ptys/astr 206outer worlds 4/19/07 the outer worlds: uranus, neptune, and pluto

PTYS/ASTR 206 Outer Worlds4/19/07

The Outer Worlds:Uranus, Neptune,

and Pluto


Announcements

• Reading Assignment– Finish Chapter 17

• 6th homework due on Tuesday, April 24

• Quiz on Tuesday, April 24 (will cover all lectures since the last exam and Chapters 14-17)

• Final Activity next Thursday – Come prepared with 2 multiple-choice final exam questions

• Next study-group session is Monday, April 23, from 10:30AM-12:00Noon – in room 330.

• Public lecture – Tuesday, April 24 7:30PM (this room)

• Movie Night – Monday, April 23, 6:30PM (this room)


Discovery of Uranus

• “Discovered” by William Herschel in 1781 (discovered by chance)– Herschel was an English

musician and amateur astronomer

– He used a 6.5” homemade reflector (it took him over 200 attempts to make it!)

– Prior to its discovery, Uranus had been seen, but not recognized as a planet (it had been designated 34 Tauri)

• Herschel made many contributions to astronomy, including the discovery of binary star systems, and the motion of the solar system in space


Discovery of Neptune

• The problem:– Uranus’s orbit didn’t match Newton’s Laws,

with the forces from all known planets at that time

• Proposed solution – there must be another planet !– Bode’s Law gives the distance to be 39 AU– Its mass must be about the same as Uranus


Neptune’s Discovery

• John Couch Adams (English student)– Predicted its location in 1843

(using Newton’s Laws including the its force on Uranus to explain Uranus’s orbit)

– George Airy (Astronomer Royal) wouldn’t look!

• Wanted a few more details worked out



• Urbain Le Verrier (French Professor)– Predicted its location in

1846 – published it– Airy (now interested) did not

have good enough star charts

– Johann Galle (German astronomer) did have good star charts

– LeVerrier convinced Galle to look for it



• Galle found it in 30 minutes !

• Galle is often credited for being the discoverer, although Adams and Le Verrier were the first to predict its presence

– Instructors note: Why did Adams get credit? He never published it !


Uranus and Neptune from Earth

• Uranus is visible with the naked eye under good (dark sky) conditions

• Neptune is not visible to the naked eye

• Both can be seen as planets with moderate-sized telescopes (rather than a point-like star)

• No surface features are visible without a filter– Only recently were a ground-

based telescope able to discern surface features on Uranus using an IR filter and adaptive optics

Uranus

Neptune


Uranus: Basic Facts

• Equatorial diameter of 51,800 kilometers – about 4 Earth diameters

• Mass is about 14.5 Earth masses

• Density =1.3 g/cm3

• Orbital period is 84.01 years– About the average lifespan of a human

• average distance from the Sun is about 19 AU

• Retrograde rotation

• Rotational period is 17 hours 14 minutes.

• It has 27 moons– The two largest are Titania and Oberon– Both are about ½ the size of our moon


Neptune: Basic Facts• Equatorial diameter of 49,500 kilometers – Nearly 4 Earth diameters – A little bit smaller than Uranus

• Mass is about 17 Earth masses– A little bit more massive than Uranus

• Density = 1.7 g/cm3

• Orbital period is about 165 years

• It is about 29.8 AU from the Sun

• Rotational period is about 16 hours

• Has 13 moons– The largest is Triton which is about

3/4 the size of our moon


Uranus’s Composition and Appearance

• The atmosphere of Uranus is composed of – 83% hydrogen, – 15% helium, – 2% methane

• Methane absorbs red light, giving Uranus its blue-green color. – Methane on Uranus is 5 times

more abundant than on Jupiter and Saturn

• It lacks an appreciable internal heat source, thus there are no visible surface features


Uranus’s Atmosphere

• Visibly, Uranus appears as a virtually featureless hazy blue ball. – Lack of internal heat. – Clouds on Uranus are cold and

don't billow up above the top haze layer.

