IAL 17: Small Bodies of the Outer Solar System

Don't Panic


  1. A Brief History of Solar System Planet Discoveries
  2. Pluto and Charon Facts
  3. Pluto: The Problematic Planet
  4. The Kuiper Belt
  5. KBOs/TNOs
  6. Sedna: Planet X?
  7. The Oort Cloud
  8. Centaurs
  9. Comets

  1. A Brief History of Solar System Planet Discoveries

  2. The ancient planets---Mercury, Venus, Earth, Mars, Jupiter, and Saturn---were all discovered in prehistory---but, of course, Earth wasn't recognized as a planet until after COPERNICUS.

    1. The Discovery of Uranus:

      Uranus was discovered on 1781mar13 by William Herschel. See figures below (local link / general link: uranus_rings.html; local link / general link: william_herschel.html).

      HERSCHEL was also the maker of his own telescopes. The most useful of these was his 0.48 m (18.7 inch) reflector erected at Slough, England near Windsor Castle (see the figure below: (
      local link / general link: telescope_william_herschel.html).

    2. The Discovery of Neptune:

      In the decades after its discovery, Uranus's orbit was found to be difficult to predict.

        Question: Why could Uranus's orbit be difficult to predict even when numerous observations were available.

        1. The 19th astronomers had a hard time doing the calculations in absence of electronic computers.
        2. A giant impactor disturbed Uranus's orbit in the early solar system.
        3. An unobserved planet beyond Uranus was perturbing its orbit.

        Answer 1 is NOT right, but it is true it was pretty tedious to do multiple gravitating body calculations in the old days.

        Answer 2 is NOT right because the orbit one observes now is the orbit that one needs to predict. Any unknown pre-impact orbit, if there was one, is lost in the past.

        Answer 3 is right.

        Remember all gravitating bodies interact.

        A large planet beyond Uranus would perturb its orbit and if one wants POVERBIAL ASTRONOMICAL ACCURACY, one must account for all significant perturbations.

      So it was hypothesized that there was a perturbing planet beyond Uranus.

      A mathematical prediction by French and English astronomers in 1846 led to the discovery of Neptune within 1 degree of predicted location by the Berlin Observatory on 1846sep23 (No-428).

      Oddly enough this discovery was partially accidental since the predicted orbit wasn't much like real orbit of Neptune (No-432).

      The mathematical prediction had got the location of Neptune in space for the time more or less right, but NOT its long-term behavior.

        Galileo had observed Neptune from 1612dec to 1613jan as a background object while observing Jupiter and Jupiter's moons. Potentially, he could have recognized it as a planet since it did move slightly over that time relative to the fixed stars. But, in fact, he didn't. See Kowal, C. T., & Drake, S. 1980, Nature, 287, 311.

    3. Planet X:

      The discovery of Neptune and continuing small difficulties in exactly predicting the orbits of Uranus and Neptune (???) stimulated the search for another perturbing planet: Planet X. Percival Lowell (1855--1916) was one of the most avid searchers for Planet X (see the figure below (local link / general link: percival_lowell_planet_x.html).

    4. The Discovery of Pluto:

      Instead of Planet X, we discovered Pluto. For who and how, see the two figures below (local link / general link: clyde_tombaugh.html; local link / general link: blink_insert.html).

      Because of its remoteness and small size,
      Pluto is very hard to observe from Earth or even from space using the HST.

      No space probe has ever been there: the Pioneer and Voyager probes missed it on there way out of the solar system. So there are no close-up pretty pictures.

      Only a few facts about Pluto were discovered before the 1975.

      The amount of light it reflected showed early on that it was a very small object: much smaller than Uranus and Neptune, and so Pluto is NOT a gas giant.

      Pluto rotation period was 6.39 days as was discovered in 1955 by studying variations in Pluto's reflected light.

        This discovery was by Merle F. Walker and Robert Hardie of Lowell Observatory (Walker, M. F., & Hardie, R. 1955, PASP, 67, 224).

        Hardie would later work at Vanderbilt University and overlap with Doug Hall who yours truly would overlap with at Vanderbilt in 1995--1996.

      Pluto's moon CHARON was discovered on 1978jul02 by James Christy of the US Naval Observatory (FMW-238).

      CHARON in Greek mythology is the ferryman who rows the dead over the River Styx to the Underworld.

        Charon also turns up in Dante's Inferno

          ... And lo! toward us in a bark
          Comes on an old man hoary white with eld,
          Crying, "Woe to you wicked spirits! hope NOT
          Ever to see the sky again. I come
          To take you to the other shore across,
          Into eternal darkness, there to dwell
          In fierce heat and in ice.

      For Dante Alighieri (1265--1321), see the figure below (local link / general link: dante_virgil_hell.html).

