A relatively sparse population of asteroids, known as Centaurs,exists between Jupiter and Neptune. Some are dark and red,similar to the tarry (tholin-covered) D-type asteroids, but others are bluer, suggesting that much of their surfaces may be freshly exposed ice. Because their orbits cross or come close to the giant planets, they are not stable, persisting for no longer than about ten million years. Probably, Centaurs are TNOs that have been scattered inwards, perhaps by a close encounter with Neptune.Further interactions with giant planets probably nudge them inwards until they become short-period comets, spending perihelion in the inner Solar System, where they are heated by the Sun and lose their volatiles in sometimes spectacular tails.
Six trojan objects have been discovered close to Neptune’s leading Lagrangian point. Dynamical arguments suggest that vast numbers await discovery (in both Lagrangian points) and that Neptune trojans may be ten times more numerous than Jupiter’s.
Beyond Neptune, we reach the Kuiper belt and all the other TNOs. One family of Kuiper belt objects travel in 3:2 orbital resonance with Neptune. Members of the class, which includes Pluto, are known informally as ‘plutinos’, not to be confused with Plutoid, which is the offi cial IAU term for any TNO large enough to be ranked as a dwarf planet. Plutoids can be plutinos, classical Kuiper belt objects (lacking orbital resonance with Neptune), or Scattered Disk objects beyond the main belt. Classical Kuiper belt objects are known alternatively as ‘cubewanos’ (‘QB 1 -os’) because the first Kuiper belt object to be discovered after Pluto bore the provisional designation 1992 QB 1.
Pluto and Charon
The properties of most TNOs are poorly known. However, Pluto and its satellite Charon are suffi ciently large and nearby to have been studied telescopically for several decades. Frozen nitrogen,methane, and carbon dioxide have been detected spectroscopically on Pluto, and the sharpest telescope images reveal dark patches that are probably tholin-rich residues. Pluto’s density suggests that rock must be about 70% of its total mass, and most likely it is internally differentiated with a rocky core (and feasibly an iron-rich inner core) overlain by a mantle made mostly of water-ice topped by a more volatile-rich crust.
Near perihelion (which happened most recently in 1989), Pluto has a nitrogen-rich atmosphere possibly denser than Triton’s.Because Pluto’s gravity is so weak, an imaginary shell enclosing99% of its atmosphere would extend to 300 kilometres above the surface, whereas for the Earth the equivalent height is only40 kilometres. Much of Pluto’s atmosphere is expected to condense onto the surface while distance from the Sun increases from 4.5 billion kilometres at perihelion to 7.4 billion kilometres at aphelion in 2113. It is a pity that we missed the chance to study
Pluto from close range during perihelion. The fl y-past by NASA’s New Horizon mission to Pluto will occur in 2015, by which time much of the atmosphere may have condensed and hidden the‘permanent’ surface beneath a seasonal shroud of nitrogen-ice.Pluto’s 6.4-day rotation period is the same as the orbital period of its largest satellite, Charon, which also rotates synchronously. This relationship is a result of strong tides, and means that Pluto and Charon permanently keep the same faces towards each other.Pluto is more evenly matched in size and mass to Charon than any other planet or dwarf planet to its own largest satellite. Charon’s mass is about 12% of Pluto’s, and it orbits at a distance of only about 17 Pluto radii from Pluto’s centre. For comparison, the Moon’s mass is only 1.2% of Earth’s and its orbital radius is60 Earth radii. Charon’s proximity to Pluto explains why it remained undetected until 1978. Pluto’s two smaller satellites, Nix and Hydra, were discovered in 2005. These orbit in Pluto’s orbital plane in close to 4:1 and 6:1 orbital resonance with Charon.
Seen from Pluto’s surface, Charon would look eight times wider than the Moon does from Earth. Because their relative masses are so similar, their common centre of mass (their ‘barycentre’) is not inside Pluto but at a point in space between the two bodies.Although double asteroids such as (90) Antiope and double Kuiper belt objects such as 2001 QW 332 (200-kilometre diameter twins) are known, Pluto-Charon is the most evenly matched pair among bodies large enough to count as planets or dwarf planets.
Charon’s surface is dominated by water-ice with traces of ammonia. Its density is less than Pluto, but still suffi cient for a substantial rocky core. Charon may turn out to be a relatively passive, heavily cratered globe, whereas Pluto might impress us all by being geologically active, as the variety in its surface materials suggests.
