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 ?
The “Cultural” definition of “planet”
A large body that orbits a star but doesn’t shine by itself
What do YOU think a “planet” is…?
The “Cultural” definition of “planet”
A large body that orbits a star but doesn’t 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 planet’s orbits?
•What if there are a bunch of them in similar orbits?
•Doesn’t shine at what level?
•Shine with what sort of energy?
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
Pluto : Size Matters?
Which of these are “real planets”?
Which one is Pluto?
The Pluto : The Orbit Problem
The Ceres Problem : a planet lost
In 1801, Piazzi finds a planet where Bode’s 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 don’t
prevent each other from being
there.
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).
Is Pluto a Planet?
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...
Clyde Tombaugh
KBO-76 1200 km
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”
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?).
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
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!
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.
Characteristics : Luminosity source
Trapped heat of formation,
radioactive decay
Gravitational contraction
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.
Thermonuclear fusion
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.
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)
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?]
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.
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”?
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?
Cosmogony – the “standard” story
Cosmogony – formation of planetesimals
As if by magic…
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).
Compositions of the Planets
Cosmogony – problems posed by extrasolar planets
1) If the planets are formed in a disk,
why don’t they have circular orbits
2) How did gas giants get to be so close to the star?
One possible answer: orbital perturbation and migration.
Lynette Cook
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…
Desirable Characteristics for the
Definition of “Planet”
1) Physical : tells what sort of object a planet is
2) Based on easily observable quantitative parameters
3) Succinct, unique and doesn’t change
(one object is not several different things)
4) Allows for new discoveries (not too specific)
5) Makes sense to the public (and to astrophysicists)
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 :
1) Objects which have core fusion are not planets.
2) 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.
“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.
The End!
You must help decide
(and now you are better informed!)
IAU Provisional Definition – Feb. 2001
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.
Objects of different mass