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What is a Planet?
Originally planet wanderer (Greek
root) refers to apparent motion of planets in
sky among stars Earth-based no astrophysical
utility How are planets distinct from moons,
asteroids, brown dwarfs, stars ?
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The Cultural definition of planet
?A large body that orbits a star but doesnt
shine by itself
What do YOU think a planet is?
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The Cultural definition of planet

• ?A large body that orbits a star but doesnt
  shine by itself
• What are the size/mass limits (both big and
  small)?
• Does it have to orbit a star (how about a brown
  dwarf?)
• Can the orbit be very non-circular, or well out
  of the plane?
• Can planets cross other planets orbits?
• What if there are a bunch of them in similar
  orbits?
• Doesnt shine at what level?
• Shine with what sort of energy?

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The case of Pluto
Pluto was first thought to be the size of Mars,
but then turned out to be icy (shiny, so rather
small) and possessing a large moon (Charon).
Radius of Pluto 1145 to 1200 km Radius of
Charon 600 to 650 km
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Pluto Size Matters?
Which of these are real planets?
Which one is Pluto?
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The Pluto The Orbit Problem
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The Ceres Problem a planet lost
In 1801, Piazzi finds a planet where Bodes Law
predicts one (though surprisingly small 1000
km). In 1802 Pallas is found, and then Vesta in
1804.
Herschel (who found Uranus) begins referring to
them as asteroids, and as more are found,
everyone agrees they are minor planets. The
demotion occurs because there are many objects in
very similar orbits, and they dont prevent each
other from being there.
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Pluto - the real problem too much company
The remains of the disk which formed the Solar
System is still out there beyond Neptune, and
Pluto is part of a large crowd of small icy
bodies (Kuiper Belt).
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Is Pluto a Planet?
Clyde Tombaugh
To be consistent with the treatment of Ceres, we
should demote Pluto. Ceres was quickly
dethroned, but Pluto has been around for decades.
Perhaps we must wait for a new generation to grow
up knowing its status as a Kuiper Belt Object.
Popular sentiment will keep it a planet for
now unless an even larger KBO is found...
KBO-76 1200 km
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Arenas in which to define Planet

• Characteristics (physical attributes)
• What determines its size and shape (pressure
  support)
• What determines its luminosity (energy flow)
• Does it shine by itself (and by what means)
• Circumstances (orbital attributes)
• What it is in orbit around (must it be orbit at
  all?)
• What shape, size, and tilt does the orbit have
• Is object in important orbit is it alone
• Cosmogony (the mode of formation)
• Was the object formed in a disk (even stars are)
• Was the object formed by merging planetesimals
• Was the object formed by direct collapse

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Characteristics ordinary pressure

• Types of pressure support
• Coulomb forces liquid or crystalline
• Due to bound electron degeneracy

What gives us volume is the electron clouds in
atoms. Electrons are only allowed to be in
certain orbitals and may not all crowd into the
same orbital (by quantum rules). A person would
be smaller than a bacterium without this
support. If you add mass, the object gets bigger.
Too small, and it is not round (or a planet?).
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Characteristics Spherical shape
If large enough, the object will be crushed to a
spherical shape by its own self-gravity. This
depends a little on what its made of.
Gas Giants
Terrestrials
Moons
Minor planets
Stern Levinson
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Characteristics degeneracy pressure
Brown dwarf 40 jupiters

• Types of pressure support
• Free electron degeneracy

Even when electrons are not bound to atoms, if
you crowd them enough they will occupy all the
low energy states. More crowding forces new
electrons into higher energy states, until they
can be moving nearly the speed of light. This
provides a pressure too.
White dwarf 600 jupiters
Adding mass makes the object smaller!
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Characteristics thermal pressure

• Types of pressure support
• Thermal gas pressure

The heat must constantly be replaced, as the star
radiates energy into space.
The size grows with the mass again.
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Characteristics Luminosity source
Objects change their sources of luminosity
depending on their mass. More massive objects
have more extreme densities and temperatures in
their core, because more material weighs down on
it.
Trapped heat of formation, radioactive decay
Thermonuclear fusion
Gravitational contraction
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Characteristics Luminosity History
Stars stabilize their luminosity with hydrogen
fusion on the main sequence for a long time
(trillions of years for the lowest mass stars).
Brown dwarfs turn some fusion on, but then
degeneracy supports them and they shine only by
gravitational contraction (and keep fading).
Planets only contract and fade.
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Characteristics segregation by mass
Pressure support Coulomb ? degeneracy 2
jupiters Pressure support degeneracy ? thermal
70-80 jupiters Luminosity source
gravitational ? deuterium fusion 13
jupiters Luminosity deuterium fusion ?
hydrogen fusion 60 jupiters
Definition A fusor is an object capable of
core fusion at some time.
Possibility 1 Planets are non-fusors. Brown
dwarfs and stars are fusors. Then planets would
be all objects below 13 jupiter
masses. (luminosity-based) Possibility 2
have 3 classes planets, degenerates, and stars.
Non-fusor degenerates might be superplanets or
grey dwarfs. Then planets would be all objects
below 2 jupiter masses. (pressure-based)
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Circumstance the orbits
The major planets in our Solar System are in
essentially circular orbits, while extrasolar
planets (so far) have been mostly in rather
elliptical orbits (as is usually the case with
binary stars). Some of them have masses
approaching or exceeding 13 jupiters. Are they
all planets?
Question does it matter what is being orbited?
Fusor or star?
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Circumstance orbital ejection
With many bodies in a system, the bigger ones
tend to kick the smaller ones around. Some are
ejected from the system. There must be lost
planets. This has also been suggested as a means
of making brown dwarfs.
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Circumstance orbital importance
Should the object be massive enough to get rid of
all other competitors near to it (orbit
clearing)? How many similar objects can there be
before it is a minor planet?
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Circumstance low mass objects not in orbit
Objects have also been found which have apparent
masses below 13 jupiters, but are freely floating
by themselves in star-forming regions (we see
them because they are so young and bright). Are
these free-floating planets? Were they
originally in orbit around a star (fusor), or
have they always been by themselves?
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Cosmogony the standard story
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Cosmogony formation of planetesimals
As if by magic
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Cosmogony formation of the Solar System
The composition of the disk around the Sun
depends on distance from it, through temperature
(can you have ice or not). Since icy material is
plentiful, you can make big planets in the outer
reaches. Once big enough, they can grab gas from
the disk (more plentiful).
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Compositions of the Planets
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Cosmogony problems posed by extrasolar planets

