Discovery of Uranus, Neptune, Pluto• The last 3 planets were discovered with

telescopes, in the last 150 years or so (too faint to see with eyes)

• Studied (glimpsed) by Voyager 2, powered by “gravity slingshot” ; explored all Jovian planets

• Uranus was discovered accidentally by William Herschel

• Neptune was discovered ‘mathematically’! Following observations of perturbations in the orbit of Uranus by an unseen object, Adams and Levellier independently predicted its existence

• Pluto is not a (Jovian) planet, more like a moon, planetesimal, or asteroid; was discovered in 1930 by Clyde Tombaugh

Uranus

View of Uranus from Earth(something strange ?)

Orbit of Uranus around the Sun: Rotation axis tilted at 98 degrees !

Uranus• Uranus and Neptune are almost twins in size ~

25,000 Kms radius• Uranus is tilted on its side by 98 degrees; each pole

faces the sun for half the orbital period (84 years)• Cause of tilt? Where on Uranus would you go to

spend summer vacation ?• Uranus rotates somewhat backwards like Venus;

rotation period = 17hr 14m• Weak magnetic field ~ 0.74 x Earth’s, could be a very

small metallic mantle• Methane atmosphere, hence greenish hue• H2O (ice) mantle

Neptune

Neptune and the Great Black Spot

Methane ice clouds on Neptune

Atmosphere of Neptune (c.f. Jupiter)

A pale imitation of Jupiter!

Neptune• Neptune’s atmosphere very active, with convection

currents• ‘Great Black Spot’ ~ 10,000 Kms long, cyclonic

storm like the GRS on Jupiter• Internal energy source (gravity) like Jupiter; emits

2.6 times the energy it receives from the sun• Tilted at 30 degrees• Strange moon Triton, which rotates backwards,

probably a captured Kuiper Belt Object• Magnetic field tilted ~ 50 degrees to rotation axis;

mag poles not aligned with geo poles (Earth and Jupiter are about 10 degrees)

Magnetic and geographical axes

Neptune’s Magnetic axis is NOT centered

Structures of Uranus and Neptune

Relatively thin atmospheres compared to Jupiter and Saturn, but still quite extensive compared to terrestrial planets

Atmospheres of Uranus and Neptune• More He than Jupiter and Saturn• Insufficient gravity loss of H• H is locked up in methane CH4, 10 x

Jovian, (also more carbon) absorbs red, reflects blue

• Hydrocarbon ‘smog’ – mainly frozen methane haze and clouds with ethane, acetylene, etc.

• Discovery of Jupiter’s ring(s) by Voyager 2

Discovery of Ring Systems in Jovian Planets

• Jupiter --------- Voyager 2

• Saturn --------- Galileo

• Uranus --------- Occultation

• Neptune --------- Occultation

- Occultation Obscuration of starlight by rings

- Jupiter’s ring is very thin and dusty material

Occultation of starlight by rings of Uranus

Dusty rings of Uranus not seen from the Earth; occultation method used following the discovery of Jupiter’s ring by Voyager 2

‘Occultation Spectrum’ due to rings of Uranus – dimming of starlight blocked

by the rings as seen from the Earth

Uranus’ rings and moons

Uraniun system with rings

Note the rotation axis and position of rings

Voyager 2 view of Rings of Uranus(dusty, extended atmosphere makes

rings unstable)

Relatively more extended atmospheres due to lower gravity than Jupiter and Saturn

Neptune’s rings (similar to Uranus – dusty ambient atmosphere)

The slit in the middle blocks out light from the main body of the planet,Thereby enabling the faint rings to be seen

Rings of Uranus and Neptune

• Both Uranus and Neptune have ring systems

• Made mainly of rocks, not ice like Saturn, therefore nearly invisible

• Owing to low gravity, the ring structures are diffuse and unstable: extend into the moons (hence heavily cratered) or spiral into the planet

• Probably last no more than 10-100 million years

Moons of Uranus and Neptune• Uranus has 5 large moons, and 10 small

ones, all heavily cratered• Miranda shows large oval structures; may

have shattered several times and re-assembled

• Neptune has 5 small moons (< 100 Kms) bunched near the rings; followed by Proteus at ~ 200 Kms, and then Triton

• Uranus and Neptune do not have large Galilean satellites; did not form moons on their own as in Jupiter and Saturn

Moons of Uranus – heavily cratered due to interaction with loose ring

particles and dust

Miranda – Broken and re-assembled ?

Triton – captured by Neptune ?

Orbits opposite to direction of rotation of Neptune

Triton

• Triton is more dense, ~ 2 g/cc, indicating rocky composition, than the moons nearer to Neptune which are mostly ice

• Reverse of the trend for Galilean satellites• Triton might have been a Kuiper Belt

Object, captured by Neptune• Has a very thin atmosphere, 0.00002 bars• T = 38 K N and CH4 freeze and “snow”• Polar caps extend to 25 degree latitude