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Trans-Neptunian Objects and
Pluto
Astronomy 311
Professor Lee Carkner
Lecture 21
Trans-Neptunian Region
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Beyond Neptune is the region of small, icy,
Trans-Neptunian Objects
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The region is populated by icy planetesimals
that either formed at the edge of the solar
system or were ejected out by the planets
Pluto -- God of the Underworld
Pluto is the God of the
Dead in Roman
mythology
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The Discovery of Pluto
In the late 1800’s it was believed that Neptune’s
orbit was being perturbed by a 9th planet
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In 1930 a young astronomer named Clyde
Tombaugh found a very faint planet near Lowell’s
predicted position
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The Discovery of Pluto
Observing Pluto
Through most telescopes Pluto simply
appears as a faint star
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Recent attempts to develop a spacecraft
have been curtailed
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Pluto Facts
Size: 2300 km
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Orbit: 39.5 AU
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Description:
Pluto’s Orbit
Pluto has the most eccentric and most
inclined orbit in the solar system
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Pluto’s orbit carries it inside the orbit of
Neptune
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Pluto is tipped on its side like Uranus
Composition of Pluto
Pluto has a density of 2000 kg/m3
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Pluto is probably composed of ice and rock
Spectra of Pluto reveal the presence of
methane, nitrogen and carbon monoxide
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HST Images Pluto
Features of Pluto
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The other bright regions may be areas
where impacts have gouged out fresh
ice
Pluto and Charon
Charon
Pluto’s moon Charon was discovered as a
small bulge in a high resolution image (1978)
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Pluto and Charon are in a close, tidally locked
orbit
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Where Do Comets Come From?
Comets are small (few km) icy bodies that
sometimes come in to the inner solar system on
highly elliptical orbits
Short period comets
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Long period comets
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The Kuiper Belt
Around 1950 Kuiper and Edgeworth
proposed a belt of comets out beyond
Neptune
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In 1992 the first (besides Pluto) Kuiper
belt object was discovered (QB1)
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The Kuiper Belt
Discovering
Kuiper Belt
Objects
The Known Kuiper Belt
 There are now hundreds of known Kuiper Belt
Objects (KBOs)
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 Total population of large KBO’s may be 70000 (larger
than 100 km)
 Kuiper belt seems to end at about 50 AU
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 Larger and larger KBO’s being detected
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 Larger than Pluto
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Large KBO Sizes
Known KBOs as of 10/2003
Plutinos and Plutos
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These orbits tend to minimize perturbations from
Neptune
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Some KBO’s have fairly large sizes
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2003 UB313 is probably larger than Pluto
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Some may have dark surfaces and be hard to see
Is Pluto a Planet?
Pro
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Con
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What Makes Something a
Planet?
Planets used to be obvious
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Needed new definition when rest of solar system
was discovered with telescopes
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The International Astronomical Union (which has
authority over these things) calls Pluto a planet
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The Oort Cloud
In 1950 Dutch astronomer Jan Oort
postulated a spherical shell of comets
surrounding the solar system at about
50,000 AU
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Population of the Oort Cloud
The Oort cloud is the source of the long
period comets
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They are too far away to see, so we only
have indirect methods of studying them
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Diagram of the Oort Cloud
Summary
Past the orbit of Neptune the solar
system is made up of many small icy
bodies
Kuiper Belt
extends from 30-500 AU
formed from left over planetesimals at the
edge of the solar system
Oort Cloud
extends from 1000-100,000 AU
formed from ejected icy planetesimals
Summary: Pluto
Description: small, cold , distant
Pluto resembles a large Kuiper belt
object more than a planet
Has a closely orbiting large moon
Charon
Properties
Thin atmosphere
Very cold (~50 K)
Bright surface features possibly composed
of fresher ice