Space, time & CosmosLecture 3:

Solar system - outer planets & (a little bit) beyond

Prof. Ken Tsang

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The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale)

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Visible Planet Orbits This diagram shows the relative size of the orbits of the seven planets visible to the naked eye. All the orbits are nearly circular (but slightly elliptical) and nearly in the same plane as Earth's orbit (called the ecliptic).The diagram is from a view out of the ecliptic plane and away from the perpendicular axis that goes through the Sun.

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Outer Planet Orbits This shows the relative sizes and positions of the orbits of the planets farther from the Sun than Earth. All the planets have orbits that are ellipses with the Sun at one of the foci, and the ellipses are in different planes. However, only Pluto has a noticeably noncircular orbit that lies in a different plane from the other planets.

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Jupiter

based on a 1979 image from the Voyager 1 spacecraft and enhanced by the U.S. Geological Survey

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The Romans named the planet after the Roman god Jupiter (the god of sky and thunder).When viewed from Earth, Jupiter is the third brightest object in the night sky after the Moon and Venus. (At certain points in its orbit, Mars can briefly exceed Jupiter's brightness.)

The most massive planet in our solar system, with four planet-sized moons and many smaller moons, Jupiter forms a kind of miniature solar system. Jupiter resembles a star in composition. In fact, if it had been about eighty times more massive, it would have become a star rather than a planet.

Jupiter

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"Jupiter et Thétis" by Jean Ingres, 1811.Jupiter or Jove was the king of the gods, and the god of sky and thunder.

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Time-lapse sequence from the approach of Voyager I to Jupiter, showing the motion of atmospheric bands, and circulation of the great red spot.

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Jupiter's upper atmosphere is composed of about 88-92% hydrogen and 8-12% helium by percent volume or fraction of gas molecules, or approximately 75% hydrogen and 24% helium by mass.The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and five percent other elements by mass. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds.Jupiter is 2.5 times more massive than all the other planets in our Solar System combined. Jupiter's volume is equal to 1,317 Earths, yet is only 318 times as massive.Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. Beyond this basic outline, there is still considerable uncertainty.

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This cut-away illustrates a model of Jupiter's interior.

In the upper layers the atmosphere transitions to a liquid state above a thick layer of metallic hydrogen. In the center there may be a solid core of heavier elements.

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Although Jupiter would need to be about 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter. This has led some astronomers to term it a "failed star“.

In the novel, 2010: Odyssey Two, written by Arthur C. Clarke in 1987, Jupiter's density was mysteriously increased until the planet achieves nuclear fusion, becoming a mini-sun which Earth eventually names "Lucifer“, and destroying the American space ship Discovery entirely.

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This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on February 25, 1979, when the spacecraft was 9.2 million km (5.7 million mi) from Jupiter. Cloud details as small as 160 km (100 mi) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion.

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Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries.

The best known feature of Jupiter is the Great Red Spot, a persistent anti-cyclonic storm located 22° south of the equator that is larger than Earth. It is known to have been in existence since at least 1831, and possibly since 1665. Mathematical models suggest that the storm is stable and may be a permanent feature of the planet. The storm is large enough to be visible through Earth-based telescopes.

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Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer "gossamer" ring. These rings appear to be made of dust, rather than ice as is the case for Saturn's rings.

Rings of Jupiter: third ring system discovered in the Solar System, after those of Saturn and Uranus, first observed in 1979 by the Voyager 1 space probe

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Rings of Jupiter

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Triple EclipseOn Earth, we witness a solar eclipse when our Moon's shadow sweeps across our planet's face as it passes in front of our Sun. Jupiter, however, has four moons roughly the same size as Earth's Moon. The shadows of three of them occasionally sweep simultaneously across Jupiter. The image was taken March 28, 2004, with Hubble's Near Infrared Camera and Multi-Object Spectrometer.

Closer inspection by NASA's Hubble Space Telescope reveals that these spots are actually a rare alignment of three of Jupiter's largest moons - Io, Ganymede, and Callisto - across the planet's face. In this image, the telltale signatures of this alignment are the shadows [the three black circles] cast by the moons. Io's shadow is located just above center and to the left; Ganymede's on the planet's left edge; and Callisto's near the right edge. Only two of the moons, however, are visible in this image. Io is the white circle in the center of the image, and Ganymede is the blue circle at upper right. Callisto is out of the image and to the right.

Seeing three shadows on Jupiter happens only about once or twice a decade. Why is this triple eclipse so unique? Io, Ganymede, and Callisto orbit Jupiter at different rates. Their shadows likewise cross Jupiter's face at different rates. For example, the outermost moon Callisto orbits the slowest of the three satellites. Callisto's shadow moves across the planet once for every 20 shadow crossings of Io. Add the crossing rate of Ganymede's shadow and the possibility of a triple eclipse becomes even more rare. Viewing the triple shadows in 2004 was even more special, because two of the moons were crossing Jupiter's face at the same time as the three shadows.

Jupiter appears in pastel colors in this photo because the observation was taken in near-infrared light. Astronomers combined images taken in three near-infrared wavelengths to make this color image. The photo shows sunlight reflected from Jupiter's clouds. In the near infrared, methane gas in Jupiter's atmosphere limits the penetration of sunlight, which causes clouds to appear in different colors depending on their altitude.

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Jupiter has 63 named natural satellites. Of these, 47 are less than 10 kilometres in diameter and have only been discovered since 1975. The four largest moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto.

Ganymede

CallistoIo

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Jupiter's four largest moons, known as the Galilean satellites. From top to bottom, the moons shown are Io, Europa, Ganymede and Callisto. The Great Red Spot, a storm in Jupiter's atmosphere, is at least 300 years old. Winds blow counter-clockwise around the Great Red Spot at about 400 kilometers per hour (250 miles per hour). The storm is larger than one Earth diameter from north to south, and more than two Earth diameters from east to west.

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Io is the most volcanically active body in our solar system. Ganymede is the largest planetary moon and is the only moon in the solar system known to have its own magnetic field. A liquid ocean may lie beneath the frozen crust of Europa. Icy oceans may also lie deep beneath the crusts of Callisto and Ganymede. In 2003 alone, astronomers discovered 23 new moons orbiting the giant planet, giving Jupiter a total moon count of 49 officially named -- the most in the solar system. The numerous small outer moons may be asteroids captured by the giant planet's gravity.

Facts about Jovian moons learned in the past 30 years

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The Laplace resonance exhibited by three inner Galilean moons. The ratios in the figure are of orbital periods.

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The Galilean moons seen with an amateur telescope

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Jupiter with three of its Galilean satellites (from lower left to upper right): Io, Europa, and Callisto. Sky & Telescope's editor in chief, Rick Fienberg, recorded this scene on March 16, 2003, using a 12-inch (30-centimeter) Meade Schmidt-Cassegrain telescope and a Canon digital camera.

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Io and GanymedeDate: 01.17.2007The New Horizons Long Range Reconnaissance Imager (LORRI) took this 4-millisecond exposure of Jupiter and two of its moons at 01:41:04 UTC on January 17, 2007. The spacecraft was 68.5 million kilometers (42.5 million miles) from Jupiter, closing in on the giant planet at 41,500 miles (66,790 kilometers) per hour. The volcanic moon Io is the closest planet to the right of Jupiter; the icy moon Ganymede is to Io's right. The shadows of each satellite are visible atop Jupiter's clouds; Ganymede's shadow is draped over Jupiter's northwestern limb.

Ganymede's average orbit distance from Jupiter is about 1.07 million kilometers (620,000 miles); Io's is 422,000 kilometers (262,000 miles). Both Io and Ganymede are larger than Earth's moon; Ganymede is larger than the planet Mercury. Cosmos3

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Eruption at Tvashtar Catena, Io, in color

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NASA's Galileo spacecraft caught this volcanic eruption in action on Jupiter's moon Io on November 25, 1999. This mosaic shows Tvashtar Catena, a chain of calderas, in enhanced color. It combines low resolution (1.3 kilometers, or .8 miles, per picture element) color images of Io taken on July 3, 1999 with the much higher resolution (180 meters, or 197 yards, per picture element) black and white images taken in November. The molten lava was hot enough, and therefore bright enough, to saturate, or overexpose, Galileo's camera (original image is inset in lower right corner).

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Capturing CallistoDate: 02.27.2007The New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter's outermost large moon, Callisto, as the spacecraft flew past Jupiter in late February. New Horizons' closest approach distance to Jupiter was 2.3 million kilometers (1.4 million miles), not far outside Callisto's orbit, which has a radius of 1.9 million kilometers (1.2 million miles). However, Callisto happened to be on the opposite side of Jupiter during the spacecraft's pass through the Jupiter system, so these images, taken from 4.7 million kilometers (3.0 million miles) and 4.2 million kilometers (2.6 million miles) away, are the closest of Callisto that New Horizons obtained.

Callisto's ancient, crater-scarred surface makes it very different from its three more active sibling satellites, Io, Europa and Ganymede. Callisto, 4,800 kilometers (3000 miles) in diameter, displays no large-scale geological features other than impact craters, and every bright spot in these images is a crater. The largest impact feature on Callisto, the huge basin Valhalla, is visible as a bright patch at the 10 o'clock position. The craters are bright because they have excavated material relatively rich in water ice from beneath the dark, dusty material that coats most of the surface.

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Jupiter's broad magnetic field is 14 times as strong as the Earth's, ranging from 4.2 gauss (0.42 mT) at the equator to 10–14 gauss (1.0–1.4 mT) at the poles, making it the strongest among the Solar planets.The field traps a sheet of ionized particles from the solar wind, generating a highly-energetic magnetic field outside the planet — the magnetosphere.Electrons within the magnetosphere generate a strong radio signature that produces bursts in the range of 0.6–30 MHz.

Magnetosphere of Jupiter

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Hubble Space Telescope image of Jupiter aurora in UVBright streaks and dots are caused by magnetic flux tubes connecting Jupiter to its largest moons-- Io: bright streak on the far left; Ganymede: bright dot below center; Europa: dot right of Ganymede dot

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Saturn

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A Hubble Space Telescope image of Saturn in true color.

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Surely one of the most gorgeous sights the solar system has to offer, Saturn sits enveloped by the full splendor of its rings.

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Saturn, named after the Roman god of agriculture and harvest, is the second largest planet in the Solar System, the most distant of the five planets known to the ancients. Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust.

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While the other three gas planets in the solar system - Jupiter, Uranus and Neptune - have rings orbiting around them, Saturn's are by far the largest and most spectacular. With a thickness of about 1 kilometer (3,200 feet) or less, they span up to 282,000 km.Named alphabetically in the order they were discovered, the rings are relatively close to each other, with the exception of the Cassini Division, a gap measuring 4,700 kilometers (2,920 miles). The main rings are, working outward from the planet, known as C, B, and A. The Cassini Division is the largest gap in the rings and separates Rings B and A. In addition a number of fainter rings have been discovered more recently. The D Ring is exceedingly faint and closest to the planet. The F Ring is a narrow feature just outside the A Ring. Beyond that are two far fainter rings named G and E. The rings show a tremendous amount of structure on all scales; some of this structure is related to gravitational perturbations by Saturn's many moons, but much of it remains unexplained.

Saturn: Rings

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Saturn's system of rings is highly structured with many interesting features. Alongside the gaps between the rings there are also moons that act like shepherds keeping the rings in shape and the gaps clear of material.

