Transcript Lec26-042407

Comets and Kuiper Belt
Objects
PTYS/ASTR 206
Comets
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Announcements
• Reading Assignment -- Chapter 30
• quiz today
• In-class activity and course evaluations on Thursday
• Public lecture tonight – 7:30PM, this room
– Prof. Alfed McEwan, Mars HiRISE
• Last Lecture – a week from today
– Extrasolar planets
– Final exam review
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• Most distant “planet”
– Most of the time, but not all of
the time! Its orbit can bring it
inside of Neptune’s (as it did
from 1979-1999)
• Only “planet” not visited by a
spacecraft
– New Horizons, launched in
Jan. 2006, will reach Pluto in
2015.
• Pluto can be seen with an
amateur telescope, but it is not
easy!
– About 14th magnitude
• 6th magnitude is the limit for
the naked eye
• Pluto is about 1600 times
dimmer than this
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Pluto
Pluto: Basic Facts
• Avg. distance to the Sun
– 39.5 AU
• Orbital Period
– About 250 years
• Retrograde rotation
– Like Venus and Uranus
• Eccentricity
– 0.25
– larger than all of the
planets
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• Avg. density
– About 1900 kg/m3
– Rock and ice
Pluto: Basic Facts
• Diameter
– 0.18 Earth Diameters
– Not that well known (+/1%)
• Mass
– 0.0021 Earth Masses
– Also not well known,
although the combined
mass of Pluto and Charon
is well known
• Tenuous atmosphere (but not
as we have defined in in class)
of N2 that is probably not in
equilibrium
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Neptune and Pluto are in a 3:2 orbital
resonance
• Pluto’s orbital period is 1.5
times Neptune’s
– Neptune is about 164
years
– Pluto is about 249 years
• This is a stable configuration
and they will never crash into
each other
• Pluto is like other objects in
this sense – comprising a
class of objects called
“Plutino’s”
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• Discovered in 1978 by Jim
Christy.
– Prior to that it was thought
that Pluto was much larger
since the images of Charon
and Pluto were blurred
together.
Charon: Pluto’s Moon
• Charon is the largest moon with
respect to its primary “planet” in
the Solar System (a distinction
once held by Earth's Moon).
• Some prefer to think of
Pluto/Charon as a double planet
rather than a planet and a
moon.
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• Pluto and Charon are in an unusual resonance.
• Charon has an orbital period equal to both its rotational
period AND Pluto’s orbital period
• From one side of Pluto, Charon just sits in the same
place in the sky (never sets, never rises)!
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Is Pluto a planet ?
• Since the discovery of thousands of
objects orbiting the Sun, called
Edgeworth-Kuiper objects (or Kuiper-belt
objects, KBOs), it has been suggested
that Pluto is simply one of these and is
not a planet
• Many KBO’s are in orbital resonances
with Neptune (3:2 resonance  “Plutino”)
• This is further complicated by the fact that
the recently discovered Eris is larger than
Pluto.
• In 2006, Pluto was downgraded to a new
class of objects called “dwarf planets”
– But the debate rages on
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Kuiper Belt
Red: All objects that are in a resonance with Neptune
(Neptune Trojans – 1:1 ; Plutinos – 2:3 ; Twotinos 1:2 )
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Blue: “Classical” objects
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The IAU’s decision in August 2006
• The IAU...resolves that planets and other bodies, except
satellites, in our Solar System be defined into three distinct
categories in the following way: (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 neighbourhood 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 neighbourhood
around its orbit, and (d) is not a satellite.
• (3) All other objects, except satellites, orbiting the Sun shall be
referred
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CometsSolar System Bodies".
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Eris
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History of planets
• Ancients recognized 7 planets
– Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn
• Earth was not a planet!
• 1543, changed from 7 to 6 planets
– Definition changed to an object orbiting the Sun
• Sun and Moon removed, added Earth
• 1852, changed from 23 to 8 planets
– Asteroids were demoted since they did not have a resolvable
disc
• Uranus and Neptune had been discovered by now
• 2006, changed from 9 to 8 planets
– Pluto demoted
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• Asteroids
– Small rocky bodies in orbit
about the Sun
Semantics
• Comets
– Small bodies that orbit the
Sun and (at least
occasionally) exhibits a coma
(or atmosphere) and/or a tail
• Meteoroids
– Small asteroids
• Meteorites
– the debris collected on Earth
• Meteors
– A brief flash of light (i.e. a
shooting star)
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Asteroids and Comets
• Asteroids
– Mostly nearly-circular
orbits
– Mostly confined to the
asteroid belt
• Close to the ecliptic
plane
– Relatively short orbital
periods
• Comets
– Highly elliptical orbits
– Random inclinations
• i.e. not-confined to the
ecliptic
– Very long orbital periods
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Comets: Harbinger’s of Doom
• Rome: marked the assassination of Julius Ceaser
• England: blamed for bringing the Black Death
• Incan Empire: foreshadowed the brutal conquering by Francisco
Pizarro
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Comet Types (based on orbital
characteristics)
• Short-Period Comets
– Comet with an orbital period of less than 200 years
– Shortest lived
• Breakup due to gravitational forces
• Life expectancy of about 12,000 years
• Long-Period Comets
– Comets with orbital periods greater than 200 years (though
typically around millions of years)
• Other Definitions
– Jupiter Family Comets (JFC)
• Orbital period less than 20 years
– Intermediate-Period Comets
Comets
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• Orbital period between 204/24/07
and 200 years
• Kuiper Belt
– Jupiter family and intermediate
period comets
– These comets have low
inclinations compared to longperiod comets
• Oort Cloud
– Spherical distribution
– Frequent observations of longand intermediate-period comets
indicate that the reservoir must
be huge
– Cannot be seen directly
because it is so far away
– Perturbed by passing stars or
galactic tides
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Comet Origin
Comet orbits are often highly inclined
• Long-period comets,
originating from the roughly
spherically distributed Oort
cloud have orbits with random
inclinations
• Intermediate and Jupiter-family
comets tend to have orbits
more confined to the ecliptic
• Asteroids seldom have highly
inclined orbits
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Appearance of Comets
• When comets are far from the Sun,
they are dark and hard to see
– Low albedos, far away
• Because of their large
eccentricities, they occasionally
come very close to the Sun
• The body outgases as it heats and
releases dust and cometary atoms
• This produces a long visible tail
– Directed away from the Sun
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