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Lecture #32 The Main Point • The Trans-Neptunian Region – (134340) Pluto: The trans-Neptunian region contains billions of icy bodies orbiting the Sun beyond the orbit of Neptune. (134340) Pluto is a large member of the trans-Neptunian region (or Kuiper Belt) and is officially classified as a dwarf planet. • Discovery. • Classification. Cl ifi i • Physical characteristics. • Charon. – Other Kuiper Belt Objects. • Reading: Chapter 12.3. Astro 102/104 1 4 Why was Pluto once considered a planet? Discovery of (134340) Pluto • Pluto was discovered in 1930 by a fortunate accident. • Percival Lowell made calculations which later turned out to be in error that predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. • Not N t kknowing i off the th error, Clyde Cl d W. W Tombaugh T b h att Lowell L ll Observatory in Arizona did a very careful sky survey. For 14 years Tombaugh scanned the skies, and discovered: More than a dozen asteroids, including (1) Ceres and (2) Pallas, seemed unique at the time of their discovery. They were classified as planets before their demotion some 50 yyears later. (134340) Pluto seemed unique at the time of its discovery, and it was hailed as a new planet. • 1 globular star cluster • 1 comet • 1 supercluster of galaxies • 5 open star clusters • 775 asteroids • 1 trans-Neptunian object (Pluto) Astro 102/104 Astro 102/104 6 Orbits of outer solar system objects known at the time of Pluto’s discovery. J, S, U, N in blue. Pluto in red. Astro 102/104 Later it was found that Pluto is quite small. Pluto's diameter is only 2320 km (smaller than the Moon, Io, Europa, Ganymede, Callisto, Titan, & Triton). 7 1 Why did Pluto lose its planet status? Astro 102/104 8 Orbits of objects known at the time of Pluto’s discovery. This diagram shows that there is a difference of five orders of magnitude (100,000) in the ability of planets and dwarf planets to gravitationally control their orbital zone. This difference is the result of the planet formation process. Objects above and below the dotted line underwent different formation scenarios. Astro 102/104 Astro 102/104 9 Orbits of objects known today. What makes a planet? S. Soter, Astronomical Journal, 132, 2006. S. Soter, Scientific Am merican, Jan 07. Starting in 1992, astronomers began to discover bodies in the trans-Neptunian region. They gradually realized that Pluto is not unique but is embedded in a vast swarm of objects sharing similar orbital and physical properties. Just like Ceres and Pallas, Pluto lost its planet status when new discoveries prompted a revision to the classification. The “scattering parameter” is relatively easy to quantify and makes classification of newly discovered10 objects practical. Decision by the International Astronomical Union The IAU is an organization of over 9,000 professional astronomers. It has been the arbiter of planetary and satellite nomenclature since its inception in 1919. It is the only administrative body with the jurisdiction to define a planet. The 2006 resolution was approved by an overwhelming majority. Astro 102/104 11 2 Basic physical properties: Pluto is far away, small, and icy • Orbit: a ≈ 6 billion km (39.5 AU); orbital period = 248.6 years. – – – – • • • • Astro 102/104 12 e = 0.25, so perihelion = 29.6 AU and aphelion = 49.4 AU. Orbit crosses Neptune's, but 3:2 resonance prevents close encounters. Orbit is inclined by 17 17° to the ecliptic ecliptic. Pluto's tilt (inclination) is 118°: Pluto "rolls" on its side like Uranus! Pluto's diameter is only 2320 km (smaller than the Moon, Io, Europa, Ganymede, Callisto, Titan, & Triton). Pluto's mass is 1.3x1022 kg (0.0022 ME); density is about 2 g/cm3. Surface temperature ranges from 40K to 60K (-235°C to -215°C). Surface consists of N2, CH4, C2H2, and CO ices (like a comet!) Astro 102/104 Pluto has satellites Charon Pluto (D = 2320 km): Named after the Greek god of the underworld who was