Feature Radius (km)

Saturn equator 60 268

D inner edge 66 900

D outer edge 74 510

C inner edge 74 568

Titan ringlet 77 871

Maxwell gap 87 491

C outer edge 92 000

B inner edge 92 000

B outer edge 117 580

Cassini division

A inner edge 122 170

Encke gap 133 589

Keeler gap 136 530

A outer edge 136 775

F ring centre 140 180

G inner edge 170 000

G outer edge 175 000

E inner edge 181 000

E outer edge 483 000

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Cassini Division

Encke gap

A ring

C ring

B ring

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This amazing close-up of Saturn's rings reveals their incredible variety. In some regions there are wavelike structures, while in other places the rings' structure appears to be more chaotic. This image shows (from top to bottom) the A ring with the Encke gap, the Cassini Division, and the B and C rings. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on 26 April 2005 at a distance of approximately 2.3 million kilometres from Saturn. The image scale is 14 kilometres per pixel.

There are thousands of rings made up of billions of particles of ice and rock. The particles range in size from a grain of sugar to the size of a house. The rings are believe to be pieces of comets, asteroids or shattered moons that broke up before they reached the planet. Each ring orbits at a different speed around the planet.

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This highly enhanced color view was assembled from clear, orange and ultraviolet frames obtained August 17, 1981 from a distance of 8.9 million kilometers (5.5 million miles).

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History of Saturn’s RingsIn 1610, Italian astronomer Galileo Galilei was the first to gaze at Saturn through a telescope. To his surprise, he saw a pair of objects on either side of the planet. He sketched them as separate spheres and wrote that Saturn appeared to be triple-bodied. Continuing his observations over the next few years, Galileo drew the lateral bodies as arms or handles attached to Saturn. In 1659, Dutch astronomer Christiaan Huygens, using a more powerful telescope than Galileo's, proposed that Saturn was surrounded by a thin, flat ring. In 1675, Italian-born astronomer Jean-Dominique Cassini discovered a 'division' between what are now called the A and B rings. It is now known that the gravitational influence of Saturn's moon Mimas is responsible for the Cassini Division, which is 4,800 kilometers (3,000 miles) wide.

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Like Jupiter, Saturn is made mostly of hydrogen and helium. Its volume is 755 times greater than that of Earth. Winds in the upper atmosphere reach 500 meters (1,600 feet) per second in the equatorial region. (In contrast, the strongest hurricane-force winds on Earth top out at about 110 meters, or 360 feet, per second.) These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in the atmosphere.

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Though Saturn's magnetic field is not as huge as Jupiter's, it is still 578 times as powerful as Earth's. Saturn, the rings, and many of the satellites lie totally within Saturn's enormous magnetosphere, the region of space in which the behavior of electrically charged particles is influenced more by Saturn's magnetic field than by the solar wind. Hubble Space Telescope images show that Saturn's polar regions have aurorae similar to Earth's. Aurorae occur when charged particles spiral into a planet's atmosphere along magnetic field lines.

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Saturn's Moons

Saturn currently has 60 known moons, five of which have been discovered in data gathered by Cassini. Two more possible discoveries by Cassini still await final confirmation, as the objects might not be proper moons but instead are transient in nature. When Cassini-Huygens was launched in 1997 there were only 18 known saturnian moons. In addition to the Cassini discoveries, reanalysis of data from the Voyager 2 spacecraft, as well as Hubble observations and ground-based observations of Saturn have led to the further discoveries.

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The spacecraft consists of two main elements: the NASA Cassini orbiter, named after the Italian-French astronomer Giovanni Domenico Cassini, and the ESA Huygens probe, named after the Dutch astronomer,

mathematician and physicist Christiaan Huygens. It was launched on October 15, 1997 and entered into orbit around Saturn on July 1, 2004. On December 25, 2004 the Huygens probe separated from the orbiter at approximately 02:00 UTC; it reached Saturn's moon Titan on January 14, 2005 where it made an atmospheric descent to the surface and relayed scientific information. On April 18, 2008, NASA announced a two year extension of the mission. Cassini is the first spacecraft to orbit Saturn and the fourth to visit it.

