1. Our Solar System

2. Sun Facts Diameter: ~870,000 miles Rotation: 25-36 days Temperature:
Sunspots 6,700F
Surface 10,000F
Core 27,000,000F
Facts
Contains ~98% of our Solar System’s mass
Could hold 1.3 million Earths
Fusion of hydrogen atoms produces light & heat

3. Sun

4. Planet A large body orbiting a central star.
‘Planetes’ is the Greek word for wanderer.
New International Astronomical Union (IAU) Definition
A planet is a celestial body that
Is in orbit around the Sun,
Is nearly round,
Has cleared the neighborhood around its orbit, and
Is not a satellite (moon).
Examples
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, & Neptune

5. Other Bodies Dwarf Planet is a celestial body that
Is in orbit around the Sun,
Is nearly round in shape,
Has not cleared the neighborhood around its orbit,
And is not a satellite.
Examples – Ceres, Pluto, & Eris (2003 UB 313)
Small Solar System Bodies
All other objects, except satellites, orbiting the Sun.
Examples – Comets, Asteroids, Meteoroids, Kuiper Belt Objects etc.

6. Inner Planets Mercury, Venus, Earth, & Mars
Also known as the “Terrestrial Planets”
Tend to be smaller, denser, & rockier.
The four innermost planets in the Solar System (Mercury, Venus, Earth, and Mars) are sometimes called the "terrestrial" planets because of their proximity to Earth ("Terra" in Latin) and their similarity as solid bodies with compact, rocky surfaces. These four planets formed from the portion of the protoplanetary disk located close to the developing Sun during the first 100 million years of the System's birth, where it was too warm for the four developing protoplanets to agglomerate water and more volatile ices and so they failed to bulk up sufficiently in gravitational might to hold onto the lighter gases of the Solar nebulae like hydrogen and helium to become "gas giants."

7. Inner Planets

8. Mercury Data Distance from the Sun: 36 million miles
Diameter: ~3,000 miles
Temperature: -280F to 800F
Moons: zero
Facts:
No atmosphere
Craters & Cliffs
Rotation perpendicular to its orbital path

9. Mercury

10. Venus Data Distance from the Sun: 67 million miles
Diameter: ~7,500 miles
Temperature: 900 F (greenhouse effect)
Moons: zero
Facts:
Rotates from east to west
A day is longer than a year!
CO2 atmosphere & sulfuric acid clouds
Craters & Volcanic Rock

11. Venus Left – Picture taken by the Mariner probe
Right – Picture taken by the Hubble Space Telescope

12. Earth Data Distance from the Sun: 93 million miles
Diameter: ~8,000miles
Temperature: -128F to 140F
Moons: One
Facts:
Life!
Water in all 3 States
Atmosphere (Nitrogen, Oxygen, & other gases)

13. Earth Left – Picture taken by the Apollo 17 crew
Right – Picture taken by Mariner 10

14. Mars Data Distance from the Sun: 142 million miles
Diameter: ~4,200 miles
Temperature: -184F to 59F
Moons: 2
Facts:
Atmosphere (CO2 & other gases)
Polar Ice Caps (frozen CO2)
Olympus Mons (largest volcano)

15. Mars
Left – Hubble image
Right – south polar cap

16. Asteroid Belt

17. Asteroid Facts Rocky, Metallic objects Orbit the sun in a “belt”
Known as “minor planets”
Range in size
From pebbles to
Ceres formerly considered an asteroid has been reclassified as a “Dwarf Planet” (~600 miles)
Theories
Remains of a destroyed planet
Planet never formed

18. Asteroids
Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona.
Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across -- less than half the diameter of our Moon.
Eros is approximately 33x13x13 kilometers (20x8x8 miles) in size and resembles a "fat banana." It is the second largest near-Earth asteroid and spins on its axis once every 5 hours, 16 minutes. Eros exhibits a heavily cratered surface with one side dominated by a huge, scallop-rimmed gouge, and the opposite side by a conspicuous sharp, raised rimmed crater.
On October 29, 1991, Galileo came within 1,600 kilometers (1,000 miles) of Gaspra. They passed each other at 8 kilometers (5 miles) per second. This was the first time that a spacecraft made a fly-by of an asteroid. Gaspra is an irregular body with dimensions of about 20 x 12 x 11 km (12.5 x 7.5 x 7 miles). Its surface reflects approximately 20 percent of the sunlight striking it. Gaspra is classified as an S-type asteroid and is likely composed of metal-rich silicates and perhaps blocks of pure metal. It is a member of the Flora family.

