What is a Planet? What is a Star? Originally: “planet” = “wanderer” (Greek root) refers to apparent motion of planets among stars Earth-based; no astrophysical utility Stars were the fixed lights; now we say they are like the Sun.

What is YOUR definition of “planet”?

What is a Planet? Originally: “planet” = “wanderer” (Greek root) now  A large body that orbits a star but doesn’t shine by itself. But what is “large”? Shines how, and how brightly? Where are the limits? On what are they based? How are planets distinct from: moons, asteroids, brown dwarfs, stars ?

Size, Mass, Density #1 Mass ~ jupiter, Size ~10 -3 km, density ~water, not round Mass ~ jupiter, Size ~10 km, density ~ 1-5 x water, not round Mass ~10 -6 jupiter, Size ~10 3 km, density ~ 1-5 x water, round

Size, Mass, Density #2 Mass ~1/300 jupiter, Size ~10,000 km, density ~ 5 x water, round Mass ~1/5 jupiter, Size ~40,000 km, density ~ 1.5 x water, round Mass ~1 jupiter, Size ~75,000 km, density ~ water, round

Size, Mass, Density #3 Mass 10 jupiters, Size 70,000km, Density ~ 15 x water Mass 50 jupiters, Size 60,000km, Density ~ 80 x water

Size, Mass, Density #4 Mass ~100 jup, Size ~100,000 km, density ~ 50 x water, round Mass ~1000 jup, Size ~1,700,000 km, density ~ water, round

“Ordinary” material 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.

The Shape of Things 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 Pluto 15 15Mimas 16Hyperion 16 Hyperion Mimas Vesta Xena Round Not round

“Ordinary” 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.

Pressure Support : Ordinary Not to scale! 1 Jupiter mass

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! Faster Slower

Pressure Support : Fully or Partially Degenerate 10 Jupiter masses 100 Jupiter masses 40 Jupiter masses

Density Behavior of Planets Hot, puffy envelopes Brown Dwarfs

Luminosity Sources #1 Chemical reactions (from food) Internal and surface temp: 300K, Stable phase : 75 years Radioactivity (very little), extremely dim; Temp very low unless heated by star, No bright stable phase Radioactivity, quite dim; Temp very low unless heated by star, No bright stable phase

Luminosity Sources #2 Radioactive decay, differentiation [gravitational], (core crystallization). Core temperature : K, Surface temp: ~ K (mass), No bright stable phase Gravitational contraction, and differentiation, Core temperature : 25000K, Surface temp: ~100K up (age), No bright stable phase

Luminosity Sources #3 Gravitational Contraction, Core temperature : 500,000K, Surface temperature : 500K, No bright stable phase Gravitational Contraction, Deuterium fusion, Core temperature : 1,500,000K, Surface temperature : 1000K No bright stable phase

Luminosity Sources #4 Mostly hydrogen fusion (gravitational contraction & deuterium fusion early on) Core temperature : 7,000,000K, Surface temperature : 3000K, Bright stable phase: 1 trillion years Mostly hydrogen fusion (gravitational contraction & deuterium fusion early on) Core temperature : 15,000,000K, Surface temperature : 6000K, Bright stable phase : 10 billion years

Thermonuclear Fusion In order to get fusion, you must overcome the electric repulsion. You can do this by having high density (lots of particles) and high temperature (particles moving very quickly). Additionally, you must also have both a proton and a neutron. Only fusion can produce new, heavier elements.

The Importance of Neutrons 1) Neutrons : can't build the elements without them § the strong nuclear force holds nuclei together even though protons repel each other § it works like velcro : only unlike particles can stick together Result : the stable elements have almost equal numbers of protons & neutrons Example: Deuterium burning (this is very quick and easy) 2)Neutrons fall apart by themselves after about 10 minutes, so there usually aren’t any free neutrons around § after deuterium is gone, you have to rely on the weak nuclear force to convert protons to neutrons (as in the Sun) § this is a slow process, so stars can last a long time P P P P N N + He 3 H2H2 H1H1 P P N N PPPP+++ Note change of protons to neutrons He 4

Luminosity Histories 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. Planets Brown dwarfs Stars Burrows et al.

Pressure support – Coulomb  degeneracy transition occurs at 2-5 jupiters Pressure support – degeneracy  thermal transition occurs at jupiters Luminosity source – purely gravitational  deuterium fusion transition occurs at 13 jupiters Luminosity source – deuterium fusion  hydrogen fusion transition occurs at 60 jupiters stable hydrogen burning at 75 jupiters Physical Characteristics : segregation by mass

Does Size Matter? Which of these are “real planets”? Which one is Pluto?

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 : The Orbit Problem

Orbital Shapes 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?

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 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.

The Kuiper Belt and Oort Cloud The protosolar nebula is not expected to have ended at Neptune’s distance (or even Pluto’s). Typical disks are AU in size (as observed around other stars).

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: the Kuiper Belt.

Kuiper Belt Objects : Reaching New Limits Scans of the Kuiper Belt are now reaching out beyond the main belt, and finding objects with strange orbits and strange sizes…

Quaoar, Sedna, and now “Xena” A classical KBO and object with a strange orbit, both with a size comparable or larger than Charon. And now Xena: bigger than the Moon, more tilted than Pluto, further than Sedna!! Xena and Gabrielle

Orbital “dominance” 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”?

Orbital ejection and migration 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. T Tauri Sb

Sub-fusor 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, or have they always been by themselves? Can you call them planets at all?

Sub-fusor Objects in Far Orbits Objects have also been found which have apparent masses below 13 jupiters, but are located too far from the central star to fit the usual giant planet formation scenario. (we see them because they are so young and bright). Are these “sub- brown dwarfs”? Should they be considered more akin to binary stars than planets? 2MASS TW Hya Association

My Answers (definitions) Fusors : objects which experience core fusion sometime brown dwarfs : fusors with no stable luminosity stars : fusors with a stable luminosity phase Planemos : round non-fusors (planetary mass objects) this can include various moons (planemos around planets), and also superplanets and free-floating objects Planets: planemos in orbit around a fusor minor planets : planets that are not dynamically dominant Implication : Pluto is a (minor) planet, so are Ceres, Vesta, Pallas, Varuna, Quaoar, Ixion, and likely other undiscovered KBOs. Our Solar System has: 8 major planets - perhaps 20 planets total.

FUSORS : Brown Dwarfs and Stars Solar-Type Stars And High-Mass Stars Red Dwarf Stars Brown Dwarfs

PLANEMOS : Planetary Mass Objects Gas Giant Planets / Superplanets Super-Earths and Ice Giant Planets Terrestrial Planets Mini-planets and Moons Objects of unknown origin