Download Pluto

• 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
• 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 of N2
that is probably not in
equilibrium
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Pluto: Basic Facts
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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, and in all
likelihood, Pluto will regain planet
status at some point
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Eris
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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 [3], except satellites, orbiting the Sun shall
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Worlds Solar System Bodies".
be
referred
to collectively asOuter
"Small
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