What is a Planet? What is a Star?

Originally: “planet” = “wanderer” (Greek root)refers to apparent motion of planets among starsEarth-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 #1Mass ~10-25 jupiter, Size ~10 -3 km, density ~water, not round

Mass ~10-12 jupiter, Size ~10 km, density ~ 1-5 x water, not round

Mass ~10-6 jupiter, Size ~103 km, density ~ 1-5 x water, round

Size, Mass, Density #2Mass ~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. Electronsare 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 ThingsIf 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

1515 Mimas16 Hyperion

16

Hyperion

Mimas

Vesta

Xena

RoundNot 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, puffyenvelopes

BrownDwarfs

Luminosity Sources #1Chemical 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 #2Radioactive decay,differentiation [gravitational], (core crystallization).Core temperature : 5000-15000K,Surface temp: ~ 20-100K (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 : 1000KNo 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 FusionIn 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 togetherResult : 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

PP N

N+

He3H2

H1

P

PNN

PPPP +++

Note change ofprotons to neutrons

He4

Luminosity HistoriesStars 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 70-80 jupiters

Luminosity source – purely gravitational deuterium fusion

transition occurs at 13 jupitersLuminosity source – deuterium fusion hydrogen fusion

transition occurs at 60 jupitersstable 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 ShapesThe 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 lostIn 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.

The Kuiper Beltand Oort Cloud

The protosolar nebula is not expected to have ended at Neptune’s distance (or even Pluto’s). Typical disks are 100-400 AU in size (as observed around other stars).

Pluto - the real problem : too much companyThe 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 1207 - TW Hya Association

My Answers (definitions)Fusors : objects which experience core fusion sometime

brown dwarfs : fusors with no stable luminositystars : 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 fusorminor 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 StarsAnd High-Mass Stars

Red Dwarf Stars

Brown Dwarfs

PLANEMOS : PLANEMOS : Planetary Mass Planetary Mass

ObjectsObjects

Gas Giant Planets / Superplanets

Super-Earths and Ice Giant Planets

Terrestrial Planets

Mini-planets and Moons

Objects of unknownorigin