Minor bodies: Pluto, trans-Neptunian Objects and centaurs

What's outside of Neptune's orbit? Lots of stuff but it's hard to see.

This was predicted since the mid-20th century - it didn't make sense that there would be so much mass in the giant planets then suddenly nothing outside of them. Models of Solar System formation suggested that mass ought to drop off more gradually. The discovery of Pluto, the "ninth planet" in 1930 did little to solve the problem. This hypothetical region of the Solar System was called the Kuiper Belt after Gerard Kuiper, who performed pioneering work in the concept.

Major trans-Neptunian objects from Wikipedia


Orbital characteristics of trans-Neptunian objects from Wikipedia

Trans-Neptunian Structure:

It now appears that Kuiper's vision of a belt of small objects orbiting the Sun in the plane of the ecliptic in regular orbits was somewhat naive. As the roster has filled out, researchers have struggled to describe the unfolding patterns. Terminology is unsettled, but we see four general classes of objects based on orbital characteristics:

Artistic rendering of Planet Nine from Wikipedia (The Sun's "halo" is the orbit of Neptune.)
Origins: Current speculation is that object of the Kuiper Belt and Scattered Disk occupy their orbits for different reasons:

  • More to come: Evidence for an even more remote reservoir of objects will be taken up in the next lecture.

    Kuiper Belt Objects up close:

    Robot spacecraft have visited seven TNOs in all:

    Triton as seen by Voyager II from Wanderingspace.net. Southern hemisphere on the left.


    Triton is Neptune's largest moon by far (2705 km diameter), discovered by amateur astronomer William Lassell in 1846 - just 17 days after the discovery of Neptune itself.

    Triton specs:

    Triton is strange:

    Because of its retrograde orbit and tidal interactions with Neptune, it is slowly losing its orbital momentum, and descending closer to Neptune. It will cross Neptune's Roche limit in roughly 3.6 billion years, yielding an impressive ring system.

    Triton as seen by Voyager II from Wanderingspace.net. Southern hemisphere on the left.


    Pluto has been called the ninth planet, a dwarf-planet, and the first known Kuiper Belt object. Whatever it is, its 2:3 resonance with Neptune clearly places it among the resonant object plutinos. Even prior to New Horizons' 2015 fly-by, Pluto was a world of odd superlatives:

    Pluto at sunset as seen by New Horizons from The Verge.
    Water-ice mountains meet nitrogen glaciers beneath the thin air.
    New Horizons: New Horizons is still slowly returning data from its 2015 fly-by, and surprises continue to pile up. (See Stern, et al., 2015) Pluto's surface reveals geologic activity:

    Charon from Wikipedia


    Charon is roughly half Pluto's diameter and larger than 1 Ceres. It has its own issues, including:

    In all, Charon seems to have a differentiated interior and active tectonics. Stay tuned.

    And for the future, 2014 MU69

    Recall that Pluto's orbit brings it (and Charon) inside the orbit of Neptune at times. If you complain that all three of the trans-Neptunian objects considered here at some point or another actually not trans-Neptune, quiet your concerns. New Horizons has been redirected toward a 2019 fly-by of the classic Kuiper Belt object 2014 MU69. At last, we will glimpse a truly trans-Neptunian body.


    Centaur color and albedo from Wikipedia
    Chiron and friends: One last thing - we did not address one component of the graph of trans-Neptunian objects presented above. In 1977, the object Chiron was discovered. An unremarkable 233 km long potato except for its orbit, which crosses those of Saturn and Uranus. At its closest approach to the sun, it acts slightly like a comet, developing an atmosphere and a tail. Subsequently many other such objects were discovered in this region of the Solar System - common but hardly as abundant as either asteroids or KBOs. Collectively we call these centaurs and name after centaurs from Greek mythology.

    Color and Albedo: But one odd thing. Centaurs exhibit a striking range of colors, from nearly white to nearly as red as Mars. Compare Chiron with the second known centaur - Pholus. Why are some covered with tholins and others not?

    Origin: So where did the centaurs originate? Current thinking has them originating in the Kuiper Belt and Scattered Disk but being tossed into the inner Solar System by Neptune's gravity. As they cross the orbits of the giant planets, it is easy to imagine them having yet more gravitational interactions with them, each one altering their orbit. Remember, minor bodies tend to "pile up" either:

    Parting qestions:

    Comet? Stay tuned.

    Key concepts and vocabulary:
    Additional reading: