PURDUE UNIVERSITY

EAS 105-THE PLANETS

Prof. Robert L. Nowack

 

Lecture 16

 

 

Uranus and Neptune

 

 

Uranus

 

 

 

 

Neptune

 

 

 

 

Uranus and Neptune are quite similar in size.  Uranus has 14.5 times more mass than the Earth and Neptune has 17.1 times.  Both planets are smaller than Jupiter and Saturn.  They can be considered as "small" gas giants.  Uranus orbits around the Sun at an average distance of 19.2 Au, and Neptune orbits 30 Au from the Sun.  Uranus has a bulk density of about 1,300 kg/m3, and Neptune has a bulk density of 1,600 kg/m3.  Both bulk densities are similar to Jupiter. 

 

Diameters of both planets are about 4 times that of Earth’s.  Uranus takes 84 years to orbit the Sun, and Neptune takes 164 years. 

 

 

 

 

Both Uranus and Neptune were discovered planets.  William Herschel, an English-German musician and amateur astronomer discovered Uranus in 1781.  He found the planet while observing stars in the constellation Gemini using a 6” telescope.  One "star" seemed to look more like a small disk instead of a point.  Subsequent successive observations showed that the object moved in the sky, but at a speed too slow to be a comet.  Soon, its path was mapped well enough to confirm that it was a new planet with a nearly circular orbit lying beyond that of Saturn.  Herschel received knighthood for this accomplishment and went on to become a productive astronomer of his day.  Herschel wanted to name the new planet after King George III, but it was named Uranus, the Grandfather of Jupiter, continuing the tradition of naming planets after Greek and Roman gods.

 

Uranus' distance from the Sun was similar to that found from an empirical relationship known as the Titius-Bode Law.  Unfortunately, this law breaks down for Neptune and Pluto.  While Uranus was found unexpectedly, Neptune was found as a result of a mathematical prediction.

 

After the discovery of Uranus, its measured orbit was compared with predicted positions.  By 1830, the predicted and measured positions were off by more than 4 planet diameters.  One possibility was that discrepancy was the result of a perturbation caused by the gravitational pull of some unknown body.  As a starting point, a distance out from the Sun of 39 Au was assumed – the next position in the empirical Titius-Bode Law.

 

Two relatively unknown mathematicians, Urbain J. J. Leverrier from France and John Couch Adams from England, worked on the problem of predicting the position in the skies and mass of this unknown body.  Adams finished his calculations in Sept. 1845 and tried delivering them personally to Sir George Airy, the Royal Astronomer.  Airy was initially unavailable and finally when he read the report was skeptical.  Adams felt rebuffed.  

 

Meanwhile, Leverrier published his preliminary calculations on November 10, 1845.  He completed his analysis in June 1846.  Airy read this report and realized Leverrier's predicted position of another planet agreed with Adam’s predicted position within several degrees.  Airy directed the Cambridge Observatory to start searching for the new planet in July, 1846.  Unfortunately, they lacked good star charts of this part of the skies.

 

Leverrier presented his results to the French Academy in August 1846.  He was unsuccessful in convincing French Astronomers to look for the new planet.  In desperation he wrote to a young assistant he knew, Johann Galle at the Berlin Observatory.  They had good charts of this part of the sky.  Using these charts and the 9-inch Berlin Telescope, Galle found the new planet on September 23, 1846 - on his first try!

 

 

 

John Couch Adams

 

 

 

 

Urbain J. J. Leverrier

 

 

A dispute ultimately developed as to who discovered Neptune.  However, today both individuals, Adams and Leverrier, are given credit for predicting its existence.  The new planet was named Neptune, after the Roman God of the Sea.  As it turned out, Galileo observed Neptune 234 years earlier.  Unfortunately, charts didn’t exist for him to identify it.

 

Continued observations of Uranus and Neptune in the 19th century suggested that yet another planet existed.  A search began in spite of less convincing evidence.  Percival Lowell was one of the most persistent observers.  In 1930, Clyde Tombaugh discovered “Planet X” using the Lowell Observatory Telescope and called it Pluto. 

 

 

Image Leading to the Discovery of Pluto

 

 

 

 

Note:  Pluto is far too small to perturb Neptune’s orbit enough to make its presence known, although Percival Lowell made endless calculations to find it.  Clyde Tombaugh found Pluto (magnitude ~13.5 ) by systematically taking pictures of the plane of the Solar System.  He took pictures in pairs one or two weeks apart and looked for anything that moved.  The idea is that planets would appear to shift against the backdrop of stars because the Earth had moved to a new viewing angle over the intervening two-week period.

 

Scientists now know Pluto is far too small to cause perturbations of Uranus and Neptune.  Recent studies suggest that the problem actually was in the observations of these two planets.  However, interest in the possibility of a 10th planet persists and a number of small bodies have been found past Pluto.

 

Neptune and Uranus are nearly twins in size - about 15 times as massive as Earth.  Their densities are comparable to Jupiter, but because of their smaller sizes, they may have a higher proportion of heavier elements.  The size and densities of Uranus and Neptune suggest that metallic hydrogen may not be present.

