New Planet Definition Proposed by IAUThe 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) shape1, 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."3

 (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 1020 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.

http://en.wikipedia.org/wiki/Center_of_mass

Satellite, Asteroid, and CometSatellite An object that orbits a planet (e.g., our Moon)

Asteroid A relatively small, rocky object that orbits a star. • Sometimes called ‘minor planet’.

Comet A 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, 102 to 107) 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 × 1013 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?