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 of Uranus deviated substantially from its predicted motion -- a so-called orbital anomaly.

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

Independently, the French Scientist Urbain Leverrier predicted a similar location for a new planet in 1846. Using the prediction of Leverrier, the German astronomer Johann Gottfriend Galle was the first to actually observe 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.

T T

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.