• If the planets are formed in a disk,
• why dont they have circular orbits
• How did gas giants get to be so close to the
  star?
• One possible answer orbital perturbation
  and migration.

Lynette Cook
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Cosmogony do we need planetesimals for gas
giants?
Perhaps we can make giant planets directly from
the disk. Then they could be carried by the tidal
gap to near the star. But that is also how you
make brown dwarfs or binary stars
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Desirable Characteristics for the Definition of
Planet

• Physical tells what sort of object a planet is
• Based on easily observable quantitative
  parameters
• Succinct, unique and doesnt change
• (one object is not several different
  things)
• Allows for new discoveries (not too specific)
• Makes sense to the public (and to astrophysicists)

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The definition of Planet

• Only the International Astronomical Union can
  make an official
• definition. All there is now is something from
  the
• WORKING GROUP ON EXTRASOLAR PLANETS
• Objects which have core fusion are not planets.
• Objects which are not in orbit around suns are
  not planets.
• (this is not really a definition, but
  establishes some parameters)

Basri, and Stern Levinson propose something
like A spherical non-fusor has planetary
mass. A planet is a body with planetary mass
born in orbit around a fusor.
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Planet can have qualifiers
historical planets (the usual nine), maybe
adding Ceres minor planets (those not in
dynamically important orbits) terrestrial,
icy,gas giant, super, ordinary or
degenerate are structural or compositional
qualifiers agglomerated, core-accretion,
direct collapse are cosmogenetic
qualifiers ejected or captured planets
(this is not assumed unless it can be
established)
Moons are formed around planets, and might have
planetary mass or not. There could be captured
planets. You perhaps have a double planet if
the center-of-mass is outside both bodies.
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The End!
You must help decide (and now you are better
informed!)
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IAU Provisional Definition Feb. 2001
WORKING GROUP ON EXTRASOLAR PLANETS (WGESP) OF
THE INTERNATIONAL ASTRONOMICAL UNION
WORKING GROUP ON EXTRASOLAR PLANETS (WGESP)
Rather than try to construct a detailed
definition of a planet which is designed to cover
all future possibilities, the WGESP has agreed to
restrict itself to developing a working
definition applicable to the cases where there
already are claimed detections, e.g., the radial
velocity surveys of companions to (mostly)
solar-type stars, and the imaging surveys for
free-floating objects in young star clusters. As
new claims are made in the future, the WGESP will
weigh their individual merits and circumstances,
and will try to fit the new objects into the
WGESP definition of a "planet", revising this
definition as necessary. This is a gradualist
approach with an evolving definition, guided by
the observations that will decide all in the end.
Emphasizing again that this is only a working
definition, subject to change as we learn more
about the census of low-mass companions, the
WGESP has agreed to the following statements 1)
Objects orbiting around solar-type stars with
true masses above the limiting mass for
thermonuclear fusion of deuterium (currently
calculated to be 13 Jupiter masses for objects of
solar metallicity) are "brown dwarfs" (no matter
how they formed) while objects with true masses
below this limiting mass are "planets". 2)
Free-floating objects in young star clusters
(which presumably formed in the same manner as
stars and have not been shown to be ejected from
planetary systems) with masses below the limiting
mass for thermonuclear fusion of deuterium are
not "planets", but are "sub-brown dwarfs" (or
whatever name is most appropriate). These
statements are a compromise between definitions
based purely on the deuterium-burning mass or on
the formation mechanism, and as such do not fully
satisfy anyone on the WGESP. However, the WGESP
agrees that these statements constitute the basis
for a reasonable working definition of a "planet"
at this time. We can expect this definition to
evolve as our knowledge improves. Note that
these statements are restricted to extrasolar
planets and are not intended to address the
question of a possible lower mass limit for
"planets" in our Solar System.
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Objects of different mass