  3. Pluto and Charon Facts

  4. Although there are no close up images of the Pluto-Charon system, orbital parameters, distant images, and spectroscopy allow some information about the system to be deduced.

    Pluto and Charon
    Quantity                            Value
    Mean distance from the Sun    39.48168677  astronomical units
    Eccentricity of orbit         0.24880766 :   this is the largest in the
                                                 solar system for a planet. 
    Mean inclination to Ecliptic  17.14175 degrees :  also the largest in the 
                                                      solar system for a planet.
    Axis tilt to orbital north pole   119.61 degrees  :  This means that
                                         the reference rotation 
                                         is clockwise??? in a crude sense
                                         around the orbital north pole.
    Orbital period                248.0208 years :  it hasn't completed 1/3 of
                                                    its orbit since discovery. 
    Rotational Period             6.38725 days : the rotation is retrograde:
    (both bodies:                                i.e., it is clockwise-ish as 
    they are mutually                            viewed from the orbital north
    synchronously tidally locked.)               pole. 
    Charon revolution period      6.38725 days : the same as the rotation periods
                                                due to synchronous tidal locking.  
                                                Each body
                                                hovers at the same point in the
                                                other's sky. 
    Charon semi-major axis    1.96 x 10**4 km = 3.07 Earth radii = 16.4 Pluto radii
    Pluto equatorial radius       1195 km = 0.180 Earth radii: 
                                                  Pluto is only the 12th largest 
                                                  rocky-icy bodies in the solar 
                                                  system:  it is smaller than 
                                                  the Moon and the Galilean 
    Charon equatorial radius       593 km = 0.0930 Earth radii = 0.496 Pluto radii
    Pluto mass                     1.3 x 10**22 kg = 0.002200 Earth masses :
                                                  Pluto is by far the least 
                                                  massive planet:  Mercury is next
                                                  with 0.055274 Earth masses.
    Charon mass                    1.62 x 10**21 kg = 0.00027 Earth masses
                                                    = 0.125 Pluto masses
    Pluto & Charon mean density    about 2 g/cm**3 :  recall water density 
                                                is 1 g/cm**3 and silicates 
                                                have about 3 g/cm**3
                                                Pluto and Charon may a 50-50
                                                mixture of rock and water ice.
                                                But the densities are NOT 
                                                well determined it seems:
                                                which means the masses and/or
                                                radii arn't either. 
                                                Probably the radii since the
                                                orbits should give masses 
                                                pretty well.
    Pluto surface temperature      about 50 to 60 K varying from aphelion to 

    References: Cox-294--297, 302--308; SRJ-233; FMW-239.


    From the above numbers one can see that the Pluto-Charon system is the closest the solar system has to a double planet.

    1. The diameter of Charon is about 1/2 of Pluto's.
    2. The semi-major axis of the Charon orbit is only 16.4 Pluto radii.

      The corresponding numbers for the Earth-Moon system are 1/4 and 60 recall. The Earth-Moon system is thus a distant second in the double planet race.

    The double planet nature is illustated in the figure below.

    Spectroscopy tells us that the surface of Pluto is mostly Ni_2 ice with a few percent methane (CH_4), carbon monoxide (CO), and ethane (C_2H_6) (HI-239).

    Pluto does have thin atmosphere which is mainly N_2 and methane that probably comes from evaporation from the surface during the near perihelion phase (FMW-239). The last exact perihelion was 1989sep05 (Cox-295).

    This atmosphere may recondense as Pluto draws away from the Sun.

    The pressure of the atmosphere is only of order 10**(-5) of Earth's.

    Pluto looks reddish. This may be caused by compounds created from methane ice by sunlight (Se-547).

    The surface features of Pluto can be marginally resolved using the HST and computer processing as illustrated in the figure below.

    Oddly, Charon's surface is different from Pluto's.

    It seems to be all water ice and dark material: some kind of soil: perhaps carbonaceous regolith??? (HI-240).

    How did the double planet system originate?

    Well just as with the Earth-Moon system, a giant impactor may have collided with Pluto and mostly merged with it, but ejected material some of which coalesced into Charon (HI-239).

    But we know far less about the Pluto-Charon system than about the Earth-Moon system, and so ideas might change.

    As for the origin of Pluto itself? We will take that up below when we consider rocky-icy bodies in general.

  5. Pluto: The Problematic Planet

  6. There is no precise definition of a planet.

    Commonly, a planet is larger body that orbits the Sun (or any star) and asteroids and smaller rocky-icy bodies---a term I use for the lack of any better one---are smaller bodies or, maybe, planetoids.

    Well PLUTO kind of straddles the line as the line now appears as illustrated in the figure below (local link / general link: rocky_icy_body.html).

    References: (
    Cox-295,305,306; HI-161; David Jewitt's Kuiper Belt site).

    PLUTO is only the 12th largest rocky-icy bodies in the solar system.

