Our Solar System Before 1930

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A Revolution in Planetary Science The Discovery
and Implications of Solar Systems Third
Zone
Alan Stern
Our Solar System Before 1930

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Our Solar System Before 1930



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It Was A Tidy Orderly Place In Which

• Planets are isolated from each other.
• Planets travel in the same direction, and in the
  same plane.
• Their orbits are nearly circular.
• And the space between them is empty.

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Our Solar System Before 1930

1929 View 4 Terrestrial Planets 4 Giant
Planets Comets Asteroids
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Then Entered Tombaugh
Pluto was discovered in 1930, by Clyde Tombaugh
at the Lowell Observatory, in Arizona.
Tombaugh searched for objects that moved slowly
relative to the stars.
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And Found Planet 9
Pluto orbits the Sun on an elliptical path,
ranging between 30 and 50 AU, and tilted 16 deg
from the plane of the planets.
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A Lone Misfit

Mid-20th Century View 4 Terrestrial Planets 4
Giant Planets 1 Misfit Pluto
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Which Puzzled

4 Terrestrial Planets 4 Giant Planets Comets
Asteroids And 1 Misfit Pluto
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Though Planet Pluto Began To Resemble Other
Planets in Key Ways

• With a solid surface, and very likely a core.
• With moons.
• With an atmosphere, and surface snows.
• And observable seasons.

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But Meanwhile, Along Came An Idea

• 1930 Leonard notes the possibility of many
  trans-Plutonian objects, just after Plutos
  discovery.
• 1943 Edgeworth postulates more objects with
  Pluto in a belt.
• 1951 Kuiper discusses the unnatural situation of
  a sharp outer edge to the planetary formation
  region.
• 1980 Fernandez predicts belt of comets and
  planetesimals as the source region for most short
  period comets.
• 1987 Duncan, Quinn, Tremaine show that the
  source of the low-inclination comets, JFCs,
  requires a disk-like, tran-Neptunian reservoir.

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Something Known As The Kuiper Belt
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A Thing That Came to Be Known As The Kuiper Belt
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Which Eventually, In 1992, Was Discovered

• And a floodgate was opened
• 1992 Jewitt Luu find an object in orbit wholly
  beyond Pluto.
• 1993 4 more KBOs found.
• 1994 10 KBOs found.
• 1997 50 KBOs were known.
• 2009 Over 1300 KBOs are known.

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What is The Kuiper Belt?

• Over 140,000 KBOs orbit their in the 30-50 AU
  region, with diameters gt100 km.
• Billions of smaller comets are also predicted to
  orbit there.

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What is The Kuiper Belt?
The Third Zone and The Largest Structure in Our
Planetary System
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E.g., At Fomalhaut
It Turns Out Such Structures Are Common Around
Stars
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It Also Turns Out That The KB Is Dynamically
Complex
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And Contains Forensic Evidence of Planet Migration
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And, Too, The KB Is Rich In Small Planets
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What are these Small Planets?

• The large KBOs are planetary embryos, bodies
  which reached the mid-stage of planetary
  accretion, but grew no further.
• They are commonly termed dwarf planets.

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ASTEROIDS EXPLORED BY SPACECRAFT NOT PLANETS
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These Dwarf Planets Are Large Compared to
Asteroids Spacecraft Have Visited
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But They Are Small Compared to Giant Planets
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Just As The Sun is Small Compared to Some Stars
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Just As The Sun is Small Compared to Some Stars
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No Longer is Pluto a Misfit Small Planets Litter
Our System




The 21st Century View 4 Terrestrial Planets 4
Giant Planets Perhaps 1000 Dwarf Planets Comets
asteroids
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And What Makes Small Planets Different?

• They are smaller and more numerous than larger
  planets.
• Often their orbits are more elliptical and/or
  more inclined.

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And What Makes Small Planets Different?

• They are smaller and more numerous than larger
  planets.
• Often their orbits are more elliptical and/or
  more inclined.
• And thats about it.

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In Contrast, What Do Small Planets Have In
Common with Their Larger Cousins?