19. Meteoroids
Small bodies that travel through SPACE.
Nasa.gov

20. Meteors
Meteors which enter earth’s atmosphere and burn up.
Nasa.gov

21. Meteorites
If meteor does not completely vaporize, it lands on earth as a meteorite.
Meteoroid (in space) – made up of solid rock-like objects that revolve around the sun. The friction between the meteorites and the atmoshere causes the meteorite to glow and be seen in the night sky.
Meteor (traveling through the Earth’s atmosphere)
Meteorite (what is left over if it is not vaporized while traveling through the Earth’s atmosphere
Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.

22. Barringer Meteorite Crater
Barringer Meteor Crater, Arizona 35°02'N, 111°01'W; diameter: kilometers (.737 miles); age: 49,000 years

23. Outer Planets Jupiter, Saturn, Uranus, & Neptune
Also known as “Gas Giants” or the “Jovian Planets”
Tend to be much larger & most are made up of gases.

24. Jupiter Data Distance from the Sun: 484 million miles
Diameter: ~89,000 miles
Temperature:
Surface -163F
Core 54,000F
Moons: 63
Facts:
Largest Planet
Faint Ring System
Atmospheric Clouds & Storms (Red Spot)

25. Jupiter
Left – Hubble image
Right – red spot

26. Saturn Data Distance from the Sun: 886 million miles
Diameter: ~74,000 miles
Temperature:
Surface -219F
Core 27,000F
Moons: 58
Facts:
Flat at the poles
Less dense than water
Ring System

27. Saturn Left – voyager 2 image Right…
Last View of Saturn Two days after its encounter with Saturn, Voyager 1 looked back on the planet from a distance of more than 5.0 million kilometers (3.0 million miles). This view of Saturn has never been seen by an earth based telescope, since the earth is so close to the Sun only the sunlit face of Saturn can be seen.

28. Uranus Data Distance from the Sun: 1.8 billion miles
Diameter: ~32,000 miles
Temperature: -330F
Moons: at least 22
Facts:
Gaseous Planet (H, He, Methane)
Bluish Color
Orbits on its Side
Faint Ring System

29. Uranus
Left – voyager 2 image
Right – uranian ring system

30. Neptune Data Distance from the Sun: 2.8 billion miles
Diameter: ~30,000 miles
Temperature: -330F
Moons: 8
Facts:
Great dark spot
Strongest winds in the solar system (1,200mph)
Orbital path crosses Pluto’s
Faint rings made of dust particles
Composed of H, He, & Methane & a core of molten rock

31. Neptune
Great Dark Spot Feathery white clouds fill the boundary between the dark and light blue regions on the Great Dark Spot. The pinwheel shape of both the dark boundary and the white cirrus suggests that the storm system rotates counterclockwise. Periodic small scale patterns in the white cloud, possibly waves, are short lived and do not persist from one Neptunian rotation to the next. (Courtesy NASA/JPL)

33. Pluto Data Distance from the Sun: ~4 billion miles
Diameter: ~1,400 miles
Temperature: -390F
Moons: 3 – Charon, Hydra, Nix
Facts:
Thin atmosphere freezes & falls to the surface
Orbits almost on its side
Orbital path crosses Neptune’s
Newly defined as a “Dwarf Planet”

