Published on December 23, 2008
Uranus was discovered by chance, but Neptune’s existence was predicted by applying Newtonian mechanics. : No one knows who discovered the five planets – they’ve been known since antiquity. Uranus was discovered by “accident” by German-born musician turned astronomer William Herschel in 1781 using a large telescope to search for comets. Neptune’s existence position was predicted mathematically due to perturbations in Uranus’ orbit by Adams and Le Verrier in 1846. Oddities in Neptune’s orbit motivated a long search for planet Pluto. Uranus was discovered by chance, but Neptune’s existence was predicted by applying Newtonian mechanics. William HerschelDiscovered Uranus 1781 : William HerschelDiscovered Uranus 1781 Herschel’s telescope. It is a Newtonian design, 6.5 ft long with a 7 inch diameter mirror. William Herschel (1752-1822) Musician and Astronomer Discovery of Uranus: A strange tale : Discovery of Uranus: A strange tale Herschel (who was a musician, not an astronomer) had built the best telescope in England. In January 1781, he discovered an object that was ‘fuzzy’ and moved with respect to the stars. He thought it was a comet. This was not the first ‘discovery’ of Uranus, but previous observers (with inferior telescopes) thought it was a star. He reported it as a comet discovery to the Astronomer Royal, who confirmed the discovery, invited Herschel to the Royal Society to present his discovery. In the next few months, measurements of the orbit made it clear that the ‘comet’ was orbiting the Sun like a planet (nearly circular orbit), not comet (nearly parabolic orbit). Herschel proposed the name ‘Georgium Sidus’ [Georgian planet] to flatter King George III, but the name Uranus (father of Saturn, grandfather of Jupiter in Greek mythology) was finally accepted. This name was only accepted after c.1850. King George made Herschel a Royal Astronomer, paid him £ 200 per year, and also purchased a telescope. The King of Spain then ordered a telescope for £ 1200! Herschel became quite famous and later built much larger telescopes (at right). His son John later named the 4 largest moons of Uranus (Oberon, Titania, Ariel, Umbriel). These are the only satellites in the solar system not named after Greek or Roman mythological characters. King George III Herschel also discovered Infrared radiation (1800) : Herschel also discovered Infrared radiation (1800) Herschel used blackened thermometers, placed them in the ‘rainbow’ produced by a prism. He found that the highest reading occurred just past the red edge of the spectrum. This was the first demonstration of ‘invisible’ radiation, i.e. not detectable with the eye. Slide 6: In visible light Uranus is nearly featureless and has an unusually tilted axis of rotation. Uranus’ tilted axis of rotation orientation remains fixed with respect to the stars : Uranus’ tilted axis of rotation orientation remains fixed with respect to the stars Slide 8: Methane gives it its character-istic blue color by absorbing red light. Neptune is a cold, bluish world with Jupiterlike atmospheric features. : Neptune is a cold, bluish world with Jupiterlike atmospheric features. Discovery of Neptune: Another strange tale and (eventually) a Triumph of Theoretical Physics : Discovery of Neptune: Another strange tale and (eventually) a Triumph of Theoretical Physics By 1840, it was clear that the motion of Uranus was not consistent with an ellipse, as expected from Kepler’s 1st law. Some astronomers suspected another planet was perturbing the orbit, but others (e.g. Airy, Astronomer Royal of England) though Newton’s law of gravity might be incorrect at large distances. John Adams, a brilliant mathematics major at Cambridge, become interested in the problem, and inferred the existence of another planet, whose position he predicted in 1845. He asked Sir George Airy [Astronomer Royal] to observe the planet, but his wife forgot to give airy the message. He also asked Challis a Cambridge Observatory to try observing. Challis missed seeing Neptune – he had actually observed Neptune but failed to recognize its motion. Meanwhile, in France Le Verrier also predicted a new planet, and tried to get the Paris Observatory to observe it without success. Finally Galle at Berlin Observatory was asked to search at found Neptune the first night. Neptune position: predicted vs. observed : Neptune position: predicted vs. observed Slide 15: High altitude cirrus clouds made of methane ice crystals. Slide 16: Neptune has zones and belts much like Jupiter. This is somewhat of a surprise considering how far it is from the Sun which drives much of the circulation given that Uranus has very little obvious structure. Neptune has more internal cloud building energy than Uranus. Uranus and Neptune contain a higher proportion of heavy elements than Jupiter and Saturn. : Jupiter and Saturn are primarily