Pluto Discovery l l l By the end

Pluto

Discovery l l l By the end of the nineteenth century, observations of the orbits of Uranus and Neptune suggested that Neptune’s influence was not sufficient to account for all the irregularities in Uranus’s motion Furthermore, it seemed that Neptune itself might be affected by some other unknown body Percival Lowell set about calculating where the supposed ninth planet should be l l l He searched for it, without success, during the decade preceding his death in 1916 Not until 14 years later did American astronomer Clyde Tombaugh finally succeed in finding Lowell’s ninth planet, only 6° away from Lowell’s predicted position The new planet was named Pluto for the Roman god of the dead who presided over eternal darkness (and also because its first two letters were Lowell’s initials) Its discovery was announced on March 13, 1930, Percival Lowell’s birthday (and also the anniversary of Herschel’s discovery of Uranus) Unfortunately, it now appears that the supposed irregularities in the motions of Uranus and Neptune did not exist and that the mass of Pluto is far too small to have caused them anyway

Orbit l l l Pluto’s orbit is quite elongated and is inclined at 17. 2° to the plane of the ecliptic At perihelion, it lies 29. 7 A. U. (4. 4 billion km) from the Sun, inside the orbit of Neptune. At aphelion, the distance is 49. 3 A. U. (7. 4 billion km), well outside Neptune’s orbit Pluto last passed perihelion in 1989, and remained inside Neptune’s orbit until February, 1999 The next perihelion passage will not occur until the middle of the 23 rd century Pluto takes 248 years to orbit around the Sun

Orbit l l l Pluto and Neptune are locked into a 3: 2 resonance (two orbits of Pluto for every three of Neptune) as they orbit the Sun As a result, even though their orbits appear to cross, Pluto and Neptune are in no danger of colliding with each other Because of the orbital resonance and Pluto’s tilted orbit plane, the distance between the two planets at closest approach is actually about 17 A. U.

Appearance l l l Pluto is often hard to distinguish from the background stars The planet is actually considerably fainter than many stars in the sky It is never visible to the naked eye

Physical Properties l l l Pluto is so far away that little is known of its physical nature In July 1978, astronomers at the U. S. Naval Observatory discovered that Pluto has a satellite It is now named Charon, after the mythical boatman who ferried the dead across the river Styx into Hades, Pluto’s domain Knowing the moon’s orbital period of 6. 4 days, astronomers could determine the mass of Pluto to much greater accuracy than had previously been possible It is 0. 0021 Earth masses l l l Far smaller than any earlier estimate More like the mass of a moon than of a planet Pluto’s diameter is about 1, 400 miles, about one-fifth the size of Earth Charon is about 800 miles across and orbits at a distance of 12, 000 miles from Pluto The Pluto–Charon system has the largest satellite-to-planet mass ratio in the solar system

Physical Properties l l l Pluto exhibits retrograde motion The mass, radius, and density of Pluto are just what we would expect for one of the icy moons of a jovian planet The planet is almost certainly made up mostly of water ice Spectroscopy reveals the presence of frozen methane as a major surface constituent Pluto is the only planet in the solar system on which methane exists in the solid state, implying that the surface temperature on Pluto is no more than 50 K (370 degrees F)

New Moons l l l Two additional moons Hydra and Nix, were discovered in 2005 In Greek mythology, Nyx was the goddess of the night and the mother of Charon Hydra is the nineheaded mythological serpent that guarded Pluto's realm

Dwarf Planet l On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet to a dwarf planet New Horizons

Pluto video l Quiz to follow

Quiz

#1 The planet Pluto was discovered in _____. l A. B. C. D. 1781 1846 1930 1978

#2 Who discovered the planet Pluto? l A. B. C. D. Jim Christy Vesto Slipher Percival Lowell Clyde Tombaugh

#3 When closest to the Sun, Pluto is just inside the orbit of _____. l A. B. C. D. Saturn Jupiter Uranus Neptune

#4 Pluto takes almost _____ years to complete one orbit of the Sun. l A. B. C. D. 150 250 350 450

#5 The mass of Pluto is _____ the mass of our Moon. l A. B. C. D. equal to less than much greater than slightly greater than

#6 Earth-based astronomers can learn most about Plutos surface when _____. l A. B. C. D. Charon eclipses Pluto eclipses Charon appears farthest from Pluto and Charon appear side by side

#7 When Pluto is farthest from the Sun, it is part of the _____. l A. B. C. D. Oort cloud Kuiper belt asteroid system exoplanetary system

#8 The space probe New Horizons is expected to send pictures of Pluto and Charon in the year _____. l A. B. C. D. 2010 2015 2020 2025

#9 l As Pluto swings away from the Sun, even its atmosphere freezes. l l true false

Answers