Planet Searching• After Uranus and Neptune, some

began searching for yet another planet, among them was P. Lowell from 1905-1916

• Search resumed in 1929 by C. 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

ClydeTombaugh

JamesChristy

Hubble Image of Pluto & Charon

Pluto and Charon to Scale

Mapping Pluto

Charon’s orbit is such as to eclipse Pluto at times, allowing us to map its surfaceDone in 1985

HST mapped Pluto to a resolution of 150 km, revealing a strong variation of reflectivity

Pluto-Charon Eclipses

Charon-Pluto Eclipse Visual

Surface Mapping with HST

A Map of Pluto

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 tenuous

CH4 atmosphere• Orbital plane is nearly perpendicular to

ecliptic• 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’ !

Structure of Pluto and Charon

Synchronous Orbits for Pluto and Charon

New Moons: Nix, Hydra,

and “P4”

New Horizons

Dwarf Planets

A "dwarf planet" is a celestial body that, within the Solar System:

1. is in orbit around the Sun2. has sufficient mass for its self-gravity to overcome

rigid body forces so that it assumes a (near-spherical) shape

3. has not cleared the neighborhood around its orbit4. 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 moon Dysnmonia(goddess of discord and daughter of

lawlessness)

GabrielleXena

Orbit of Eris

Comparison of Pluto with Eris

Ceres Ceres is now a dwarf

planet that 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.

Rotation and Surface of Ceres

Interior of Ceres

Two Other Dwarf Planets

Haumea

Makemake

Future Dwarf Planets?