    It is smaller than many of the larger moons, including the Earth's moon (i.e., the Moon).

    But it is larger than any of the other rocky-icy bodies that orbit the Sun including the trans-Neptunian objects (TNOs) Sedna, Orcus (AKA 2004 DW), Quaoar (pronounced kwah-o-wahr), and Ixion.

    And it is significantly larger than the largest asteroid: i.e., Ceres.

    So size-wise, PLUTO could go either way.

    But as a physical object PLUTO is probably the same in nature and origin to the other TNOs that we will discuss below.

    PLUTO is only distinguished by its size and its early discovery (i.e., 1930).

    But traditionally PLUTO is classed as a planet and the International Astronomical Union (IAU) asserted in 1999 that PLUTO would continue to be a planet (PF-158).

    But larger TNOs than PLUTO may be discovered someday---maybe even someday soon---and then the status of PLUTO will probably be debated again.

  7. The Kuiper Belt

  8. COMETS are icy/rocky bodies on highly elliptical orbits that plunge into the inner solar system from the outer solar system.

    Their ices evaporate somewhat explosively as they approach the Sun which leads to them being bright, large, ``long-haired'' objects: the name comet is from the Greek kometes meaning long-haired (Ba-241).

    Because they lose volatile material every time they enter the inner solar system, COMETS CANNOT live forever.

    They may last only a thousand orbits: i.e., a thousand perihelions (Se-569)

    Assuming they do NOT hit a planet or even the Sun (Se-568), COMETS become dead rocks probably with carbonaceous material (Se-568) without volatiles or break-up altogether.

    The dead comets too can hit planets and probably end up doing so at some point since they keep crossing planet orbits.

    Debris from live or dead COMETS can become path of small particles that hit the Earth as METEORS in a METEOR SHOWER when the Earth passes through the path (Se-553).

    Since COMETS do NOT live forever, they must be resupplied in order to still have them so long after the formation of the solar system.

    There are two classes of COMETS: short-period and long-period.

    The short-period COMETS have orbital periods of less than 200 years and their orbital inclinations tend to lie within 30 degrees of the ECLIPTIC PLANE (Se-569).

    In the 1940s and 1950s, Edgeworth and Kuiper (pronounced koyper) proposed that there must be a belt of rocky-icy bodies beyond about Neptune to resupply the short-period COMETS (HI_249; PF-157).

    Perturbations from the outer planets and, perhaps, collisions??? occasionally causes one of these rocky-icy bodies to go onto a plunging orbit and become a short-period COMETS (HI_249).

    The KUIPER BELT (as it is now called) is thought to extend from about 30 to 100 AU from the Sun and be about 10 AU thick (HI_249; PF-157).

    The Kuiper belt objects were probably created in situ. They are icy/rocky planetesimals or protoplanets or fragments thereof that were never accreted into larger bodies for some reason (HI_249).

    Until 1992, the KUIPER BELT was entirely theoretical unless one counts Pluto and Charon as Kuiper belt objects (KBOs) as we now do, but we didn't do then.

    KBOs are also called TRANSNEPTUNIAN OBJECTS (TNOs) since the orbital semi-major axis of Neptune is 30.069 AU (Cox-294). To me neither name is entirely satisfactory since there can be icy bodies that are neither KBOs of TNOs.

    In 1992, David Jewitt and Jane Luu discovered the first KBO: which has the catchy name 1998 QB1.

  9. KBOs/TNOs

  10. Many KBOs/TNOs have discovered since 1992 by dedicated searches.

    As of 2004apr23, there are 786 TNOs, NOT counting Pluto, Charon, or Sedna ( Minor Planet Center list of trans-Neptunian objects). Batches of new discoveries seem to be anounced monthly or bimonthly.

    Most of known KBOs are estimated to have diameters of about 100 km ( David Jewitt's Kuiper Belt Page).

    But the sizes of KBOs/TNOs are very uncertain. Except for Pluto and Charon, they are NOT resolved.

    The sizes must be estimated by their reflected brightness visual or thermal emission in the IR.

    This estimation requires making some uncertain assumptions.

    It has been estimated that that there maybe 35,000 KBOs larger than 100 km in diameter (HI-249).

    Of course, the largest KBOs/TNOs attract the most natural curiosity: we are always looking for new record setters.