34. Dwarf Planet Pluto
Pluto & Charon This view of Pluto was taken by the Hubble Space Telescope. It shows a rare image of tiny Pluto with its moon Charon, which is slightly smaller than the planet. Because Pluto has not yet been visited by any spacecraft, it remains a mysterious planet. Due to its great distance from the sun, Pluto's surface is believed to reach temperatures as low as -240°C (-400°F). From Pluto's surface, the Sun appears as only a very bright star. (Courtesy NASA)
Pluto's Rocky Road
by Kelly Reeves 
Percival Lowell died in 1916, unaware that his photographic plates had captured Pluto. Lowell Observatory © 2006
Aug. 10, 2006
How Mars Mania Led to the Discovery of Pluto
NPR.org, August 24, 2006 · 1905: Percival Lowell starts the search for Planet X. The planets, including the newly discovered Neptune, didn't move around the sun in quite the way gravitational laws predicted, and Lowell proposes that an undiscovered planet must be the reason why. He never finds Planet X before his death in 1916.
Feb. 18, 1930: Clyde Tombaugh takes up the search in 1929 at Lowell's observatory and proves that discovering new planets is not glamorous work. For a year, he photographs the same section of sky several nights apart and then searches the images for any objects that move like a planet should. On Feb. 18 he looks at his photographic plates and knows right away that one of the dots is Planet X.
May 1930: A little girl in Britain interested in Greek and Roman mythology tells her grandfather over breakfast that the new planet should be named Pluto. He cables the Lowell Observatory, and they unanimously vote for the name because Pluto is the god of the underworld, which seems appropriate for such a cold and remote planet, and the first two letters of Pluto are Percival Lowell's initials.
June 22, 1978: The U.S. Naval Observatory's James Christy discovers that Pluto has a moon. He names it Charon, after the ferryman who take souls into the underworld in Greek mythology, but pronounces it Sharon because his wife's name is Charlene. The existence of Charon meant that scientists could get a better read on the mass of Pluto. They found that Pluto and Charon are actually pretty small. Together, they're smaller than Earth's moon, even.
Aug. 30, 1992: Pluto's tiny size didn't disqualify it from being a planet, but then David Jewitt, an astronomer at the University of Hawaii, and Jane Luu, a researcher at the University of California, Berkeley, discover Pluto isn't the only chunk of rock and out there in the Kuiper Belt beyond Neptune. Scientists have found hundreds of these objects since 1992, so some astronomers start to think that maybe Pluto isn’t so special after all.
Feb. 3, 1999: Pluto's debated status as a planet gets publicity. The International Astronomical Union calms stargazers worried by recent media reports saying the IAU is planning to demote Pluto. Not so, they say in a press statement. They only want to include it in their numbering system for Kuiper Belt objects.
May 11, 2000: Scientists may debate whether Pluto is a planet, but it's place in the classical music canon gets secured. Composer Colin Matthews writes a movement for Pluto into Gustav Holst's The Planets. Although Pluto was discovered in Holst's lifetime, he declined to add it to his suite.
Feb. 19, 2000: The Rose Center for Earth and Space at the American Museum of Natural History in New York City opens -- and sneakily omits Pluto from its list of planets. No one seems to notice until the next year, when the New York Times writes a front page article about it.
Jan. 5, 2005: Michael Brown, a planetary scientist at the California Institute of Technology, discovers what might be the 10th planet, Xena. He says it's rocky and icy like Pluto. When he announces his discovery on July 29, he forces astronomers to decide what makes a planet.
Oct. 31, 2005: The Hubble Space Telescope Pluto Companion Search Team discovers that Pluto has three moons, not just one. Moons don't qualify an object to be a planet, but having a couple moons doesn't hurt Pluto's case.
April 11, 2006: The Hubble Space Telescope finds that Xena is slightly larger than Pluto. Astronomers now have to make a decision: either Xena and Pluto are both planets or neither is a planet.
August 24, 2006: The International Astronomical Union strips Pluto of its planetary status. The group says a planet must, among other things, have "cleared the neighborhood around its orbit." Because Pluto's orbit overlaps Neptune's, Pluto is out. The celestial body formerly known as the ninth planet will be reclassified as a "dwarf planet."