On the other hand, there may be a reason for Charon to be more active than its larger cousin. This is because Pluto’s axial inclination is 119.6° (being greater than 90°, this means its rotation is retrograde). Charon’s orbit is exactly in Pluto’s equatorial plane, so shares the high inclination relative to their joint orbit about the Sun. Competing tidal pulls on Charon from the Sun and Pluto might feasibly be strong enough to cause melting somewhere within Charon’s icy mantle. If this is the case,then we are faced with the intriguing prospect of a Europa-like surface for Charon and even a potentially life-bearing ocean beneath it. The best hint we have so far comes from infrared spectra of Charon obtained in 2007 which found water-ice on Charon that is still in pristine crystalline form, in contrast to the amorphous submicroscopic state of ice that has been exposed to solar ultraviolet radiation and cosmic ray bombardment for more than a few tens of thousands of years. The simplest explanation for this is geysers, spraying out fresh ice from the interior, like the plumes on Enceladus.
The rest
Table 7 lists Pluto and the ten largest other TNOs as ranked at the time of writing. Of these, Eris, Makemake, and Haumea are offi cially recognized as dwarf planets. The latter is fl attened, either because of its rapid rotation (less than 4 hours) or resulting from collision. These are classical Kuiper belt objects, except Eris and2007 OR 10 (Scattered Disk objects), 2002 TC 302 (5:2 orbital resonance with Neptune), Ixion (plutino), and Sedna, which is an oddity way beyond the Scattered Disk, in a highly elliptical orbit with aphelion at 975 AU.
Table 7 The largest trans-Neptunian objects
Apart from Pluto, the sizes of these objects are poorly known(even those for which a single round number is given in the table).Their dimensions are estimates based on assumptions about their albedo (the percentage of the incident sunlight they refl ect). If they are less refl ective than assumed, they must be larger, but if they are more refl ective, then they must be smaller. Size estimates can be improved by measuring thermal radiation from their surfaces, but they are so cold (–230 °C or less) that this can be achieved only by telescopes in space, above the Earth’s atmosphere. Given the uncertainties, it is unlikely that all of these objects will survive into future ‘top ten’ lists.
TNOs range in colour from red (probably widespread tholins across their surfaces) to blue-grey (exposed ice or amorphous carbon). Haumea is one of the blue-grey ones, and its mass(derived from the orbits of its satellites) shows that its density is greater than Pluto’s, so it must have a relatively high non-ice content. On Quaoar, crystalline ice and ammonia hydrate have been detected spectroscopically, suggesting recent resurfacing(using arguments similar to those advanced for Charon). This would require either geological activity or a major impact event to generate ejecta suffi ciently widespread to dominate the spectrum.
Between 2% and 3% of TNOs are known to have satellites, which is similar to the abundance of asteroids with satellites. The proportion is higher among the larger TNOs and poses problems in trying to account for their origin.
If NASA’s New Horizon mission remains healthy after its 2015fl y-by of Pluto-Charon, it will be directed onwards to a more distant TNO. The target has yet to be determined, but ideally will be a blue-grey object to contrast with Pluto’s reddish nature.
A trans-Neptunian planet?
Most astronomers accept that we have discovered all the large objects belonging to our Solar System. Certainly, there can be nothing of planetary size hiding in the Kuiper belt. If such an object were present, then the Kuiper belt would be unstable.However, there remain two possibilities for an outlying planet(popularly dubbed ‘Planet X’) that have not yet quite gone away.One is that there is an Earth-mass object in an inclined and eccentric orbit between 80 and 170 AU from the Sun. The presence of such a large body (perhaps originally ejected outwards by a close encounter with Neptune) could explain an observed sudden drop-off in the population of the Kuiper belt beyond 48AU, known as the ‘Kuiper cliff ’. It might also account for the extreme scattering evidenced by objects such as Sedna.
The second possibility arises because long-period comets originate preferentially from a particular region of the sky, rather than coming in from random directions. It has been suggested that these were dislodged from the Oort Cloud by a Jupiter-mass body about 32,000 AU from the Sun. This would be hard, but not impossible, to detect by telescope. A ‘planet’ so far out need not be gravitationally bound to the Sun, but could be just a chance wanderer through interstellar space, possibly escaped from the planetary system of another star.