    Largest KBOs/TNOs as of 2004apr
    Object 	   Diameter    semi-major  ecc.  Incl.   Year     Discovery date 
                 (km)      axis (AU)        (deg.)   (Jyr)    
    Pluto 	   2320           39.4817 0.2488 17.14  248.0208  1930feb18
    Orcus     1500?          39.473  0.218  20.6   248.00    2004feb17 
    Sedna     <1500?         532.     0.857  11.9 10500.      2003nov14 
    Charon 	   1270                                           1978jul02 
    Quaoar 	   1200±200       43.377  0.034   8.0   285.69    2002jun04
    Ixion	   1065±165       39.485  0.242  19.6   248.12    2001may22
    Varuna	    900±140       43.129  0.051  17.2   283.24    2000nov28
    2002 AW197  890±120       47.501  0.130  24.3   327.39    2002jan10 

    References: David Jewitt's Kuiper Belt Page; Minor Planet Center list of trans-Neptunian objects. Notes:

    1. The periods, except for Pluto, are rather approximate. I calculated them myself using Kepler's 3rd law.
    2. The Pluto and Charon diameters 2380 km and 1186 km, respectively, are given by Cox-306. Who is right?
    3. Orcus's provisional name was 2004 DW.
    4. Sedna is NOT officially a KBO since its orbit overall is too far out. It is a TNO for sure. Sedna's perihelion and aphelion distances are 76 AU and 990 AU, respectively.

      Kepler's 3rd law gives the Sedna year as 12300 Julian years, but the NASA press release gives 10500 years and presumably NASA knows best ( NASA: Sedna press release).


  11. Sedna: Planet X?

  12. SEDNA was discovered at Palomar Observatory in California in 2003nov14, but the discovery was only announced 2004mar15---the Ides of March.

    It was discovered as all remote small solar system objects are by observing motion against the background of fixed stars as discussed in the figure below.

    SEDNA has been observed also by the HST and the Spitzer Telescope which is an infrared satellite telescope.

    SEDNA is NOT a KBO nor an Oort Cloud object (see the Oort Cloud section below).

    It's mean distance from the Sun (532 AU) is too far out for the former and to close for the latter.

    So SEDNA may need to be put in a new class of objects.

    But for the nonce we can put it in the wide class of TNOs.

    Sedna and Pluto are compared in the figure below.

    Initially, it was thought that Sedna like Pluto had a moon, because of Sedna's very slow rotation rate: about 20 days. Synchronous tidal locking with a fairly distant moon would slow Sedna down to such a long period.

    But close observation reveals NO moon: maybe the moon is very dark or was eclipsed by Sedna when they looked for it or was lost in some gravitational interaction in the past.

    Currently, Sedna is at about 90 AU from the Sun NASA press release.

    It will get closer to the Sun for another 72 years.

    For an artist's conception of Sedna, see the figure below.

    At present, yours truly does NOT be believe that Sedna or any comparable TNO will ever be classed as a planet.

    In the future if solar system bodies larger than Pluto are found, an official rule about what constitutes a planet may evolve.

  13. The Oort Cloud

  14. LONG-PERIOD COMETS have periods of order 10**6 to 10**8 years (Se-569; HI-248).

    The longest-lived ones can outlive the solar the solar system if they can make of order 1000 orbits (Se-569).

          10**8 years x 1000 = 10**11 years = 100 Gyr
          and the solar system is only going to live about 10 Gyr
          10**6 x 1000 = 10**9 Gyr
          and so the shorter-lived LONG-PERIOD COMETS must be renewed.

    Another aspect about LONG-PERIOD COMETS is that they can plunge on their highly elliptical orbits from from any direction.

    LONG-PERIOD COMETS are NOT confined to being near the Ecliptic Plane like short-period comets. See the figure below (local link / general link: comet_orbits.html).

    These long-period comet facts led Dutch astronomer
    Jan Oort (1900--1992) in 1950 to propose a spherical reservoir of rocky-icy bodies extending from about 50,000 AU to 200,000 AU from the Sun (see Wikipedia: Jan Oort: A few of Oort's discoveries).

    This reservoir has come to be called the Oort cloud---which is not a very good name since it is NOT a cloud---it is more of swarm.

    See the figure below For more on the Oort cloud, see the figure below (local link / general link: oort_cloud.html).

  15. Centaurs

  16. The CENTAURS are smaller grouping of rocky-icy bodies/comets whose orbits cross those of the gas giants.

    They are named CENTAURS are named after the prototype object Chiron, which was named after a centaur (HI-249).

    The Chiron was first identified as an asteroid (i.e., a rocky body), but then started showing cometary behavior: i.e., it started outgassing and developed a comet head. It was clearly an rocky-icy bodies.

    It has been estimated that of order 2500 centaurs of diameter scale a 75 km may exist (HI-249).

    There were 7 as of 2000 (Cox-327).

    The Minor Planet Center lists 149 Centaurs and scatterd-disk objects has of 2004apr25 ( Minor Planet Center list of trans-Neptunian objects). But they don't say which are which.

    Centaur orbits are unstable due to gravitational perturbations of the gas giants and they may only survive a few million years before they collide with something or are ejected (HI-249).

    Centaurs may have originated in the Kuiper Belt and have been kicked inward into the gas giant range by gravitational collisions or the perturbation of passing stars.

  17. Comets