35. Orbits of Pluto & Eris (aka 2003 UB 313)

36. Kuiper Belt
Starting in 1992, astronomers have become aware of a vast population of small bodies orbiting the sun beyond Neptune. There are at least 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune (at 30 AU) to 50 AU. Observations show that the trans-Neptunians are mostly confined within a thick band around the ecliptic, leading to the realization that they occupy a ring or belt surrounding the sun. This ring is generally referred to as the Kuiper Belt.
The Kuiper Belt holds significance for the study of the planetary system on at least two levels. First, it is likely that the Kuiper Belt objects are extremely primitive remnants from the early accretional phases of the solar system. The inner, dense parts of the pre-planetary disk condensed into the major planets, probably within a few millions to tens of millions of years. The outer parts were less dense, and accretion progressed slowly. Evidently, a great many small objects were formed. Second, it is widely believed that the Kuiper Belt is the source of the short-period comets. It acts as a reservoir for these bodies in the same way that the Oort Cloud acts as a reservoir for the long-period comets.
The study of the trans-Neptunians is a rapidly evolving field, with major observational and theoretical advances in the last few years. A partial list of relevant papers is included on this Web page. You can also find a Table of the known trans-Neptunians, the discovery images for 1992 QB1, and a blink sequence which shows how these objects are identified.
Occasionally the orbit of a Kuiper Belt object will be disturbed by the interactions of the giant planets in such a way as to cause the object to cross the orbit of Neptune. It will then very likely have a close encounter with Neptune sending it out of the solar system or into an orbit crossing those of the other giant planets or even into the inner solar system.

37. Kuiper Belt Disk-shaped region past the orbit of Neptune
Contains icy “Small Solar System Objects” & “Dwarf Planets” that orbit the Sun
Contains over 35,000 objects over 100 km (62 miles) in diameter
Possibly where Short-Period Comets Originate
Orbits of Kuiper Belt Objects may be affected by interactions with the Gas Giants
Examples – Pluto & Eris
Occasionally the orbit of a Kuiper Belt object will be disturbed by the interactions of the giant planets in such a way as to cause the object to cross the orbit of Neptune. It will then very likely have a close encounter with Neptune sending it out of the solar system or into an orbit crossing those of the other giant planets or even into the inner solar system.

38. Eris (2003 UB 313) Discovered: July-August, 2005
Distance: >8 billion miles
Diameter: At least as large as Pluto
Dwarf Planet
Dwarf Planet Formerly Known As Xena Officially Named 'Eris'
Science Daily — The International Astronomical Union (IAU) has announced that the dwarf planet known as Xena since its 2005 discovery has been named Eris, after the Greek goddess of discord.
Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. (Credit: Robert Hurt / IPAC, Caltech)
Eris's moon will be known as Dysnomia, the demon goddess of lawlessness and the daughter of Eris.
The names are those suggested by the discoverers of the dwarf planet--Mike Brown, a professor of planetary astronomy at the California Institute of Technology, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University, and by the discoverers of the moon--Brown and the engineering team of Keck Observatory where the observations were made.
"Eris is the Greek goddess of discord and strife," explains Brown. "She stirs up jealousy and envy to cause fighting and anger among men. At the wedding of Peleus and Thetis, all the gods were invited with the exception of Eris, and, enraged at her exclusion, she spitefully caused a quarrel among the goddesses that led to the Trojan War.
"She's quite a fun goddess, really," Brown adds. "And, for the Xena fans out there who are sad to see the name go, Eris appeared in her Latin version of Discordia as a recurring character on Xena: Warrior Princess."
True to its name, the dwarf planet Eris has stirred up a great deal of trouble among the international astronomical community, most recently last month when the question of its proper designation led to a raucous meeting of the IAU in Prague. At the end of the conference, IAU members voted to demote Pluto to dwarf-planet status, leaving the solar system with eight planets.
However, the ruling effectively settled the year-long controversy about whether Eris would rise to planetary status. Somewhat larger than Pluto, the body was formally announced to the world on July 29, With the August IAU ruling, Eris is the largest dwarf planet.
Eris, about 2,400 kilometers in diameter, was discovered on January 8, 2005, at Palomar Observatory with the NASA-funded 48-inch Samuel Oschin Telescope. A Kuiper-belt object like Pluto, but slightly less reddish-yellow, Eris is currently visible in the constellation Cetus to anyone with a top-quality amateur telescope.
Eris is now about 97 astronomical units from the sun (an astronomical unit is the distance between the sun and Earth), which means that it is some nine billion miles away at present. On a highly elliptical 560-year orbit, Eris sweeps in as close to the sun as 38 astronomical units. At present, however, it is nearly as far away as it ever gets.
Pluto's own elliptical orbit takes it as far away as 50 astronomical units from the sun during its 250-year revolution. This means that Eris is sometimes much closer to Earth than Pluto--although never closer than Neptune.
Dysnomia, the only satellite of Eris discovered so far, is about 250 kilometers in diameter and reflects only about 1 percent of the sunlight that its parent reflects. The name is both a nod to Lucy Lawless, the actress who played Xena on the TV show, and to the astronomical tradition of naming the first satellites of dwarf planets.
Based on spectral data, the researchers think Eris is covered with a layer of methane that has seeped from the interior and frozen on the surface. As in the case of Pluto, the methane has undergone chemical transformations, probably due to the faint solar radiation, causing the methane layer to redden. But the methane surface on Eris is somewhat more yellowish than the reddish-yellow surface of Pluto, perhaps because Eris is farther from the sun.
Brown, Trujillo, and Rabinowitz first photographed Eris with the Samuel Oschin Telescope on October 31, However, the object was so far away that its motion was not detected until they reanalyzed the data in January of 2005.

