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Planet Searching • After Uranus and Neptune, some began searching for yet another planet, among them was Percival Lowell from 1905-1916 • Search resumed in 1929 by Clyde Tombaugh, who identified a candidate on 2/18/1930 • The name “Pluto” was suggested by an English girl of age 11. Discoveries: Pluto at left and moon Charon at right Clyde Tombaugh James Christy Hubble Image of Pluto & Charon Pluto Charon a 39.5 AU 19600 km P 248 yr 6.39 d i (orb) 17o 99o e 0.25 0 Prot 6.39 d 6.39 d i (rot) 123o 0o R 0.67 RM 0.34 RM M 0.18 MM 0.02 MM r 2.0 g/cc 2.0 g/cc vesc 1.1 km/s 0.6 km/s A 0.5 0.4 Pluto and Charon to Scale Mapping Pluto Charon’s orbit is such as to eclipse Pluto at times, allowing us to map its surface (Last occurrence was in 1985) HST mapped Pluto to a resolution of 150 km, revealing a strong variation of reflectivity Pluto-Charon Eclipses Charon-Pluto Eclipse: Visual Aid Surface Mapping with HST Pluto is ODD! • • • • • Similar to Triton Cold at T=50K Has frozen CH4 Mostly rock and ice Largest eccentricity, with D ~ 30-50 AU (sometimes closer than Neptune) • Large orbital inclination • Smaller than planets • Large moon in relative size Origin of Pluto-Charon: • Pluto ejected from Neptune – problem is Charon • Pluto and Charon formed together (?) • Pluto captured Charon (?) • Giant impact like Earth-Moon system (?) Orbits for Pluto and Neptune Atmosphere of Pluto It is possible to study Pluto’s atmosphere from Earth using occultation of starlight. The light passes through the tenuous atmosphere. In this way we can “see” the atmosphere. Spectroscopy reveals composition of Pluto and Charon Charon • • • • • • • Discovered in 1978 by J. Christy Has an icy surface of frozen N and a tenuous CH4 atmosphere Orbital plane is nearly perpendicular to the ecliptic Charon is in synchronous orbit with Pluto (and vice versa) Ratio of masses: MC/MP=0.13 Ratio of sizes: RC/RP=0.51 As seen from Pluto, Charon subtends an angle of 4o, but the Sun is only 1’ ! Synchronous Orbits for Pluto and Charon Structure of Pluto and Charon New Moons: Nix, Hydra, and “P4” New Horizons (July 2015!) Dwarf Planets A "dwarf planet" is a celestial body that, within the Solar System: 1. 2. 3. 4. is in orbit around the Sun has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a (near-spherical) shape has not cleared the neighborhood around its orbit is not a satellite The term "dwarf planet" applies only to objects in the Solar System and is distinct from "planet" and "small solar system body". Eris and its moon Dysnmonia (goddess of discord and daughter of lawlessness) Xena Gabrielle Orbit of Eris Comparison of Pluto with Eris • Ceres is now a dwarf planet but was previously the largest of the asteroids. • It was discovered on Jan 1, 1801, by Giuseppe Piazzi. With a diameter of about 950 km. • Ceres is in the asteroid belt, and it was the largest and most massive body there, but now is the 3rd of the dwarf planets. • Interestingly, it contains approximately a third of the belt's total mass. Ceres Dawn Visits Ceres • Dawn visited Ceres in spring 2015 • The white splotch is a “cryovolcano”: a salty-mud volcano that spews salt water • Ceres is about 70% rock and 30% ice • Surface gravity is 1/34 of Earth’s • Dawn remains in “perpetual” orbit about Ceres Two Other Dwarf Planets Haumea Makemake Future Dwarf Planets? 2015 RR245 81 AU, 733 year orbit, highly elliptical, about 30% as large as Pluto