Cassini–Huygens is a joint NASA/ESA robotic spacecraft mission currently studying the planet Saturn and its moons.

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A Cassini spacecraft image of Titan's thick atmosphere.

Saturn has 52 known natural satellites (moons) and there are probably many more waiting to be discovered.

Saturn's largest satellite, Titan, is a bit bigger than the planet Mercury. (Titan is the second-largest moon in the solar system; only Jupiter's moon Ganymede is bigger.) Titan is shrouded in a thick, nitrogen-rich atmosphere that might be similar to what Earth's was like long ago. Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth. Saturn also has many smaller 'icy' satellites.

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Titan's internal structure.

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Atmosphere of TitanVisible light cannot escape from the veil of orange smog that covers Titan's surface. The moon's dry cold atmosphere causes a 300 km thick layer of smog to build up. The smog, just like on Earth, forms when sunlight interacts with hydrocarbon molecules.

Visit the NASA page for more detail on Titan: http://saturn.jpl.nasa.gov/multimedia/flash/Titan/index.html

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Saturn and two of its moons, Tethys (above) and Dione

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Cassini spacecraft captured this enduring portrait of a near-alignment of four of Saturn's restless moons

Titan

Dione

Prometheus

Telesto

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Uranus

DiscoveredMarch 13, 1781

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Cronus (Saturn) defeats his father Uranusfresco by Giorgio Vasari and Cristofano

is named after the ancient Greek deity of the sky (Uranus, Οὐρανός). Uranus

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Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers because of its dimness and slow orbit. It was generally mistaken as a star. Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system for the first time in modern history. This was the first discovery of a planet made using a telescope.

Uranus

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Once considered one of the blander-looking planets, Uranus has been revealed as a dynamic world with some of the brightest clouds in the outer solar system and 11 rings. The seventh planet from the Sun is so distant that it takes 84 years to complete one orbit. Uranus, with no solid surface, is one of the gas giant planets (the others are Jupiter, Saturn, and Neptune).The atmosphere of Uranus is composed primarily of hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas. Sunlight is reflected from Uranus' cloud tops, which lie beneath a layer of methane gas. As the reflected sunlight passes back through this layer, the methane gas absorbs the red portion of the light, allowing the blue portion to pass through, resulting in the blue-green color that we see. The planet's atmospheric details are very difficult to see in visible light. The bulk (80 per-cent or more) of the mass of Uranus is contained in an extended liquid core consisting primarily of 'icy' materials (water, methane, and ammonia), with higher-density material at depth.

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Uranus and Moons

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Taking its first peek at Uranus, NASA Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) has detected six distinct clouds in images taken July 28,1997.

The image on the right, taken 90 minutes after the left-hand image, shows the planet's rotation. Each image is a composite of three near-infrared images. They are called false-color images because the human eye cannot detect infrared light. Therefore, colors corresponding to visible light were assigned to the images. (The wavelengths for the 'blue,' 'green,' and 'red' exposures are 1.1, 1.6, and 1.9 micrometers, respectively.)At visible and near-infrared light, sunlight is reflected from hazes and clouds in the atmosphere of Uranus. However, at near-infrared light, absorption by gases in the Uranian atmosphere limits the view to different altitudes, causing intense contrasts and colors.

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The power of the Keck telescope's adaptive optics system is clear in this image of Uranus, its rings and the moon Miranda. Date: 07.09.2004

Lower: Uranus and its atmospheric details as seen in near infrared wavelengths of 1.6 microns. The image has been doubled in size.

Upper: Uranus, its rings and moon Miranda at near infrared wavelengths of 2.2 microns.