39. Eris
UB313
A faint, slowly moving dot discovered by computer shows clear signs of being a deep Solar System object at least as large as Pluto. The object, designated 2003 UB313, is currently situated nearly 100 times the Earth-Sun distance -- over twice the average Pluto-Sun distance. That far out, the only way a single round object could be as bright as 2003 UB313 would be if it is at least as large as Pluto and completely reflective. Since 2003 UB313 is surely not completely reflective, it could be substantially larger. One of the discovery frames is shown above digitally expanded and artificially brightened UB313 was identified initially on frames taken by the automated 1.2-meter Samuel Oschin Telescope at Palomar Observatory in California, USA.
Has a tenth planet been discovered? A newly discovered object, designated 2003 UB313 and located more than twice the distance of Pluto, is expected to be at least as large as Pluto and probably larger, given current measurements UB313's dimness and highly tilted orbit (44 degrees) prevented it from being discovered sooner. Many astronomers speculate that numerous other icy objects larger than Pluto likely exist in the Kuiper Belt of the far distant Solar System. If so, and if some are found closer in than 2003 UB313, it may be premature to call 2003 UB313 the tenth planet. Illustrated above is an artist's drawing showing how 2003 UB313 might look. The unusually bright star on the right is the Sun. Much of the world eagerly await the decision by the International Astronomical Union on whether 2003 UB313 will be designated a planet or given a name without subscripts.
Object UB313, also known as Xena. Its diameter is 1,490 miles, making it slightly larger than Pluto. Xena is 10 billion miles from Earth and takes 560 years to orbit the sun. Its discovery cast further doubt on Pluto's qualifications to be a planet. NASA, ESA, Mike Brown (CalTech)

40. Comet Facts Small, fragile, irregularly shaped bodies
Made of ice, dust, and gases (“dirty snowballs”)
Highly elliptical orbits around the sun
Coma – surrounding cloud which grows in size & brightness as it approaches the Sun (& melts)

41. Comets
Comet West (1975) This photograph was taken by amateur astronomer John Loborde on March 9, This picture shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles. (Courtesy John Laborde)
Comet Hale-Bopp