• Seen in false color (at the right), Uranus reveals atmospheric features

• The atmosphere is arranged into clouds running at constant latitudes (like Jupiter and Saturn)


Uranus has an unusual tilt

• Herschel discovered that moons were orbiting in a plane perpendicular to its orbit– Moons orbit about their parent

body’s equator

• Uranus’s rotation axis is tilted 98o with respect to the plane of its orbit about the Sun

• Thought to be the result of a collision with a planet sized body early in the solar system formation


Neptune’s Atmosphere

• Neptune's atmosphere is made up of hydrogen, helium, and methane.

• Neptune's has clouds and storm systems that revolve around the planet, but with wind speeds of 300 m/sec (700 mi/hr

• These winds are driven by an internal heat source (unlike Uranus)


Neptune’s Clouds

• The clouds seen on Neptune are frozen methane in the atmosphere

• Much more cloud activity is seen on Neptune than on Uranus

• This is because Uranus lacks a substantial internal heat source while Neptune does– Neptune radiates more

than twice the energy it receives from the Sun


The Giant Dark Spot on Neptune

• Similar to Jupiter’s Giant Red Spot (GRS)

• However, the giant dark spot on Neptune is not as long-lived as the GRS on Jupiter– Disappeared in 1994– Another re-appeared in the

northern hemisphere in 1995

• Rotates counterclockwise, as does the GRS on Jupiter


Uranus and Neptune contain a higher proportionof heavy elements than Jupiter and Saturn

• Both Uranus and Neptune may have a rocky core (about the size of Earth) surrounded by a mantle of water and ammonia (like Windex!)

• Electric currents in the mantles may generate the magnetic fields of the planets


The Origin of Magnetic Fields on Uranus and Neptune

• Not massive enough to possess a liquid metallic hydrogen layer like Jupiter

• H2O in the interior is under enough pressure to induce ionization– Ionization increases its

electrical conductivity– An ionized-water

dynamo!


• The magnetic axes of both Uranus and Neptune are steeply inclined from their axes of rotation

• The magnetic and rotational axes of all the other planets are more nearly parallel

• The magnetic fields of Uranus and Neptune are also offset from the centers of the planets


Uranus and Neptune each have a system of thin,

dark rings


Triton (Moon of Neptune)

• Discovered by William Lassell only a month after Neptune was discovered

• Orbits Neptune in a retrograde orbit !– opposite to the rotation

direction of Neptune– The only large moon to do

this

• The orbit is also highly inclined relative to Neptune’s equator

• Density = 2066 kg/m3

– More rock than in Saturn’s moons

Probably a captured asteroid!


Triton (Moon of Neptune)

• Shows evidence of geological activity

– Lacks large craters – it is a very young surface

– Probably tidal heating from Neptune

– Perhaps extreme tidal heating in the past

• It also has a tenuous nitrogen (N2) atmosphere

– 70,000 times less atmospheric pressure than Earth

– about 500 times thinner than the Martian atmosphere


Pluto and Charon

• How was Pluto Discovered?• What is the nature of the

planet and its moon?• Is Pluto a planet?


Planet “X”

• Percival Lowell repeated Adams & Le Verrier calculations for planet “X”

– He looked, but never found it

• After Lowell Died, there was a tedious search


• Lowell hired Clyde Tombaugh who discovered Pluto in 1930

• Pluto was named after the lord of Hades (in Greek mythology), “PL” are the initials of Percival Lowell


Pluto’s Discovery

• Only “planet” found by an American, and only planet found in the 20th century

• Too small to effect the orbits of other planets– The calculations of

Lowell didn’t matter

ptys/astr 206outer worlds 4/19/07 the outer worlds: uranus, neptune, and pluto

Documents

gcm3orbital period

earth diameters mass

earth massesdensity

sunrotational period

good star chartsleverrier

earth massesa little

basic factsequatorial

moonsthe largest