Science 3210 001 : Introduction to Astronomy

Lecture 6 : The Outer Planets

Robert Fisher

Items

! Solution set 4 has been posted to the website.

! Reading/Homework set 5 has been posted to the website.

! Lunar eclipse tomorrow!

! Visible from moonrise (at about 5:50 PM) though about 7:15 PM.

! Look just north of due East to see the moon rise.

! Observational project.

Today -- Outer Solar System

! Jupiter

! Saturn

! Uranus

! Neptune

! Pluto (Ok -- not a planet anymore, but a dwarf planet in the

Kuiper belt!)

Comparison of Outer Planets

! A 3D computer rendering of the outer planets, compared with the

inner planets, all to-scale :

Jupiter

! Jupiter, the “king of the planets,” is the largest planet in the solar system.

! It has a mass 320 times that of the Earth.

! It has a radius 10 times that of the Earth, and is visibly oblate due to its

strong rotation.

! Its enormous volume has allowed it to retain the heat left over from its

formation even today -- it radiates away more energy than it takes in

from the sun.

Jupiter

! Jupiter’s cloud layers and red spot.

Giant Red Spot

Moons of Jupiter

! Jupiter has 63 known moons, including 4 major ones which are

large enough to be spherical.

! The four major moons of Jupiter were discovered by Galileo in

1610 and are referred to as the Galilean moons.

Io

Ganymede Callisto

Europa

5,000 km

Io

! Jupiter’s moon Io is volcanically active, spewing sulfur plumes.

Europa

! Detailed images of the surface of Europa taken with spacecraft

reveal a highly-fractured icy surface, possibly with an underlying

oceanic layer.

Ganymede

! Ganymede, as photographed by Voyager and Galileo, reveals a

complex surface activity similar to Europa.

Jupiter’s Cloud Layer

! Like the Earth, the Coriolis effect due to Jupiter’s rotation divides

the circulation patterns into a number of bands or cells.

! Unlike the Earth, the greater rotation of Jupiter leads to a larger

number of bands.

! The colorations we see are caused when either uprising currents

allow the top layer of ammonia to be visible (as white) or the

lower-lying ammonium hydrosulfide layer to be visible (as

reddish-brown).

Jupiter’s Ring System

! Jupiter has a ring system like Saturn’s, although much less

spectacular.

Saturn

! Saturn is the second-largest planet in the solar system, and is the

only planet in the solar system to have an average density less

than that of liquid water.

! Mass 95 times that of the Earth.

! Radius about 4.5 times that of Earth, and is visibly oblate due to

its strong rotation, similar to Jupiter.

Artist’s Conception of Saturn’s RIngs

Saturn’s Rings Imaged by Voyager

Shepherd Moons

! Two small moons can “shepherd” ring material into a particularly

sharp ring, as is the case with Saturn’s F ring :

Saturn’s Moons

! Saturn has a rich system of satellites -- some 60 in total.

! Of these, seven are major satellites which are large enough to be

spherical.

Titan

! Several years ago, NASA and the European Space Agency

(ESA) sent the Cassini/Huygens mission to Saturn. The Huygens

space probe entered the atmosphere of Saturn and took detailed

measurements of its properties on its descent.

Titan

! Titan is unique in the solar system -- it is a terrestrial-sized world

with an atmosphere containing ethane and methane, both of

which can be solid, liquid, and gaseous, just as water on Earth.

Liquid Ethane Lakes?

Uranus

! Uranus was the first planet to have been discovered, and not

known from antiquity.

! It was discovered by accident by William Herschel

! It has roughly 14 times the mass of the Earth, with a radius about

4 times the radius of the Earth.

Uranus as Photographed by Voyager 2

! In January of 1986, shortly before the Challenger disaster,

Voyager 2 flew by Uranus -- the first space mission to have ever

visited it.

! The atmosphere of the planet consists of a methane haze and

has virtually no distinguishable atmospheric features -- faint cloud

patterns can only be brought out in “false color”.

Moons of Uranus

! Uranus has at least twenty-seven known moons, including five

major ones.

! The names of the moons are chosen not from classical mythology

but rather Shakespeare.

ArialMiranda Umbriel Titania Oberon

1500 km

The Surprising Miranda Moon

! On its flyby, Voyager 2 approached the moon Miranda the

closest.

! Given the distance from the Sun and the extremely cold

temperatures (-335 degrees Fahrenheit), scientists expected a

cold dead world.

! Instead, they were stunned to find a surprising world with a

remarkably complex surface.

Miranda as Imaged by Voyager 2

A Closeup of the Surface of Miranda

Neptune

! Neptune is similar in more similar to Uranus than it is to either

Jupiter and Saturn.

! Mass of 17 times the Earth.

