet club.The last I heard Pluto was considered an asteroid. Asteroids are usually depicted as, rude misshapen piles, to quote Sir Thomas Jefferson. Pluto appears anything but.

  4. Bloke down the pub says:

    The ‘anti-Pluto’ label arose from the fact that the orbit of probable dwarf planet Orcus looks like a mirror-image of that of Pluto (as shown above), and is less than three years shorter than Pluto’s 248 years.

    Orcus is like an anti-Pluto, however, because the two objects always remain across the Solar System from each other.’

    Is it a silly question to ask how, if the orbit of Orcus is getting on for three years shorter than Pluto, how come it will ‘always remain across the solar system’ from Pluto?

  5. oldbrew says:

    Bloke: no, it’s a good question.

    Assuming the orbit periods are correct and stay the same, the cycle of synodic periods could in theory be about 22215 years. At that time 89.46~ Pluto orbits = 90.46~ Orcus orbits i.e. one more. [but see ‘Update’ below]

    So something would have to change to prevent a future close(r) encounter, on those numbers. Of course there could be other complications like precession that might muddy the waters. Hard to measure such things when one orbit takes nearly 2.5 centuries, although techniques always improve.

    Note also the orbits have different orientations to prevent any problems.
    ——-
    Update: looks like Wikipedia’s Orcus page is out of date, with the JPL small-body database browser giving Orcus an orbit period of 247.88 years, or about 15 days less than Pluto.[JPL solution date = 2015-Jun-04]

    That would greatly extend the time before any putative close(r) encounter.

  6. p.g.sharrow says:

    I think you should consider Orcus and Pluto as being satellites or at least part of the swarm of bodies that occupy the 8th position in the solar system, of which Neptune is the main occupant. Their actual orbits are too erratic to exactly plot due to their gravitational interactions with Neptune. pg

  7. p.g.sharrow says:

    @oldbrew; Quite a collection and I would suspect more to be discovered.

    I should have said that this is the 9th orbital position as the asteroid belts occupy the 5th position. pg

  8. Tom van Flandern in “Dark Matter, Missing Planets & New Comets” regards both the asteroid belt and the Kuiper belt due to exploded large solar system bodies. His theories could explain the rotation of Venus by a collision, and the size of Mars which could have been a moon of the exploded body and the orbit of Neptune.

  9. oldbrew says:

    cementafriend – Tallbloke prefers a different theory of the rotation of Venus:
    https://tallbloke.wordpress.com/2014/06/28/venus-surface-temp-correctly-predicted-from-lapse-rate-in-1967-but-is-it-the-whole-story/comment-page-1/#comment-96231

    NB this is off-topic so we don’t want to get into Venus theories in detail here.

    pg: ‘There is something out there — part 1’ – by Mike Brown (the day Brown spotted Sedna)
    http://www.mikebrownsplanets.com/2010/10/there-is-something-out-there.html

    Brown’s lecture that day:
    “Come back and take my class again next year, and I’ll have it all figured out,” I confidently told them.

    That was seven years ago. Any poor student taking my advice would have sat through the last six years of lectures and still not learned what put Sedna where it is, since I still don’t know the answer.

  10. oldbrew says:

    Review of Mike Brown’s book: ‘”How I Killed Pluto and Why It Had It Coming” by the Wall Street Journal

    The Man Who Made a Planet Vanish – An astronomer writes wittily about how his discoveries accidentally earned Pluto an unpleasant demotion
    http://www.wsj.com/articles/SB10001424052748704243904575630683559145518