New Planet Definition Proposed by IAU The IAU therefore resolves that planets and other Solar System bodies be defined in the following way: (1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape 1, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet. 2 (2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a "dwarf planet." 3Ceres (3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutons". (4) All non-planet objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". 4 1 This generally applies to objects with mass above 5 x kg and diameter greater than 800 km. An IAU process will be established to evaluate planet candidates near this boundary. 2 For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet. 3 If Pallas, Vesta, and/or Hygeia are found to be in hydrostatic equilibrium, they are also planets, and may be referred to as "dwarf planets". 4 This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets. In the new nomenclature the concept "minor planet" is not used.

Why is Charon Also a Planet? 2.For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto- Charon a double planet.barycentre

Satellite, Asteroid, and Comet SatelliteAn object that orbits a planet (e.g., our Moon) AsteroidA relatively small, rocky object that orbits a star. Sometimes called ‘minor planet’. CometA relatively small, icy object that orbits a star.

Star Cluster The stars in a star cluster are generally formed at approximately the same time from the same cloud of interstellar gas. Globular Cluster M80 A Star Cluster is a group of stars (from a few hundred to a few million, 10 2 to 10 7 ) that are closely associated in space.

Interstellar Medium Interstellar Medium (or Interstellar Cloud) is the gas and dust that fill the space in between stars. Interstellar Dust Grains The dust that fill the space in between stars. Orion Nebular

Galaxies Galaxies are islands of stars in space, containing from a few hundred millions to a trillion or more stars. Spirals Ellipticals Irregular

Supercluster, the Large-Scale Structure of the Universe Galaxies are not uniformly distributed in space. Some regions have higher concentration of galaxies (cluster and supercluster of galaxies). Some regions have very few galaxies (voids)…

Questions?

The universe is the sum total of all matter and energy. That is, it encompasses the superclusters and voids, and everything within them. Matter Energy Space The Universe How big is the universe? Click it!

How big is the universe? A.The universe has a finite size B.The universe is infinite in size C.No Can Tell! Maybe the universe wrap around itself…like the surface of earth? We can start from the north pole, traveling toward the south. When we reach the south pole, the earth doesn’t end there. If we keep going in the same direction, we actually start going toward the north, and eventually end up back to the north pole. The surface of a sphere is a two-dimensional space embedded in a three-dimensional space. Can we consider the universe as a four-dimensional space (three dimensional space and time) embedded in a five-dimensional hyperspace?

How big is the universe? A.The universe has a finite size B.The universe is infinite in size C.No Can Tell! It doesn’t look like we can give any sensible answer to this question…we simply don’t have the information to answer it.

Although we cannot talk about how big the universe is, we can actually talk about how big the observable universe is… To talk about the size of the observable universe, we first need to talk about speed of light

How fast does light travel? A.Infinite speed B.1 billion km per second C.1 million km per second D.It depends E.We cannot measure it

How fast does light travel? A.Infinite speed B.1 billion km per second C.1 million km per second D.It depends C = 300,000 km/sec in vacuum Light travels slower in glass and water E.We cannot measure it

Does light with different color travel with different speed? A.Yes B.No

Does light with different color travel with different speed? A.Yes B.No

If you can travel with the speed of light, you can… Fly around the world 7 ½ times in one second. Go to the Moon in 1.2 second (384,400 km) Go to the Sun in 8 minutes 20 seconds (150 millions km) Go to Pluto in ~ 5 ½ hours (5,800 millions km) Go to the next star (Alpha Centauri) in ~ 5 years… Go to the closest galaxy (Andromeda) in ~ 2.5 million years! Got to the edge of the universe in......

The ‘Observable’ Universe Because 1.Light travels at a finite speed, and… 2.The universe has a finite age ~ 14 billions years…  we can only see a limited portion of the universe—the observable universe. The furthest object we can see today is at a distance equal to the speed of light times the age of the universe ~ 14 billion light-years. But how do we know the age of the universe? In a minute…

What is Light-Year? The distance light travels in one year. The time it takes light to travels one complete circle at the edge of the universe.

What is Light-Year? The distance light travels in one year. The time it takes light to travels one complete circle at the edge of the universe. 1 light-year ~ 10 trillion km = 1 × km

The Size of the Observable Universe 14 billion light-years The invisible Universe today The observable Universe today Distance light signals have traveled if they were emitted when the universe was born. 14 billion light-years NO CONTACT TODAY because the universe isn’t old enough. 18 billion light-years to another planet in another galaxy

20 billion light-years The observable universe when the age of the universe is 20 billion years. Distance light signals has traveled if they were emitted when the universe was born. 20 billion light-years 18 billion light-years The Observable Universe 20 Billion Years After Big Band

The Lookback Time Another important consequence of the fact that the speed of light is constant and finite is that when we observe (receiving light signals emitted from) a distance place, we actually only see things that happened in the past at that place. When we look at a distant object, we look back in time. –For example, when we look at the Andromeda galaxy (2.5 million light-years away), we see things that happened 2.5 million years ago. The lookback time is 2.5 million years.

What is Outside the Observable Universe? NO CAN TELL! Questions?