PlanetPlanet CategorizationCategorization && PlanetaryPlanetary Science:Science: ComingComing ofof AgeAge inin thethe 2121stst CenturyCentury Alan Stern Stern/March 09 Planet Classification Stern/March 09 Planet Classification Stern/March 09 Some Planets Are Small, Some Are Freakishly Large Just As Some Stars Are Small and Some Are Very Large Stern/March 09 Nicolaus Copernicus 1473-1543 AAS Meeting, 1930 Stern/March 09 Our Solar System Before 1930: A Tidy & Orderly Place There are just two types of planets: terrestrials and gas giants. All planets travel in essentially the same plane. All planets are in nearly circular orbits. Planets are believed to orbit where they were born. Before 1930 The Census: 4 Terrestrial Planets 4 Giant Planets Asteroids & Comets Our ideas were simple but naïve, because our data was so limited—limited by the technology of the times. But That Year Tombaugh Discovered Pluto A Misfit Which Puzzled …. Mid-Century Census: 4 Terrestrial Planets 4 Giant Planets 1 Misfit Pluto Asteroids & Comets Stern/March 09 Until ‘92, When the Kuiper Belt Was Revealed, Giving Context Stern/March 09 But Make No Mistake—These “Dwarfs” Are Not Small Bodies They Are Sizeable Worlds Stern/March 09 So, You Ask: What Sets Small Planets Apart From Large Ones? They are smaller and more numerous than larger planets. Often their orbits are more elliptical and/or more inclined. But that’s about it. And What Do Small Planets Have In Common with Larger Cousins? They are believed to have formed like Earth, Mars, & Venus. They are made of rock and ice—as are both Earth and Mars. Many have moons—like other planets. Many likely have cores—like all of the known larger planets. Some have atmospheres—just like larger planets. Their surfaces are solid—again, like the terrestrial planets. They are expected to have active surface geology & even tectonics— as do the terrestrial planets. Simply Put—Small Planets Have No Distinguishing Intrinsic Characteristics From Larger Planets Stern/March 09 And What Do Small Planets Have In Common with Larger Cousins? They are believed to have formed like Earth, Mars, & Venus. They are made of rock and ice—as are both Earth and Mars. Many have moons—like other planets. Many likely have cores—like all of the known larger planets. Some have atmospheres—just like larger planets. Their surfaces are solid—again, like the terrestrial planets. They are expected to have active surface geology & even tectonics— as do the terrestrial planets. Simply Put—Small Planets Have No Distinguishing Intrinsic Characteristics From Larger Planets Except Their Size. Since ‘92 Nothing Less Than A Revolution Has Transpired …In Planetary Science Nicolaus Copernicus 1473-1543 Our View of the Solar System’s Architecture Was Transformed And So Has Our View of the Solar System’s Population The 21st Century View: 4 Terrestrial Planets 4 Giant Planets Perhaps 1000 Dwarf Planets Comets & Asteroids Stern/March 09 But the Revolution in New Planet Types Continues, Afar Including Hot Jupiters Including Pulsars with Planets And Systems With Highly Eccentric Orbits And Even Super Earths & Balsa-Wood Density Giants OGLE-2005-BLG-390Lb is a “super-Earth” extra- solar planet, weighing 5.5 Earth masses. And TrES-4 is 84% the mass of Jupiter but with an average density of only about 0.24 gm/cm3. What Is The Message Here? The diversity of planetary types—both far and near—is exploding before our eyes. Host stellar, orbital, and compositional, atmospheric types, as well as planetary sizes range over wide parameter space. And our own Earth has been further displaced as an archetype or centerpiece. Somewhere, I Think, Copernicus is Smiling Nicolaus Copernicus 1473-1543 Result: Planet Classification Is a Challenge Now Upon Us How shall the milieu be organized? What properties determine planethood? What subtypes make most sense? And who should decide? In 2006 the IAU Voted A Controversial Planet Definition 1. A celestial body that: is in orbit around the Sun, 2. Has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and 3. Has "cleared the neighborhood" around its orbit. A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet", whilst a non- satellite body fulfilling only the first criterion is termed a "small solar system body." What’s The Problem With Dynamical Clearing? First, it has nothing to do with the attributes and nature of the body. Worse, it depends on the stellar mass and the -3/4 1/4 9/4 system’s age: Mplanet > ~G TsystemM* aplanet Which fundamentally biases against both young and distant planets. Consider: A reordering the planets in our system would change which objects are classified as planets! Should a “Planet” Be More Massive To Qualify The Farther Out It Orbits? Planets: Capable of Clearing + Clearing By Scattering Not Capable of Clearing Clearing By Accretion If Earth were at 40 AU, it would not be an IAU Stern/MarchH.F. Levison 09 (2006) planet! Imagine: This Does Not Always Qualify as a Planet EARTH Is It Really So Hard? ?????????????????????? Stern/March 09 I Have a Different Test: My “Star Trek” Test. Stern/March 09 DO WE JUDGE A HOUSE A HOME BASED ON ITS LOCATION? OR BASED ON ITS ATTRIBUTES? AND DO WE JUDGE A STAR A STAR BASED ON ITS ATTRIBUTES? SO ENTER GPD: AN ATTRIBUTE-BASED DEFINITION The “Geophysical Planet Definition” (GPD) says a planet is 1. A celestial body that: has sufficient mass so that it can assume a hydrostatic equilibrium (nearly round) shape due to its gravity overwhelming material strength. 2. But with insufficient mass to initiate sustained fusion in its interior at any time. AND WHERE IS THIS HYDROSTATIC DIVIDING LINE? Planets: Capable of HSE Not WHAT’S SO ATTRACTIVE ABOUT THE GPD DEFINITION? It’s simple, intuitive, and far less ambiguous. It embraces a diversity of planetary sizes and types which share a fundamental physical trait in common: shape controlled by gravity rather than material strength. It does not rely on having a complete census of a system to classify its objects. Objects do not reclassify based on orbital location. Instead, objects are classified purely on the basis of their nature, as are stars, stellar remnants, etc. AND WITH THE GPD DEFINITION, AN EARTH IS ALWAYS A PLANET A Census Gives 20 Solar Orbiting Planets, 2/3 of Which Are Dwarfs The Terrestrials: Mercury, Venus, Earth, and Mars. The Giants: Jupiter, Saturn, Uranus, and Neptune. The Rocky & Icy Dwarfs: Ceres, Pallas, Juno, Vesta, Pluto, Charon, Quaoar, Ixion, EL61, Eris, Makemake, and Sedna. Science Is About Discovering New Paradigms Stern/March 09 And Some Planet Orbiting Planets Too The Terrestrials: Mercury, Venus, Earth, and Mars. The Giants: Jupiter, Saturn, Uranus, and Neptune. The Rocky & Icy Dwarfs: Ceres, Pallas, Juno, Vesta, Pluto, Charon, Quaoar, Ixion, EL61, Eris, Makemake, and Sedna. And Satellite Planets, Like: Io, Europa, Ganymede, Callisto, Titan, Triton, & Luna. Science is About Discovering New Paradigms. Stern/March 09 Science Reaches Consensus One Person at a Time But Remember, Copernicus Is Watching Nicolaus Copernicus 1473-1543 Stern/March 09.