 

Uranus and Neptune may also have cores of rock and ice, the same size as Jupiter and Saturn’s.   Some scientists suggest an intermediate layer of dense water vapor clouds for Uranus and Neptune.

 

Still, Uranus and Neptune are not identical.  There densities are somewhat different.  Also, Uranus appears to lack a significant internal heat source, while Neptune (similar to Jupiter and Saturn) generates some internal heat.  This could result from gravitational settling or loss of residual primordial heat.  Another puzzle is the peculiar rotation axis for Uranus which is almost in the orbit plane.

 

 

 

 

Hydrogen and helium appear to dominate the atmospheres of Uranus and Neptune.  However, they have more methane (CH4) than Jupiter or Saturn, consistent with being smaller and having more heavier elements compared to Jupiter or Saturn.

 

 

 

 

Voyager results indicate that Uranus has an upper atmosphere haze over its tilted rotational pole. 

 

An image of Uranus shows high altitude haze over the South Polar area.  A pair of high clouds (or plume-type) labeled A and B are 4,300 and 3,100 km across respectively.  Notice how they rotate with the planet.

 

 

 

 

Uranus has very peculiar seasons because its axis of rotation nearly aligns with the plane of orbit.  Uranus takes 84 years to orbit the Sun.  This means one pole receives sunlight for 42 years then the other pole for 42 years.  One might think this would effect atmospheric circulation.  However, Voyager found that Uranus’ circulation and clouds are dominated by the planet's rotation.

 

Seasons last 21 years on Uranus.

 

 

 

 

On Neptune, the inclination of the equator to the orbit planet is 29.6° (similar to Earth and Saturn).  Thus, Neptune would have similar types of "seasons".  Neptune's blue color results from the fractional amounts of Methane (CH4) in the atmosphere.  It has zonal bands and giant storms reminiscent of Jupiter.

 

 

 

The Great Dark Spot

 

 

 

 

Close-up view of Great Dark Spot on Neptune shows associated white clouds that occur at higher altitudes.

 

Rotation of their magnetic fields was used to determine internal rotation period for Uranus and Neptune.  They turned out to be 17.24 hours for Uranus and 16.11 hours for Neptune.  However, outer atmosphere on Uranus rotates at about 16 hours.  Fastest winds blow at the north and south poles, not at the equator.  Finally temperature at the poles of Uranus seem about the same even though one pole has been in the dark for 42 years.  (Note that Uranus rotates in a retrograde manner).

 

On Neptune, the outer atmosphere also rotates at different rates.  The Great Dark Spot at 22°S rotates about the planet every 18.3 hours.  At 55°S, a small dark spot takes 16 hours.  Also, the Great Dark Spot tends to circulate about in a counterclockwise fashion.  While the white clouds continue to be seen, subsequent high-resolution Earth-based images reveal that the Great Dark Spot has faded.

 

Uranus has both a magnetic field and a magnetosphere.

 

 

 

 

However, Uranus' magnetic pole is inclined by 60° to the rotation axis.  In addition, it is offset from the center of the planet by 1/3 of a planetary radius.  The question is still open as to what causes the magnetic field.  There probably is no metallic hydrogen so there must be some other type of dynamo effect.

 

In 1989, Neptune was also found to have a magnetic field.  It likewise inclines to the axis of rotation by 47°, similar to Uranus.  Thus, an inclined off-center magnetic pole is not unique to Uranus.  Both planets have a magnetosphere containing trapped plasma.

 

Pluto is a special case.  Pluto is smaller than our Moon and has a bulk density of 2000 kg/m3.  This suggests that Pluto is composed partly of water, ice, and rock.  Its rotational period is about 6 days and 9 hours.

 

 

 

 

Pluto has a highly eccentric orbit which crosses the path of Neptune.  Thus, there are periods when Pluto is actually closer to the Sun than Neptune.  It was closer to the Sun than Neptune until 1999.

 

 

 

Pluto’s Orbit

 

 

 

 

In 1978, scientists discovered that Pluto has a Moon, Charon.  It has a mass 1/10 that of Pluto and appears to be tidally locked with a revolution period the same as Pluto’s rotation period.  Pluto’s equator was also found to be inclined by 118° to its orbit plane.  Similar to Uranus (assuming Charon orbits the planet Pluto in its equator plane), Pluto would also be retrograde.

 

 

 

 

To an observer positioned above the plane of the Solar System, both Uranus and Pluto would seem to rotate in a retrograde direction. (Sizes not to scale in diagram)

 

It has been suggested that Pluto is actually an escaped satellite of Neptune by some catastrophic collision.  However, this hypothesis has problems since Pluto has its own small Moon, Charon.  Nevertheless, Pluto and Neptune's Moon, Triton, appear to be similar in size and bulk density.  Possibly both are composed partly of methane ice.

 

 

 

 

            On July 29, 2005, a large object was found on the order of the size of Pluto with an orbit about 3 times Pluto’s distance from the Sun.  This “tenth planet” is just one of a large collection of objects in the Kuiper Belt.  It was later found that it also has a small Moon.

 

            In October 2005, the Hubble Space Telescope was used to identify two additional small Moon’s of Pluto which orbit in the same plane as Charon.  These Moons, along with the larger Charon, suggest a violent history for Pluto.