• They formed similarly, much like Earth, Mars,
  and Venus.
• They are made of rock and iceas are both Earth
  and Mars.
• Many have moonslike other planets.
• Many have coreslike all of the known larger
  planets.
• Some have atmospheresjust like larger planets.
• Their surfaces are solidagain, like the
  terrestrial planets.
• Many are expected to have active surface geology
  even tectonics
• as do the terrestrial planets.

Simply PutSmall Planets Have No Distinguishing
Characteristics From Larger Planets
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In Contrast, What Do Small Planets Have In
Common with Their Larger Cousins?

• They formed similarly, much like Earth, Mars,
  and Venus.
• They are made of rock and iceas are both Earth
  and Mars.
• Many have moonslike other planets.
• Many have coreslike all of the known larger
  planets.
• Some have atmospheresjust like larger planets.
• Their surfaces are solidagain, like the
  terrestrial planets.
• Many are expected to have active surface geology
  even tectonics
• as do the terrestrial planets.

Simply PutSmall Planets Have No Distinguishing
Characteristics From Larger Planets Except
Their Size.
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So How Should We Classify DwarfPlanets?
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RECENT TRENDS IN PLANET CLASSIFICATON
Two Broad Themes Have Been Advanced1.
Dynamical i.e., Location Based.2. Intrinsic
i.e., Attribute Based.
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NOW DO WE JUDGE A HOUSE A HOME BASED ON ITS
LOCATION?
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OR DO WE JUDGE A HOUSE A HOME BASED ON ITS
ATTRIBUTES?
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AND DO WE JUDGE A STAR A STAR BASED ON ITS
ATTRIBUTES?
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OR DO WE BASE THAT ON ITS LOCATION?
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THE IAUs HAS A DYNAMICALLY-BASED, LOCATIONAL
DEFINITION

• The 2006 definition of "planet" by the (IAU)
  states that a planet is
• A celestial body that is in orbit around the
  Sun,
• Has sufficient mass so that it assumes a
  hydrostatic equilibrium (nearly round) shape, and
  
• Has "cleared the neighborhood" around its orbit.
• A non-satellite body fulfilling only the first
  two of these criteria is classified as a "dwarf
  planet", whilst a non-satellite body fulfilling
  only the first criterion is termed a "small solar
  system body."

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THE IAUs HAS A DYNAMICALLY-BASED, LOCATIONAL
DEFINITION

• The 2006 definition of "planet" by the (IAU)
  states that a planet is
• A celestial body that is in orbit around the
  Sun,
• Has sufficient mass so that it assumes a
  hydrostatic equilibrium (nearly round) shape, and
  
• Has "cleared the neighborhood" around its orbit.
• A non-satellite body fulfilling only the first
  two of these criteria is classified as a "dwarf
  planet", whilst a non-satellite body fulfilling
  only the first criterion is termed a "small solar
  system body."

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WHATS THE PROBLEM WITH SUCH A LOCATION-BASED
DEFINITION?

• First, it has nothing to do with the nature of
  the body.
• Further, it depends on the stellar mass and the
  systems age Mpl2/3 gt G-1/2TsysM1/6 apl3/2
• Which fundamentally biases against distant
  planets.
• Even a reordering the planets in our system
• would change which are classified as planets!

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SO IN THE IAUS VIEW THIS IS NOT ALWAYS A PLANET
EARTH
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Why? Because under the IAU definition, a planet
must be more and more massive the farther it is
from the Sun.
Planets Capable of Clearing

Clearing By Scattering
Not So
Clearing By Accretion
If Earth were in the outer solar system, it would
not be a planet!
H.F. Levison (2006)
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NOW ENTER THE GPD AN ATTRIBUTE-BASED DEFINITION

• The Geophysical Planet Definition says a planet
  is
• A celestial body that has sufficient mass so
  that it can assume a hydrostatic equilibrium
  (nearly round) shape due to its gravity
  overwhelming material strength.
• But with insufficient mass to initiate sustained
  fusion in its interior at any time.