able to render himself invisible. • Discovered in 1978. • Charon is large relative to Pluto: – – – – Charon (D = 1270 km): Named after the mythological boatman who y to ferried souls across the river Styx Pluto for judgment. Groundbased, 1978 1270 km diameter (more than half Pluto's size!). Orbital separation ~20,000 km. Revolves around Pluto in 6.4 days. Pluto's spin period is 6.4 days. • Charon is bright: icy. • But spectra show H2O ice, not CO, N2, or CH4 like Pluto. • Density about 1.6 g/cm3. • Probably formed by a giant impact. Nix (D = 40 – 160 km): Goddess of darkness and night, mother of Charon. Hydra (D = 40 – 160 km): Terrifying monster with the body of a serpent and nine heads that guarded the Underworld. Astro 102/104 13 14 Astro 102/104 15 3 Surface Composition Pluto/Charon Eclipses! • • • • Spectroscopy. Phase Diagrams. Modeling. Methane ice (CH4) was discovered first in 1976. • Surface is dominated by Nitrogen ice (N2) with traces of CH4 and CO. • Very low pressures and temperatures: • Earth passed through the plane of the Pluto/Charon orbit in the late 1980s. • Resulting eclipses provided a way to study & map both objects bj t in i greatt detail. d t il • Won't happen again until 2110! 118° inclination – Comparable to Triton. Astro 102/104 17 Astro 102/104 Pluto has a thin atmosphere Pluto and the Kuiper Belt • Between 1979 and 1999, Pluto was near perihelion and closer to the Sun than Neptune. • Surface temperatures warm enough for some CO, CH4, and N2 ices to sublimate sublimate. • Atmosphere discovered by stellar occultations by Pluto. • Surface pressure is only a few microbars (10-6 bar). • As Pluto recedes from the Sun, the atmosphere may freeze out back onto the surface. Astro 102/104 18 • Pluto is a large member of a class of small bodies known as Kuiper Belt Objects (KBOs) or transNeptunian objects (TNOs) (TNOs). • The discovery of the second TNO was in 1992! • Currently more than 1000 TNOs known. • Most known TNOs have sizes above 100 km. • Icy compositions. 19 Astro 102/104 Pluto Neptune 20 4 Trans-Neptunian Objects Classes of TNOs • Primitive objects with semi-major axis between 30 AU and hundreds of AU. • Many protected from encounters with the planets by orbital resonances (like Pluto). Pluto) • TNOs are the source of short period comets. • There may be 100,000 TNOs larger than 100 km, and the total mass may be ~0.01 ME. • Still much debate about differences in color and composition--they are hard to measure. • But there are classes based on orbit parameters: – "Classical" Classical TNOs: Objects with circular orbits never too close to Neptune; formed by "quiet" slow accretion of primordial outer solar nebula materials? – "Scattered" TNOs: Objects on eccentric, inclined orbits; these objects have once encountered a giant planet? – “Resonant" TNOs: Objects that occupy an orbital resonance with Neptune (like Pluto). – This is more than the number and mass of asteroids in the main asteroid belt! Astro 102/104 22 Overhead view of the known trans-Neptunian region as of 2000. (Blue=classical, Red=resonant, Black=scattered). Astro 102/104 23 Mission to the trans-Neptunian region NASA selected the “New Horizons” mission to fly by Pluto and another TNO. Launch in 2006, arrival at Pluto in 2015. Mission cost ~$650 million. Plot of TNO semi-major axis (x-axis) vs. eccentricity (y-axis), showing classical & resonant TNOs. Astro 102/104 24 Astro 102/104 26 5 Summary • (134340) Pluto is a dwarf planet. It is part of the Kuiper belt or trans-Neptunian region. • Pluto is a small icy outer solar system world: – Surface composed of N2, CO, CO CH4 ices. ices – Thin atmosphere formed by ice sublimation. • It has an eccentric and inclined orbit similar to that of many TNOs. Pluto is "protected" in a 3:2 resonance with Neptune. • Pluto has a moon (Charon) half its size, and two smaller moons (Nix and Hydra). • It remains a fascinating object worthy of scientific inquiry. Astro 102/104 29 6