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 Radius of Pluto = 1145 to 1200 km Radius of Charon = 600 to 650 km 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).
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? Clyde Tombaugh KBO-76 1200 km 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...
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. Stern & Levinson Gas Giants Terrestrials Moons Minor planets
Characteristics : degeneracy pressure
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. Brown dwarf: 40 jupiters White dwarf : 600 jupiters Adding mass makes the object smaller! Energy
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 Thermonuclear fusion 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.
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 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 ) Definition : A “fusor” is an object capable of core fusion at some time.
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
If the planets are formed in a disk,
why don’t they have circular orbits
How did gas giants get to be so close to the star?
One possible answer: orbital perturbation and migration.
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”
Physical : tells what sort of object a planet is
Based on easily observable quantitative parameters
Succinct, unique and doesn’t change
(one object is not several different things)
Allows for new discoveries (not too specific)
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 :
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.
“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) OF THE INTERNATIONAL ASTRONOMICAL UNION 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. WORKING GROUP ON EXTRASOLAR PLANETS (WGESP)