! Radius of about 4 times that of Earth.

The Story of the Discovery of Neptune

! If Uranus was the first planet to be discovered, Neptune was the first(and actually only) planet to be predicted.

! By the early 1840s, it became evident that the actual observed motion ofUranus deviated substantially from its predicted motion -- a so-calledorbital anomaly.

! British scientist John Couch Adams first inferred a predicted location forthe new planet in 1843. Adams sent the British Royal Astronomer SirGeorge Airy the prediction. The British observational effort, however,floundered.

! Independently, the French Scientist Urbain Leverrier predicted a similarlocation for a new planet in 1846. Using the prediction of Leverrier, theGerman astronomer Johann Gottfriend Galle was the first to actuallyobserve Neptune, on September 23, 1846.

Neptune

! The Voyager 2 encounter with Neptune on August 25, 1989 was

its last major encounter with a planet before leaving the solar

system.

Dark Spot

Moons of Neptune

! Neptune has 13 known moons, including the major moon Triton,

which is unique among the major moons in the solar system for

its retrograde motion.

Triton as Imaged by Voyager 2

! Like Miranda, Triton revealed a remarkably complex, geologically-

active surface.

“Canteloupe”

Triton as a Kuiper Belt Object ?

! The unusual orbital and surface properties of Triton have led

some astronomers to suggest that in fact it is a captured Kuiper

Belt Object, like Pluto.

! The most common model suggests that Triton may have had its

own satellite prior to the encounter, which was lost during the

capture. Other “primordial moons” of Neptune would also have

been lost in the process.

TT

Pluto / Kuiper Belt Objects

! Pluto (now recognized by the International Astronomical Union as

a “dwarf planet” and a Kuiper Belt Object) was discovered by

Clyde Tombaugh in 1931.

The Saga of Planet X

! Neptune had been discovered by Adams and Leverrier by the

careful analysis of unexplained anomalies in the orbit of Uranus.

! By about 1900, the orbit of Neptune began to exhibit apparent

anomalies in its orbit, much like Uranus had nearly in the early

part of the 19th century.

! Some astronomers believed that apparent anomalies in the orbit

of Neptune could be explained by the existence of a ninth, yet-

undiscovered planet -- Planet X.

Percival Lowell (1855 - 1916)

! Percival Lowell was an astronomer, mathematician, and writer

born to an elite Boston clan. He is perhaps one of the best and

one of the last examples of a Renaissance scholar who funded

his own research efforts and constructed his own observatory.

And Planet X Inspired Bad Science Fiction…

Clyde Tombaugh (1906 - 1997)

! Tombaugh was born on a farm in Streator, Illinois. His family later

moved to Kansas, where he built his own telescope.

! After sending his astronomical drawings to the Lowell observatory

in Arizona, he was offered a position as an observer -- with just a

high school diploma!

! He discovered Pluto in 1930 while searching for the mysterious

“Planet X” which had been predicted by Percival Lowell almost 30

years earlier.

! After discovering Pluto, he went on to attend college and

graduate school!

Clyde Tombaugh with Homemade Telescope --

c. 1930

Blink Comparator

! By careful comparison of two photographic plates of the same

portion of the sky taken at different times, Tombaugh found Pluto

-- initially thought to be the mysterious Planet X.

Pluto Discovery Plates

! After inspection of the discovery plates in January, 1930, it

became clear that Pluto had been seen on other plates taken by

other more senior professional astronomers at Lowell, but

missed.

Pluto’s Orbit

! The orbit of Pluto is highly unusual in comparison to the other

planets in the solar system -- it is both highly eccentric and

inclined with respect to the plane of the ecliptic.

Pluto Compared with the Earth

! Pluto is a tiny world, much smaller than any of the other planets in

the Solar System.

Pluto as Compared to Some Other Major Moons

in Solar System

! Pluto is much smaller even than most major moons in the solar

system -- its mass is equivalent to 0.2 times the mass of our our

moon.

Planet X ?

! Eventually it became clear that Pluto was far too small to account

for the anomalies of the much-larger planet of Neptune.

! The Planet X hypothesis remained for decades to come -- though

now it was hypothesized that Planet X was a tenth planet outside

Pluto.

! It was only years later, in the 1990s, that astronomers finally

understood that the “anomalies” in Neptune’s orbit were actually

the direct consequence of a few early inaccurate measurements

of Neptune’s orbit.

Next Week -- New Planetary Systems, and the

Formation of the Sun and the Planets

! Next week we will discuss the radical new detections of planets

outside our solar system that has emerged in the last ten years --

revolutionizing our understanding of how planets form.

! We will also discuss how stars form, and explore some of the

riddles of star formation that scientists are exploring today.