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This series of images from NASA's Hubble Space Telescope shows how the ring system around the distant planet Uranus appears at ever more oblique (shallower) tilts as viewed from Earth - culminating in the rings being seen edge-on in three observing opportunities in 2007. The best of these events appears in the far right image taken with Hubble's Wide Field Planetary Camera 2 on August 14, 2007.The edge-on rings appear as two spikes above and below the planet. The rings cannot be seen running fully across the face of the planet because the bright glare of the planet has been blocked out in the Hubble photo (a small amount of residual glare appears as a fan- shaped image artifact). A much shorter color exposure of the planet has been photo- composited to show its size and position relative to the ring plane.Earthbound astronomers only see the rings' edge every 42 years as the planet follows a leisurely 84-year orbit about the Sun. However, the last time the rings were tilted edge-on to Earth astronomers didn't even know they existed.

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The six best-known moons of Uranus compared at their proper relative sizes.

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From left to right: Puck, Miranda, Ariel, Umbriel, Titania, and Oberon.

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Neptune

DiscoveredSep 23, 1846

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Neptune from Voyager 2

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Neptune statue in Bologna

Neptune was the only planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led astronomers to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants Jupiter and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia and methane.

In Roman mythology, Neptune was the god of the sea.

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In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbor Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing the orbit through gravitational interaction. In 1843, John Couch Adams calculated the orbit of a hypothesized eighth planet that would account for Uranus's motion.In 1845–46, Urbain Le Verrier, independently of Adams, rapidly developed his own calculations but also experienced difficulties in stimulating any enthusiasm in his compatriots.Le Verrier urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor. Heinrich d'Arrest, a student at the observatory, suggested to Galle that they could compare a recently drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. The very evening of the day of receipt of Le Verrier's letter on September 23, 1846, Neptune was discovered within 1° of where Le Verrier had predicted it to be.

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Discovery of Neptune

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The outer planet takes more time to complete an orbit than the inner planet, so once per orbit the inner planet overtakes the outer planet. When the planets are at a, the outer planet exerts a gravitational perturbation that accelerates the inner planet, advancing the body ahead of its normal path. When the planets reach b, the reverse is true and the inner planet is decelarated. This perturbing influence is what led to the discovery of the planet Neptune.

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1. Upper atmosphere, top clouds2. Atmosphere consisting of hydrogen, helium and methane gas3. Mantle consisting of water, ammonia and methane ices4. Core consisting of rock and ice

The internal structure of Neptune:

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Neptune HurricanesDate: 08.24.1989Voyager 2 sent back this stunning image of storms at work in Neptune's windy atmosphere in August 1989.

This photograph of Neptune was reconstructed from two images taken by Voyager 2's narrow-angle camera, through the green and clear filters. The image shows three of the features that Voyager 2 photographed during its Neptune flyby. At the north (top) is the Great Dark Spot, accompanied by bright, white clouds that undergo rapid changes in appearance. To the south of the Great Dark Spot is the bright feature that Voyager scientists nicknamed "Scooter." Still farther south is the feature called "Dark Spot 2," which has a bright core. Each feature moves eastward at a different velocity, so it is only occasionally that they appear close to each other, such as at the time this picture was taken.

Voyager 2 is the only spacecraft to visit Neptune.

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Neptune has thirteen known moons.

The largest by far is Triton, discovered by William Lassell just seventeen days after the discovery of Neptune itself. About one hundred years later American astronomer Gerard Kuiper found Neptune's third-largest moon, Nereid, in 1949. He missed Proteus, the second-largest, because it's too dark and too close to Neptune for telescopes of that era. No further moons were found until Voyager 2 flew by Neptune in 1989 and discovered five new inner moons, bringing the total of known moons to eight.From 2002 onwards, telescopic surveys found the remaining five outer moons.

Moons of NeptuneImage of Triton from Voyager 2.

