Transcript New Planet Definition Proposed by IAU

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) 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,
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 PlutoCharon a double planet.
http://en.wikipedia.org/wiki/Center_of_mass
Satellite, Asteroid, and Comet
Satellite
Asteroid
An object that orbits a planet (e.g., our Moon)
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
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.
•
The stars in a star cluster are
generally formed at
approximately the same time
from the same cloud of
interstellar gas.
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
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
Click it!
How big is the universe?
• Space
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.
B.
C.
D.
E.
Infinite speed
1 billion km per second
1 million km per second
It depends
We cannot measure it
How fast does light travel?
A.
B.
C.
D.
Infinite speed
1 billion km per second
1 million km per second
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
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
18 billion light-years to another planet in another galaxy
14 billion light-years
NO CONTACT TODAY because
the universe isn’t old enough.
The Observable Universe 20 Billion Years
After Big Band
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
20 billion light-years
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?