42. Sedna
Artist's conception of the cold distant Sedna. The sun is a tiny point of light 8 billion miles away from the red planetoid. A hypothesized tiny moon appears nearby.
On 15 March 2004, astronomers from Caltech, Gemini Observatory, and Yale University announced the discovery of the coldest, most distant object known to orbit the sun. The object was found at
Sedna is the most distant solar system object ever discovered. It is twice as far from the sun as any other solar system object and three times farther than Pluto or Neptune. Standing on the surface of Sedna, you could block the entire sun with the head of a pin held at arm's length.
Even more interestingly, the orbit of Sedna is extreme elliptical, in contrast to all of the much closer planets, and it takes 10,500 years to circle the sun.
Sedna lies extremely far from the Sun, in the coldest known region of our solar system, where the temperature never rises above minus 240 degrees Celsius (minus 400 Fahrenheit).
INFORMATION REGARDING SEDNA
The object 2003 VB_12
The object designated 2003 VB_12 was discovered with the Palomar 1.2-m Schmidt telescope on November 14, 2003 and has also been found on sky photographs taken in 2001 and Orbit computations show that the object's distance from the Sun varies between 76 and about 900 Astronomical Units (1 AU is close to 150 million kilometers). Its current distance is about 86 AU, about twice Pluto's distance. A very uncertain diameter of around 1800 km has been estimated from the brightness of the object.
Since the object appears to be somewhat smaller than Pluto and has an extremely elongated orbit with a period of around 10,000 years, compared to 248 years for Pluto, it seems appropriate to designate the object as an asteroid and not as the 10th planet.
The name Sedna has been proposed after an Inuit ocean goddess, but the name has not yet been endorsed by the IAU and needs to be reviewed first by the IAU Committee on Small Bodies Nomenclature.

43. Sedna Data Distance from the Sun: ~12 billion miles
Diameter: ,100 miles
Discovered in 2004
Facts:
Moon?
Rocky?
Highly Elliptical Orbit (10,000 years to orbit the Sun)
Brown said Sedna occupies a region of space beyond the Kuiper Belt but inside the theorized Oort Cloud, a distant reservoir of icy comets that are detected only when they zoom through the inner solar system on occasion. The Oort Cloud is thought to extend halfway to the next known star, but scientists know almost nothing about its scope, density or composition.
Brown said the discovery suggests the Oort Cloud might be more dense -- containing more objects -- than was previously thought.

44. Sedna Size Comparison
The artist's rendition shows "Sedna" in relation to other bodies in the solar system, including Earth and its Moon; Pluto; and Quaoar, a planetoid beyond Pluto that was until now the largest known object beyond Pluto.
Scientists used the fact that even the Spitzer telescope was unable to detect the heat of the extremely distant, cold object to determine that it must be no more than 1,700 kilometers (about 1,000 miles) in diameter, smaller than Pluto. By combining all available data, Brown estimates the size at about halfway between that of Pluto and Quaoar, the planetoid discovered by the same team in Until "Sedna" was detected, Quaoar was the largest known body beyond Pluto.