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NOW ENTER THE GPD AN ATTRIBUTE-BASED DEFINITION

• The Geophysical Planet Definition says a planet
  is
• A celestial body that has sufficient mass so
  that it can assume a hydrostatic equilibrium
  (nearly round) shape due to its gravity
  overwhelming material strength.
• But with insufficient mass to initiate sustained
  fusion in its interior at any time.

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WHAT DO WE GAIN WITH THE GPD DEFINITION?

• Its simple, intuitive, and far less ambiguous.
• It embraces a diversity of planetary sizes and
  types which share a fundamental physical trait in
  common shape controlled by gravity rather than
  material strength.
• It does not rely on having a complete census of a
  system to classify its objects.
• Objects do not reclassify based on orbital
  location.
• Instead, objects are classified purely on the
  basis of their nature, as are stars, stellar
  remnants, etc.

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AND WHERE IS THIS HYDROSTATIC DIVIDING LINE?
Planets Capable of HSE
Not
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AND WITH THE GPD DEFINITION, AN EARTH IS ALWAYS,
ALWAYS
A PLANET
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SO DISCOVERING PLUTO LED TO THREE SEPARATE
REVOLUTIONS
The Discovery of The Kuiper BeltThe Third And
Largest Zone of Our Planetary System.
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SO DISCOVERING PLUTO LED TO THREE SEPARATE
REVOLUTIONS
The Discovery of The Kuiper BeltThe Third And
Largest Zone of Our Planetary System. The
Discovery That the Planets Migrated From Their
Formation Sites.
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SO DISCOVERING PLUTO LED TO THREE SEPARATE
REVOLUTIONS
The Discovery of The Kuiper BeltThe Third And
Largest Zone of Our Planetary System. The
Discovery That the Planets Migrated From Their
Formation Sites. And the Discovery of a Third
Class of Planets The Dwarfs, Which Dominate the
Solar Systems Population.
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SOMEWHERE, I THINK,COPERNICUS IS SMILING.
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THE EXPLORATION OF THE KUIPER BELT BEGINS IN
JULY, 2015
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SOMEWHERE, I THINK,COPERNICUS IS SMILING.
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SOMEWHERE, I THINK,COPERNICUS IS SMILING.
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BACKUPS
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A Census Gives 20 Solar-Orbiting Planets, 2/3 of
Which Are Dwarfs

• The Terrestrials Mercury, Venus, Earth, and
  Mars.
• The Giants Jupiter, Saturn, Uranus, and
  Neptune.
• The Rocky Icy Dwarfs Ceres, Pallas, Juno,
  Vesta, Pluto, Charon,
• Quaoar, Ixion, EL61, Eris, Makemake, and
  Sedna.
• And Satellite Planets, Like Io, Europa,
  Ganymede, Callisto,
• Titan, Triton, Luna.

Science is About Discovering New Paradigms.

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A Census Gives 20 Solar Orbiting Planets, 2/3 of
Which Are Dwarfs

• The Terrestrials Mercury, Venus, Earth, and
  Mars.
• The Giants Jupiter, Saturn, Uranus, and
  Neptune.
• The Rocky Icy Dwarfs Ceres, Pallas, Juno,
  Vesta, Pluto, Charon,
• Quaoar, Ixion, EL61, Eris, Makemake, and
  Sedna.

Science Is About Discovering New Paradigms

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THE GEOPHYSICALPLANET DEFINITION

• Is also parallel to the definition of stars in
  ways that unifies planet classification with
  other astronomical bodies
• 1. Stars are stars based on a single unifying
  attribute (ability to burn elements by fusion),
  without regard to orbit or location or size.
• 2. The ability to do fusion is fundamentally a
  gravitational criterion varying only with regard
  to composition.

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Primary Comet Reservoirs of the Solar System

• Kuiper Belt
• Formed In Situ
• Reservoir For the JFCs
• Oort Cloud
• Ejection Formation
• Reservoir for LP, HFCs

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Global Architecture 102
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Primary Comet Reservoirs of the Solar System

• Kuiper Belt JFCs
• Oort Cloud LP, HFCs

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How Do We Know the Oort Cloud Exists?
Estimated Oort Cloud Mass 0.1 to 10 Earth Masses
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YOU DECIDE.