Planetary Systems

Pluto, Dwarf Planets, and SSSB

Pluto -- As shown in Figure 07-20a, Pluto is really a double planet system, since its moon, Charon, is very close and about half of its size. There are some who think Pluto should be classified as a large asteroid or comet. Some consider it to be the largest of the Kuiper Belt

(Trans-Neptune) objects. Unlike the other planets, Pluto's orbit is highly eccentric; it has an inclination angle of 17^o; and the spin axis has a tilt of 120^o relative to the ecliptic plane. Pluto's atmosphere is extremely tenuous. It may exist as gas only when Pluto is near its perihelion (the nearest approach to the Sun), where it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. For the majority of Pluto's long year, the atmospheric gases are frozen into ice.

Figure 07-20a Pluto [view large image]

NASA launched the New Horizons spacecraft on January 19, 2006. Its mission is to survey the outer region of the Solar system including Pluto and the Kuiper Belt. It is scheduled to rendezvous with Pluto during Summer 2015. Its closest approach is on time in July 14, 2015. The images in Figure 07-20b were released on July 17. The image on the left shows plains sprawl outward from the base of ice

		mountains (up to 3.5 km). The right image shows frozen polygons surrounded by shallow troughs, some of them filled with darker material. These pictures reveal a young surface that appears to be constantly re-shaped without a trace of impact craters to indicate any past activities. Figure 07-02c is a high resolution image of Pluto and Chron. A click on this image will present a flyby video.
Figure 07-20b Pluto, 2015 [view large image]	Figure 07-20c Pluto and Chron

After lively debates in a meeting, the International Astronomical Union has demoted Pluto to "dwarf planet" in August, 2006 mainly because its oblong orbit overlaps with Neptune's. The class of "dwarf planet" also includes Ceres, 2005 FY9 (now renamed to Makemake

		- Pronounced MAH-kay MAH-kay, see Fiugre 07-20d), 2003EL61 (renamed to Haumea in September 2008, also in Figure 07-20d) and UB313 (nicknamed Xena, Figure 07-20d). Meanwhile, the universe unfolds itself as usual; it is indifferent to labeling. On 14 September 2006, the trouble-making UB313 that forced astronomers to reconsider Pluto's planetary status has finally received its official name - Eris. The name, taken from a Greek goddess of discord and strife (her golden apple ultimately ignited the Trojan war, Figure 07-20e), is most appropriate for this celestial object.
Figure 07-20d Eris (UB313) & its Moon [view large image]	Figure 07-20e Mythological Eris [view large image]

The IAU has now defined planets and other bodies in our Solar System into three categories in the following way:

1. A planet is a celestial body that
    (a) is in orbit around the Sun,
    (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly
          round) shape, and
    (c) has cleared the neighbourhood around its orbit.

2. A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite (of a planet).
3. All other objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies” (SSSB).

Figure 07-20f TNO, KBO
[view large image]

Figure 07-20f shows the artist concepts of the three dwarf planets and some SSSB, which are also referred to as Trans-Neptunian or Kuiper Belt Objects (TNO, KBO).

	The 10^th planet in the solar system Sedna has been discovered in 2004 (Figure 07-20g). It is a dark red object over twice the distance to Pluto, making it a candidate for the long-hypothesized Oort cloud objects thought to extend to the Solar System's edge. It is estimated to be about three-quarters the size of Pluto and therefore the largest Solar System object found since Pluto in 1930.
Figure 07-20g Sedna