Frozen Worlds - Uranus, Neptune, Pluto

Reorganized text plus new material - see other web entry.

Discovery of New Worlds

Uranus - 1781 - accidential - William Herschel - period of 84 years - amateur astronomer, first thought was comet, named for King George but not accepted. Uranus father of Saturn. Note Uranium named after new planet.

Neptune - 1846 - period 165 years - predicted from difference predicted and observed position of Uranus - by Adams (English) and Leverrier (French) found by Galle (German) - named after Roman god of the sea.

Charon - 1978 - satellite of Pluto - accidental.

Comparison of Uranus and Neptune

Mostly New



Slow orbit about Sun - far away

84 years

165 years


Blue - Green Methane Ice


Blue - Green Methane Ice

Small size for giant planet

4 RE

RE = Earth radius

Jupiter = 11 RE

4 RE

RE = Earth radius

Jupiter = 11 RE

Low Density

1.3 g/cm3

But not mostly hydrogen and helium because of small size requires greater mass for density

1.6 g/cm3

But not mostly hydrogen and helium because of small size requires greater mass for density

Surface Temperature



Excess Heat


2.6 times heat from sunlight





Thin, Empty

Thin, Empty


Self-generated heat of Neptune = 2.6 times heat due to absorbed sunlight, greatest such ratio known, yet Uranus has no detectable excess heat.

Uranus and Neptune - internal structure and magnetic fields - (also see other web entry)

Outer envelope hydrogen and helium about one Earth mass.

Inside cannot be mainly hydrogen and helium - if so less massive than observed for their size.

Mantle - global internal sea of water, methane and ammonia.

Core ? - small, rocky, about one earth mass, maybe.

Large tilt between rotation axis and magnetic axis. Dipolar field offset from planet center.

Magnetism generated in moving, electrically conducting fluid, mostly water mantle.

New Neptune - not in text

Dynamic, giant spots come and go.

Great Dark Spot - large as Earth - disappeared (without a trace), another one arose on opposite side of the lively world.

New Text - ignore direction of winds on Neptune and Uranus.

In contrast, Jupiter's Great red Spot stable for 300 years.

Small moon's exist inside Neptune's Roche limit.

Comparison of Titan and Triton







5,150 km

2,700 km

Density - half rock/half ice

1.9 g/cm3

2.1 g/cm3

Temperature at Surface

94o K

38o K

Pressure of Atmosphere at Surface

1.5 bars

0.000015 bars


Nitrogen, methane

Nitrogen (mostly ice)

Organic Compounds

Orange Color

Pink Color


Rings of Uranus and Neptune - also see other web entry

For example Neptune's rings can only survive for 500 million years.

Uranus' rings discovered by stellar occultation in 1977, Neptune's rings confirmed in 1989 during Voyager 2 encounter.

Pluto, Charon and Triton

Pluto is smaller than our Moon.

Pluto is tipped on its side.

Pluto might not be a full-fledged planet - all we learned in school is a lie - but Pluto is round with an atmosphere and satellite.

Pluto's satellite Charon about half the size of Pluto.

Pluto has an atmosphere, discovered by gradual stellar occultation. Like Triton, Pluto's atmosphere is about one ten thousandth of Earth's surface pressure. Contains nitrogen, methane and carbon monoxide. Pluto's atmosphere may expand when it moves closer to the Sun, receiving greater heat in slow orbit.

Pluto radius = 1123 km

Charon radius = 560 km

Earth radius = 6378 km

Charon may have collided with Pluto. Hubble Space Telescope - bright polar caps (nitrogen ice) and dark equatorial regions (darkened methane ice).

Neptune's Satellite Triton - (also see other web entry)

Triton in retrograde, inclined orbit.

Triton headed for collision with Neptune.

Triton's frozen surface at 38o K coldest yet measured, since far from Sun and surface ice reflects the feeble sunlight.

Triton has towering nitrogen - powered geysers, and bright polar caps.

Triton has a thin atmosphere of nitrogen and methane - about one ten thousandth Earth's surface pressure.

Triton either cleaned out Neptune's former satellites or kept them from forming.

Pluto, its satellite Charon, and Neptune's satellite Triton all have unusual orbits and lie on the inner edge of the Kuiper belt of icy-rocky objects. They are all also small, and denser than the giant planets.

Comparison of Triton and Pluto - (also see other web entry)



Remote Distance

30 AU

29AU (now)

Small Size (diameter)

2700 km

2300 km

Low, but not too low, Density. Half rock and half ice.

2.1 g/cm3

~2 g/cm3

Thin atmosphere

(1/10,000=10-5 bars)

Nitrogen and Methane

Methane and Nitrogen