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Pluto's Surface from 3 Billion MilesImages of Pluto taken by the NASA Hubble Space Telescope with the ESA Faint Object Camera. These images, taken in late June and early July, 1994 are the first views which allow resolution of features on Pluto's surface.

Pluto Discovered February 18, 1930

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Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet in addition to Neptune. An extensive project in search of a possible ninth planet, termed "Planet X“, was started by some astronomers. Finally it was discovered by a young American astronomer Clyde Tombaugh on February 18, 1930, after nearly a year of searching.

Pluto was the Roman god of the underworld.

Discovery of PlutoIn the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus.

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Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005.The large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet.The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon. If one were standing on Pluto's near side, Charon would hover in the sky without moving; if one were to travel to the far side, one would never see Charon at all.

Moons of Pluto

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Once known as the smallest, coldest, and most distant planet from the Sun, Pluto has been enshrouded in controversy since its discovery in 1930.

On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet to a dwarf planet.

According to the new rules a planet meets three criteria: it must orbit the Sun, it must be big enough for gravity to squash it into a round ball, and it must have cleared other things out of the way in its orbital neighborhood. The latter measure knocks out Pluto and 2003UB313 (Eris), which orbit among the icy wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt.

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(1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

Planet or Dwarf Planet ?

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Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. It has a highly eccentric and highly inclined orbit.Some astronomers believe that Triton, Charon are merely the largest examples of Kuiper Belt objects.

Kuiper belt and Kuiper Belt objects

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The Kuiper belt, is a disk-shaped region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger -- 20 times as wide and 20–200 times as massive. It is home to at least three dwarf planets – Pluto, Haumea and Makemake. But while the asteroid belt is composed primarily of rock and metal, the Kuiper belt objects are composed largely of frozen volatiles (dubbed "ices"), such as methane, ammonia and water. It is now considered to be the source of the short-period comets. astronomical unit (AU) = 149,597,870.691 km; the average distance from the Earth to the Sun. 1 AU is a long way -- at 100 miles per hour (160 kmph) it would take over 100 years to go 1 AU.

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Positions are accurate for January 1st, 2000

Green:Kuiper belt object

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Plutoids

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Plutoids

Pluto and Eris are the first objects to be classified as Plutoids, a special class of dwarf planets that orbit our Sun in the icy region beyond the orbit of Neptune.

The name plutoid was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), and approved by the IAU Executive Committee at a June 2008 meeting in Oslo, Norway.

Plutoids are celestial bodies in orbit around the Sun at a semi-major axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. The dwarf planet Ceres is not a plutoid as it is located in the asteroid belt between Mars and Jupiter. Current scientific knowledge lends credence to the belief that Ceres is the only object of its kind.

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is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly, roughly 2 500 km in diameter and 27% more massive than Pluto.

Eris

Eris (centre) and Dysnomia (left of centre, the only known moon of Eris). --Hubble Space Telescope

Because Eris is larger than Pluto, its discoverers and NASA called it the Solar system’s tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term planet for the first time.

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The IAU's formal definition of 'plutoid,' announced 11 June 2008, is:Plutoids are celestial bodies in orbit around the Sun at a semi-major axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit. Satellites of plutoids are not plutoids themselves.

Accordingly, plutoids can be thought of as the intersection of the set of dwarf planets and the set of trans-Neptunian objects. As of 2008[update], Pluto, Eris, Haumea, and Makemake are the only objects classified as plutoids.

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Also includes the Main Asteroid Belt (the white donut-shaped cloud), the Hildas (the orange "triangle" just inside the orbit of Jupiter) and the Jovian Trojans (green). The group that leads Jupiter are called the "Greeks" and the trailing group are called the "Trojans"

The inner Solar System, from the Sun to Jupiter.

