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Extra Credit

• Observing: Fill out form

• Attend Theme Semester event:Write 1-page summary, including description, your impressions, and relation to class

Friday, Jan 23 7:30http://www.lsa.umich.edu/universe

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Our Solar System Before 1930

A Revolution in Planetary Science:A Revolution in Planetary Science:The Discovery and Implications of The Discovery and Implications of Solar System’s Third ZoneSolar System’s Third Zone

Alan Stern

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Our Solar System Before 1930

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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.

It Was A Tidy & Orderly Place

In Which

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1929 View:4 Terrestrial Planets

4 Giant PlanetsComets & Asteroids

Our Solar System Before 1930

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Tombaugh searched for objects that moved slowly relative to the stars.

Pluto was discovered in 1930, by Clyde Tombaugh at the Lowell Observatory, in Arizona.

Then Entered Tombaugh

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Pluto orbits the Sun on an elliptical path, ranging between 30 and 50 AU, and tilted 16 deg from the plane of the planets.

And Found Planet 9…

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A Lone Misfit

Mid-20th Century View:4 Terrestrial Planets

4 Giant Planets1 Misfit Pluto

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4 Terrestrial Planets4 Giant Planets

Comets & Asteroids

And 1 Misfit Pluto

…Which Puzzled

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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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1930– Leonard notes the possibility of many “trans-Plutonian” objects, just after Pluto’s 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.

But Meanwhile, Along Came An Idea

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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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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.

Which Eventually, In 1992, Was Discovered

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Over 140,000 KBOs orbit their in the 30-50 AU region, with diameters >100 km.

Billions of smaller comets are also predicted to orbit there.

What is The Kuiper Belt?

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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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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.

What are these Small 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 PlanetsPerhaps 1000 Dwarf

PlanetsComets & asteroids

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They are smaller and more numerous than larger planets.

Often their orbits are more elliptical and/or more inclined.

And What Makes Small Planets

Different?

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They are smaller and more numerous than larger planets.

Often their orbits are more elliptical and/or more inclined.

And that’s about it.

And What Makes Small Planets

Different?

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They formed similarly, much like Earth, Mars, and Venus. They are made of rock and ice—as are both Earth and Mars. Many have moons—like other planets. Many have cores—like all of the known larger planets. Some have atmospheres—just like larger planets. Their surfaces are solid—again, like the terrestrial planets. Many are expected to have active surface geology & even tectonics— as do the terrestrial planets.Simply Put—Small Planets Have

No Distinguishing Characteristics From Larger

Planets

In Contrast, What Do Small Planets Have

In Common with Their Larger Cousins?

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Simply Put—Small Planets Have No Distinguishing

Characteristics From Larger Planets

Except Their Size.

They formed similarly, much like Earth, Mars, and Venus. They are made of rock and ice—as are both Earth and Mars. Many have moons—like other planets. Many have cores—like all of the known larger planets. Some have atmospheres—just like larger planets. Their surfaces are solid—again, like the terrestrial planets. Many are expected to have active surface geology & even tectonics— as do the terrestrial planets.

In Contrast, What Do Small Planets Have

In Common with Their Larger Cousins?

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So How Should We Classify Dwarf

Planets?

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RECENT TRENDS IN PLANET CLASSIFICATON

Two Broad Themes Have Been Advanced:

1. 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 IAU’s HAS A DYNAMICALLY-BASED, “LOCATIONAL”

DEFINITIONThe 2006 definition of "planet" by the (IAU) states that a

planet is:

1. A celestial body that: is in orbit around the Sun,2. Has sufficient mass so that it assumes a hydrostatic

equilibrium (nearly round) shape, and 3. 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 2006 definition of "planet" by the (IAU) states that a planet is:

1. A celestial body that: is in orbit around the Sun,2. Has sufficient mass so that it assumes a hydrostatic

equilibrium (nearly round) shape, and 3. 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."

THE IAU’s HAS A DYNAMICALLY-BASED, “LOCATIONAL” DEFINITION

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First, it has nothing to do with the nature of the body.

Further, it depends on the stellar mass and the system’s age: Mpl

2/3 > ~G-1/2TsysM*1/6 apl

3/2

Which fundamentally biases against distant planets.

Even a reordering the planets in our system would change which are classified as planets!

WHAT’S THE PROBLEM WITH SUCH A LOCATION-BASED DEFINITION?

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EARTH

SO IN THE IAU’S VIEW THIS IS NOT ALWAYS A PLANET

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Clearing By Accretion

Clearing By Scattering

Why? Because under the IAU definition, a planet must be more and more

massive the farther it is from the Sun.

If Earth were in the outer solar system, it would not be a planet!

+

H.F. Levison (2006)

Planets: Capable of Clearing

Not So

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NOW ENTER THE GPD: AN ATTRIBUTE-BASED DEFINITION

The Geophysical Planet Definition says a planet is:

1. 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.

2. 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:

1. 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.

2. But with insufficient mass to initiate sustained fusion in its interior at any time.

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It’s 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.

WHAT DO WE GAIN WITH THE GPD DEFINITION?

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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, ALWAYSA PLANET

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SO DISCOVERING PLUTO LED TO THREE SEPARATE

REVOLUTIONSThe Discovery of The Kuiper Belt—The Third And Largest Zone of Our Planetary System.

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SO DISCOVERING PLUTO LED TO THREE SEPARATE

REVOLUTIONSThe Discovery of The Kuiper Belt—The 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

REVOLUTIONSThe Discovery of The Kuiper Belt—The 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 SolarSystem’s 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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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.

THE GEOPHYSICALPLANET DEFINITION

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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.