hydrogen. Uranus and Neptune are about half the size, but have much higher average densities, therefore they must have a different composition. Uranus and Neptune likely took longer to build their relatively larger cores than Jupiter and Saturn. Uranus and Neptune contain a higher proportion of heavy elements than Jupiter and Saturn. The magnetic fields of both Uranus and Neptune are oriented at unusual angles. : The magnetic fields of both Uranus and Neptune are oriented at unusual angles. Uranus and Neptune each have a system of thin, dark rings. : Uranus and Neptune each have a system of thin, dark rings. Slide 22: Uranus and Neptune each have a system of thin, dark rings. Slide 23: This view of Uranus’ rings was taken looking back at the Sun showing significant dust between the rings. The streaks are stars blurred because of the spacecraft’s motion. Some of Uranus’s satellites show evidence of past tidal heating. : Some of Uranus’s satellites show evidence of past tidal heating. Named for characters in Shakespeare’s plays. Have densities around 1.5 g/cm3 (mixture of rock and ice). Uranus’ moons are quite dark (low albedo). Some of Uranus’s satellites show evidence of past tidal heating. : MIRANDA Partially covered by ancient cratered terrain. Partially covered by parallel networks of valleys and ridges. Lower left hand side seems to have a “bite” removed. Possibly was partially shattered by an enormous impact and then recombined under its own gravity. Some of Uranus’s satellites show evidence of past tidal heating. Triton is a frigid, icy world with a young surface and a tenuous atmosphere. : Triton is a frigid, icy world with a young surface and a tenuous atmosphere. Pluto was discovered after a laborious search of the heavens. : After Neptune was discovered using perturbations in Uranus’ orbit, everyone was interested in finding a planet beyond Neptune causing perturbations in Neptune’s orbit. The effort was called the search for the trans-Neptunian planet, or planet X. Pluto was discovered after a laborious search of the heavens. Slide 31: Pluto was discovered after a laborious search of the heavens. Pluto was discovered by American Clyde Tombaugh in 1930 by comparing pictures taken of the sky several days apart and looking for the object that “moves.” Clyde Tombaugh: Discoverer of Pluto (1930) : Clyde Tombaugh: Discoverer of Pluto (1930) Slide 34: 1978 discovery image of Pluto’s Moon Charon. Pluto and its moon Charon may be typical of a thousand icy objects that orbit far from the Sun. : Pluto and its moon Charon may be typical of a thousand icy objects that orbit far from the Sun. Pluto and Charon have average densities of about 2000 kg/m3 (nearly the same as Tritan). Spectroscopy of Pluto’s surface suggests solid ice covers the planet (N2, CH4, CO). This ice probably sublimates into a thin atmosphere during the day and falls as snow at night. Pluto and Tritan are probably the largest representatives of a large group of icy bodies that orbit beyond Neptune in a region called the Kuiper Belt. Pluto and Charon are tidally locked in a dance that takes about 6 days. New Horizons Mission to Pluto : New Horizons Mission to Pluto NASA approval April 9, 2003. Launch 2006, arrive at Pluto by 2020. Many of the key questions about Pluto and its satellite Charon await the close-up observation of a space flight mission. Beyond Pluto lies the recently-discovered Kuiper Disk of "ice dwarfs" or minor planets. If the encounter with Pluto is successful, the mission may be extended to encounter one or more icy, asteroid-sized objects in the Kuiper Disk. HST image of Pluto, Charon Outer Planets Review Questions : Outer Planets Review Questions How was Uranus discovered? Accidentally by Herschel 1781, who originally thought it was a comet. How was Neptune discovered? Both Adams and le Verrier (1846) independently calculated the perturbations in Uranus orbit and predicted it was due to a new planet: Neptune. What gives Uranus and Neptune their distinctive greenish-blue color? Methane gas, which absorbs red, leaving blue-green color Why are the clouds on Neptune so much more visible than those on Uranus? Neptune, like Jupiter and Saturn, emits more radiation than it receives, due to contraction. This heats the interior, and drives ‘weather’ on the surface. What is so unusual about the magnetic fields of Uranus and Neptune? Both are offset from center of planet and highly tilted. Why are the rings of Uranus and Neptune so difficult to see? Both are very thin and narrow. Do the moons of Uranus show any signs of geologic activity? Miranda’s southern hemisphere appears cracked and streaked, perhaps from tidel resonance and internal heating What makes Neptune’s moon Triton unique in the solar system? Only satellite with a retrograde orbit: probably captured I t has a young icy surface perhaps caused by tectonic activity due to tidal heating.