45. View from Sedna
Scientists Find Another Huge Mini-World in Outer Solar System By Robert Roy Britt Senior Science Writer posted: 03:51 pm ET 15 March UPDATE: Story first posted 9 a.m. EST, March 15, 2004
The most distant object ever seen orbiting the Sun is nearly as large as Pluto, expanding astronomers notions of how the solar system formed and what resides in its outskirts.
The round world is currently three times farther away than Pluto from the Sun, a distance that expands even further on its 10,000-year orbit. It sits in a part of the solar system that some astronomers had thought empty. It is redder and brighter than anything astronomers have seen in the outer solar system, and scientists don't know why.
The object may even have its own little moon.
"There's absolutely nothing else like it known in the solar system," said Michael Brown of the California Institute of Technology in Pasadena, Calif.
Other researchers say they're not even sure how to classify the object, and the puzzling discovery is just the beginning of many years of investigation that will be needed to figure out the nature of space beyond Neptune.
Meet Sedna
The object is catalogued as 2003 VB12 and has unofficially been dubbed Sedna, goddess of the sea for Arctic dwellers. Brown thought that appropriate given the frigid conditions under which the solar system body has probably always existed. The International Astronomical Union would have to approve the name.
The discovery was led by Brown, who discussed it today at a NASA press conference.
Brown does not consider Sedna to be a planet. He and many other astronomers maintain that Pluto should not have ever received planet status, either, since astronomers are now finding myriad round objects beyond the orbit of Neptune, and several of them are quite large.
Pluto is about 1,413 miles (2,274 kilometers) wide. Sedna is estimated at no more than 1,100 miles (1,770 kilometers) in diameter. It may be the largest object in the solar system after Pluto, but more observations are needed to pin that down.
Sedna is some 8 billion miles away, or 86 astronomical units (AU) from the Sun. One AU is the distance from Earth to the Sun (93 million miles). Pluto is, on average, 39.5 AU from the Sun. But Sedna's orbit, tracked since November when it was first spotted, can bring the object out to some 84 billion miles. It is a very elliptical orbit.
"If you were stand on the surface of Sedna today and you held a pin at arms length, you cold cover the entire Sun with the head of that pin," Brown said. Even the largest backyard telescopes would have a hard time spotting Sedna from Earth, he said.
The region beyond Pluto is commonly called the Kuiper Belt. It is loaded with icy objects large and small.
Most primordial object
Brown said Sedna may be the most primordial object ever detected, having undergone very little heating by the Sun and having had few collisions in the sparse region of space where it resides. Other objects in the solar system, according to the latest thinking, have typically been transformed significantly since their formation.
The next two largest Kuiper Belt Objects were also discovered by Brown's group. Both were much closer.Last month, the team announced 2004 DW, which was estimated to be between 520-1,170 miles wide (840 to 1,880 kilometers). The best estimate is that 2004 DW is 994 miles across (1,600 kilometers). It is nearly 47 AU from the Sun.
In 2002, the group found 2002 LM60, also named Quaoar (KWAH-o-ar). It is roughly 780 miles (1,250 kilometers) wide, about half as big as Pluto. Quaoar is 42 AU from the Sun.
Eugene Chiang, an astronomer at the University of California at Berkeley, told SPACE.com that the discovery of Sedna adds to the case that Pluto is more like a Kuiper Belt Object than a planet. He called Pluto's discovery, in 1930, an accident of history.
"If Pluto were discovered after all these discoveries, would we have called it a planet? No," said Chiang, who was not involved in the Sedna discovery.
Strange origins
Brown said Sedna occupies a region of space beyond the Kuiper Belt but inside the theorized Oort Cloud, a distant reservoir of icy comets that are detected only when they zoom through the inner solar system on occasion. The Oort Cloud is thought to extend halfway to the next known star, but scientists know almost nothing about its scope, density or composition.
Brown said the discovery suggests the Oort Cloud might be more dense -- containing more objects -- than was previously thought.
"It is very likely that there are more inner Oort cloud objects like Sedna," Brown says, noting that only 15 percent of the sky has been surveyed for objects so dim as this.
Sedna probably was formed nearer to the Sun, in what's now the Kuiper Belt. Like other objects there, it would have been gravitationally booted outward by the giant planets early in the 4.6-billion-year history of the solar system. Many such objects should have been ejected from the solar system. But interactions with very distant stars could have forced some to remain in the Oort Cloud.
Sedna's presence suggests, Brown said, that the Sun might have formed in a neighborhood more tightly packed with stars than what's evident today. Other theories of planet formation have suggested this, too, holding that the Sun was long ago booted out of the star cluster in which it was born.
Another view
It is not certain, however, that Sedna should be considered part of the Oort cloud, said Alan Stern of the Southwest Research Institute (SwRI).
"I think it's a really cool find," Stern said in a telephone interview, but he added that it was "not unexpected."
Stern heads up NASA's New Horizons mission that will launch in 2006 and explore Pluto and the Kuiper Belt. He expects the spacecraft to be functional at least out to 50 AU -- short of Sedna's distant location.
Stern and others have long theorized that there would be many objects beyond the orbit of Neptune. He said Sedna might turn out to be a Kuiper Belt Object that has been scattered outward. The problem, he said, is that scientists don't know enough about either region to say for sure what belongs where and what is or isn't in between.
"I think the jury is out on whether it's a scattered Kuiper Belt Object or an Oort Cloud object," Stern said. "There's a fair chance Mike [Brown] is right." Stern said it is not even clear whether there is actually a gap between the two regions.
Stern said only multiple missions to the Kuiper Belt and beyond would answer all the questions he has about the far reaches of the solar system.
Composition unknown
Scientists don't know what Sedna is made of, but they presume it is about half ice and half rock, like other distant solar system bodies. But Sedna appears redder than all but Mars, Brown said, and observations over the next six months or so aim to learn why.
Observations suggest Sedna has a satellite -- a small moon, but further study is needed to determine if that's the case, Brown said. He and other astronomers would not be overly surprised, as several Kuiper Belt Objects, as well as Pluto, have satellites.
Sedna's surface temperature is about minus 400 degrees Fahrenheit (-240 Celsius), the coldest known place in the solar system.
Sedna was found using the 48-inch Samuel Oschin Telescope at Caltech's Palomar Observatory near San Diego. The discovery was confirmed with other observatories, and the object's size was pinned down using NASA's new Spitzer Space Telescope.

