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Trans-Neptunian Objects and
Pluto
Astronomy 311
Professor Lee Carkner
Lecture 21
Gas Giant Moons
Kinetic energy of launch equals potential
energy at peak
PE = KE
mgh = ½mv2
h = ½mv2/mg = ½v2/g
Gravity on Io
g = GM/R2
g = [(6.67X10-11)(8.94X1022)]/(1.82X106)2
g= 1.8 m/s2
Final height
h = [(½)(6002)]/1.8 = 100000 m = 100 km
Pluto -- God of the Underworld
Pluto is the God of the
Dead in Roman
mythology

Pluto was discovered
at Lowell Observatory
and its first 2 letters
commemorate
Percival Lowell
The Discovery of Pluto
In the late 1800’s it was believed that
Neptune’s orbit was being perturbed by a
9th planet

Many astronomers tried to determine its
position, including Percival Lowell

The position turned out to be a coincidence,
Pluto is too small to effect Neptune’s orbit
The Discovery of Pluto

No spacecraft has ever
visited it

But will not get to Pluto
until 2015
The best information
comes from HST
Pluto Facts
Size: 2300 km

Smaller than the 7 largest moons
Orbit: 39.5 AU

Description: Very small, very cold, very
distant
Composition of Pluto
Pluto has a density of 2000 kg/m3

Pluto is probably composed of ice and rock
Spectra of Pluto reveal the presence of
methane, nitrogen and carbon monoxide ice

The temperature on Pluto is only ~50 K so the
atmosphere can’t escape
Spectra of Pluto Showing
Methane Ice
Pluto’s Moons
Pluto’s largest moon Charon was discovered
as a small bulge in a high resolution image
(1978)

Their sizes are closer than any planet and moon

They have very similar densities, masses and
sizes

Two other smaller moons Nix and Hydra
were discovered by HST in 2005
Each is about 50 km in diameter
Is Pluto a Planet?
 Pro

Spherical

Tradition

 Con

Eccentric orbit

Not largest TNO

Pluto’s Orbit
Pluto’s orbit is much more eccentric
and much more inclined than any
planet
Eccentricity =
Most other planets e<0.1
Inclination =
Pluto’s orbit carries it inside the orbit of
Neptune

Pluto is tipped on its side like Uranus
Small, Icy Bodies

Small icy bodies in the outer solar system (beyond
Jupiter) have no good name

Lets call all of them “Trans-Neptunian Objects” or
“TNOs”

They are all similar to Pluto (but usually much
smaller)
Most are only recently discovered and not well
characterized or organized
Discovering TNOs
Around 1950 Kuiper and Edgeworth
proposed a belt of comets out beyond
Neptune

In 1992 the first (besides Pluto) TNO was
discovered (QB1)

Discovered via long exposures with large
telescopes (including HST)

Total population of large TNOs may be 70000
(larger than 100 km)
Discovering
TNOs
Centaur:
Resonant: in an orbital resonance with Neptune
Classical Kuiper Belt:
Scattered Disk: large distances and eccentricities
Classical Kuiper Belt

Most of the objects have nearly circular orbits,
low inclinations and are not effected by
Neptune’s gravity


Probably formed in place from the leftover
material at the edge of the solar nebula
Resonant Objects

TNOs tend to collect on these orbits
Examples:

Pluto is in this group so they are called Plutinos

Marks the edge of the classical Kuiper Belt, few TNOs
beyond this point
Theory: Neptune formed closer to the Sun
and then migrated outwards
Swept up TNOs into resonances as it moved out
TNOs and Resonance
Scattered Disk Objects
Some TNOs have very irregular orbits

These objects are thought to have been
scattered by gravitational interaction
with a gas giant (mostly Neptune)
Can be hard to find due to their odd
orbits
Eris
 The largest TNO currently known is called
Eris

 Larger than Pluto

 Semi-major axis of 68 AU, but is currently at
97 AU due to high eccentricity

 Part of the scattered disk

 Has a small moon, Dysnomia
 formerly called “Gabrielle”
Large KBO Size Comparisons
Centaurs
Some TNOs are inside the orbit of Neptune
Called Centaurs

Have a wide range of orbital parameters

Centaurs are thought to be former Kuiper belt
objects that have been ejected inward into the
gas giant region
Will eventually collide with something or be
ejected from the solar system altogether
The Oort Cloud
Spherical shell of comets surrounding
the solar system at about 50,000 AU

They are too far away to see, so we only
have indirect methods of studying them

There may be as many as 1 trillion
comets in the Oort cloud
Diagram of the Oort Cloud
Tentative Origin of the TNOs

The gas giants and TNOs gravitationally interact
with each other
Some TNOs are flung very far out and form the Oort
cloud

Some TNOs are swept up in Neptune’s resonances as
Neptune migrates out and form the Resonant TNOs
Some TNOs form between 40-50 AU and are not much
affected by gravitational interaction and form the Kuiper
belt

Next Time
Read Chapter 14.2
Quiz 3 Monday Oct 24
Summary: Pluto
Description: small, cold , distant
Pluto resembles a large TNO 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
Summary: TNOs
Past the orbit of Neptune the solar
system is made up of many small icy
bodies
About 1000 found in the last 15 years
Are organized into many different
classes based on orbits
Theories on their origin and evolution
still under development