It Might Be a Planet If . . .Steve Case

North Mississippi NSF GK-8

November 2006

What is a planet?

• Until recently, there was no exact definition.

• There were historically five planets.• Uranus, Neptune, and Pluto were discovered after the invention of the telescope.

Our Nine-Planet Solar System

Mercury• Closest planet to the Sun

• Very hot and rocky

Venus

• About the same size as Earth

• Covered with thick clouds

• Farther from the Sun than Mercury, but hotter because of clouds

Earth

• Our home planet• Contains water necessary for life

• Surrounded by a protective atmosphere

Mars

• Smaller than Earth

• May have once had surface water

• Planned destination for human exploration

Jupiter

• Largest planet in the Solar System

• Storms appearing as streaks and bands

• One large storm bigger than Earth: Great Red Spot

• First of the “Gas Giants”

Saturn

• Known for beautiful rings of rock and ice debris

• Least dense planet (would float in water)

• Farthest planet known to ancient astronomers

Uranus

• Orbits on its “side”

• Blue color comes from methane in atmosphere

Neptune

• Most distant of the “Gas Giants”

• Very similar to Uranus in size and composition

• Great Dark Spot similar to Jupiter’s Red Spot

Pluto

• Until recently, considered farthest planet from the Sun

• Smaller than any other planet

• Too distant to see surface features

What About A 10th planet?

• In 2005, Eris discovered

• Larger and farther from the Sun than Pluto

• Publicized at first as 10th planet

• More and more objects discovered beyond Pluto

A New Definition Needed

• If there were many objects about the same size as Pluto located near Pluto’s orbit, should they all be considered planets?

• If Pluto was a planet, why weren’t these new objects planets as well?

So what is a planet?

• In 2006 the International Astronomical Union came up with three rules for determining whether or not a body is a planet:– Must orbit the Sun– Must have spherical (ball-like) shape– Must clear orbit of other debris

1. Planets orbit their stars.

• All planets in the Solar System orbit the Sun.

• If a body in the Solar System orbits around something other than the Sun, it is not a planet.

2. Planets are shaped like a ball (spherical).

• Planets have enough gravity that they were pulled into a spherical shape when they formed.

• If the body does not have a generally spherical shape, it is not considered a planet.

3. Planets “sweep up” their area around the

Sun.

• Planets have enough gravity to clear their orbits of smaller debris.

• This debris is either captured by the planet and becomes that planet’s moon or else it is pushed out of the planet’s orbit.

What about Pluto?• Pluto is roughly spherically shaped, and it does orbit the

Sun.• Pluto has not cleared its orbit of other debris.• Scientists have found many other Pluto-like objects near

Pluto’s orbit.• Pluto was given a new classification as a dwarf planet.

Introducing the Dwarf Planets

• Dwarf planets only fulfill two of the three requirements to be a planet:– They orbit the Sun.– They are roughly spherically shaped.– They have NOT cleared their area of all other debris.

Our Eight-Planet Solar System

• Scientists now say that our Solar System has only eight planets. Pluto is no longer considered a planet.

• Our Solar System now has three dwarf planets: Pluto, Eris, and Ceres. Pluto and Eris are located beyond Neptune. Ceres is the largest body in the asteroid belt between Mars and Jupiter.

Other Members of the Solar Family

• Besides planets and dwarf planets, there are many other objects in the Solar System.

• Some of the major non-planet objects include moons, asteroids, and comets.

Moons• Bodies that orbit other things in the Solar System besides the Sun.

• Many of the planets have moons. Earth has one moon. Mars has two. Jupiter has over fifty.

• Even some smaller objects like dwarf planets and asteroids have moons.

Asteroids

• Large pieces of rocky debris that orbit the Sun.

• Many are found in the Asteroid Belt between Mars and Jupiter.

• Thought to be debris from a planet that never formed.

Comets

• Large pieces of rock and ice that orbit the Sun beyond Neptune.

• Occasionally are “bumped” out of orbit and fall in toward the Sun.

• As they approach the Sun, ice and dust is burned off to form a tail stretching away from the Sun.

Our Solar System:• Eight planets

• Three dwarf planets• Moons

• Asteroids• Comets