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The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The

asteroid belt region is also termed the main belt to distinguish it from other concentrations of minor planets within the Solar System, such as the Kuiper belt and scattered disc.More than half the mass of the main belt is contained in the four largest

objects: Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. All of these have mean

diameters of more than 400 km, while Ceres, the main belt's only dwarf planet, is about 950 km in diameter.The asteroid belt formed from the primordial solar nebula as a group of planetesimals, the smaller precursors of the planets. Between Mars and Jupiter, however, gravitational perturbations from the giant planet imbued the planetesimals with too much orbital energy for them to accrete into a planet. Collisions became too violent, and instead of sticking together, the planetesimals shattered.Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets.

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The asteroid 951 Gaspra, the first ever imaged by a spacecraft, taken by Galileo as it passed by it in 1991

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The Hilda family of asteroids consists of asteroids with a semi-major axis between 3.7 AU and 4.2 AU, an eccentricity greater than 0.07, and an inclination less than 20°.Hildas move in their elliptical orbits so that their aphelia (point on the orbit of a celestial body that is farthest from the sun) put them opposite Jupiter, or 60 degrees ahead of or behind Jupiter.

The Jupiter Trojans, commonly called simply Trojans or Trojan asteroids, are a large group of objects that share the orbit of the planet Jupiter around the Sun.

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The Hildas (black) and the Trojans viewed from ecliptic plane.

Cosmos3 89distances in AU

Positions are accurate for January 1st, 2000

Green:Kuiper belt object

Scattered disc object or Centaur in orange

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Centaur (minor planet)

The common definition of a Centaur is an object that orbits the Sun between Jupiter and Neptune. They generally have a semi-major axis between Jupiter and Neptune, crossing the orbits of one or more the giant planets. and have dynamical lifetimes of a few million years. The centaurs are an unstable orbital class of minor planets. The name was chosen because they behave as half asteroid and half comet.

No centaur has been photographed up close, although there is evidence that Saturn's moon Phoebe, imaged by the Cassini probe in 2004, may be a captured centaur.Any centaur that is perturbed close enough to the Sun is expected to become a comet.

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90377 Sedna is a trans-Neptunian object and a likely dwarf planet, discovered by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale University) on November 14, 2003. It is currently 88 AU from the Sun, about three times farther than Neptune. For most of its orbit Sedna is farther from the Sun than any other known dwarf planet candidate.

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Sedna, highlighted by the green circle.

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In 1950 Jan Oort noticed that --no comet has been observed with an orbit that indicates that it came from interstellar space, --there is a strong tendency for aphelia of long period comet orbits to lie at a distance of about 50,000 AU, and --there is no preferential direction from which comets come.

From this he proposed that comets reside in a vast cloud at the outer reaches of the solar system. This has come to be known as the Oort Cloud. The statistics imply that it may contain as many as a trillion (1e12) comets. Unfortunately, since the individual comets are so small and at such large distances, we have no direct evidence about the Oort Cloud yet.

Oort Cloud

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In 2004, the discovery of an object known as 2003 VB12 "Sedna" was announced. Its orbit is intermediate between the Kuiper Belt and what was previously thought to be the inner part of the Oort Cloud. Perhaps this object is the first of a new class of "inner Oort Cloud" objects.

Some astronomers believe the Oort Cloud may account for a significant fraction of the mass of the solar system, perhaps as much or even more than Jupiter.

But Kuiper Belt and the Oort Cloud are more than distant curiosities. They are relatively pristine remnants of the nebula from which the entire solar system was formed. Their composition and distribution places important constraints on models of the early evolution of the solar system.

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Artists rendering of the Kuiper Belt and Oort Cloud.

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The presumed distance of the Oort cloud compared to the rest of the Solar System

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Home work for Lecture 3:Go to the following web-site and follow the instruction there to download the needed software so that you can start the Cassini Virtual Tour.http://saturn.jpl.nasa.gov/video/cassinivirtualtour/ Enjoy flying around Saturn and investigate the moons of Saturn in your Virtual Tour. Write a report on the Saturn moons.

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