46. Oort Cloud
In 1950, Jan Oort proposed that comets reside in a vast could at the outer reaches of the solar system.
No direct evidence of an Oort Cloud
Possibly where Long-Period Comets Originate
Possibly extends halfway to the next known star
Recent discoveries (such as Sedna) indicate that the Oort Cloud might contain more objects than originally thought
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.
The Oort Cloud may account for a significant fraction of the mass of the solar system, perhaps as much or even more than Jupiter. (This is highly speculative, however; we don't know how many comets there are out there nor how big they are.)
Brown said Sedna occupies a region of space beyond the Kuiper Belt but inside the theorized Oort Cloud, a distant reservoir of icy comets that are detected only when they zoom through the inner solar system on occasion. The Oort Cloud is thought to extend halfway to the next known star, but scientists know almost nothing about its scope, density or composition.
Brown said the discovery suggests the Oort Cloud might be more dense -- containing more objects -- than was previously thought.

47. Pluto’s Other Satellites?
PLUTO'S SATELLITES
In 1930, tiny, icy world Pluto was discovered orbiting in the distant solar system. In 1978, its relatively large companion Charon was detected by ground-based observations. This year, the Hubble Space Telescope may well have detected two further members of the Pluto system. Provisionally designated S/2005 P1 and S/2005 P2, the two potential new moons are seen orbiting in a counterclockwise direction about 44,000 kilometers (27,000 miles) from Pluto in these deep Hubble images recorded only three days apart. The diminutive and faint companions are also apparently detected on Hubble images of Pluto from 2002, but this coming February follow-up observations are planned in an effort to confirm the discovery of the new moons. Compared to Pluto's and Charon's diameters of 2,300 and 1,300 kilometers respectively, these moons are estimated to be between 60 and 200 kilometers across. Well within the Kuiper Belt, an extensive region beyond the orbit of Neptune, the Pluto system could be the first quadruple Kuiper Belt object known.

48. Centaurs
Nine known “Small Solar System Objects,” orbiting between Jupiter & Neptune
Unstable orbits
Thought to have been part of the Kuiper Belt at one time
Examples
Chiron & Pholus
There are presently nine known objects orbiting between Jupiter and Neptune (including 2060 Chiron (aka 95 P/Chiron) and 5145 Pholus; see the MPC's list). The IAU has designated this class of objects as Centaurs. These orbits are not stable. These objects are almost certainly "refugees" from the Kuiper Belt. Their future fate is not known. Some of these show some cometary activity (ie, their images are a little fuzzy indicating the presence of a diffuse coma). The largest of these is Chiron which is about 170 km in diameter, 20 times larger than Halley. If it ever is perturbed into an orbit that approaches the Sun it will be a truly spectacular comet.