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Pla nets in Our Solar System

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：

Planets in Our Solar System

The

Sun

is

the

hub

of

a

huge

rotating

system

consisting

of

nine

planets,

their

satellites,

and

numerous

small

bodies,

including

asteroids,

comets,

and

meteoroids.

An estimated 99.85 percent of the mass of our solar system is contained within the

Sun, while the planets collectively make up most of the remaining 0.15 percent. The

planets,

in

order

of

their

distance

from

the

Sun,

are

Mercury,

Venus,

Earth,

Mars,

Jupiter,

Saturn,

Uranus,

Neptune,

and

Pluto.

Under

the

control

of

the

Sun's

gravitational force, each planet maintains an elliptical orbit and all of them travel in

the same direction.

The planets in our solar system fall into two groups: the terrestrial (Earth-like)

planets

(Mercury,

Venus,

Earth,

and

Mars)

and

the

Jovian

(Jupiter-like)

planets

(Jupiter,

Saturn,

Uranus,

and

Neptune).

Pluto

is

not

included

in

either

category,

because its great distance from Earth and its small size make this planet's true nature a

mystery.

The

most

obvious

difference

between

the

terrestrial

and

the

Jovian

planets

is

their size. The largest terrestrial planet, Earth has a diameter only one quarter as great

as

the

diameter

of

the

smallest

Jovian

planet,

Neptune,

and

its

mass

is

only

one

seventeenth as great. Hence, the Jovian planets are often called giants. Also, because

of

their

relative

locations,

the

four

Jovian

planets

are

known

as

the

outer

planets,

while

the

terrestrial

planets

are

known

as

the

inner

planets.

There

appears

to

be

a

correlation between the positions of these planets and their sizes.

Other dimensions

along which the two

groups differ markedly are

density

and

composition. The densities of the terrestrial planets average about 5 times the density

of

water,

whereas

the

Jovian

planets

have

densities

that

average

only

1.5

times

the

density of water. One of the outer planets, Saturn, has a density of only

0.7 that of

water, which means that Saturn would float in water. Variations in the composition of

the

planets

are

largely

responsible

for

the

density

differences.

The

substances

that

make

up

both

groups

of

planets

are

divided

into

three

groups

—

gases,

rocks,

and

ices

—

based

on

their

melting

points.

The

terrestrial

planets

are

mostly

rocks:

dense

rocky and metallic material, with minor amounts of gases. The Jovian planets, on the

other hand, contain a large percentage of the gases hydrogen and helium, with varying

amounts of ices: mostly water, ammonia, and methane ices.

The Jovian planets have very thick atmospheres consisting of varying amounts of

hydrogen, helium, methane, and ammonia. By comparison, the terrestrial planets have

meager atmospheres at best. A planet's ability to retain an atmosphere depends on its

temperature and mass. Simply stated, a gas molecule can

it

reaches

a

speed

known

as

the

escape

velocity.

For

Earth,

this

velocity

is

11

kilometers per second. Any material, including a rocket, must reach this speed before

it

can

leave

Earth

and

go

into

space.

The

Jovian

planets,

because

of

their

greater

masses

and

thus

higher

surface

gravities,

have

higher

escape

velocities

(21-60

kilometers per second) than the terrestrial planets. Consequently, it is more difficult

for

gases

to

from

them.

Also,

because

the

molecular

motion

of

a

gas

depends

on

temperature,

at

the

low

temperatures

of

the

Jovian

planets

even

the

lightest gases are unlikely to acquire the speed needed to escape. On the other hand, a

comparatively warm body with a small surface gravity, like Earth's moon, is unable to

hold even the heaviest gas and thus lacks an atmosphere. The slightly larger terrestrial

planets Earth, Venus, and Mars retain some heavy gases like carbon dioxide, but even

their atmospheres make up only an infinitesimally small portion of their total mass.

The orderly nature of our solar system leads most astronomers to conclude that

the planets formed at essentially the same time and from the same material as the Sun.

It is hypothesized that the primordial cloud of dust and gas from which all the planets

are thought to have condensed had a composition somewhat similar to that of Jupiter.

However, unlike Jupiter, the terrestrial planets today are nearly void of light gases and

ices. The explanation may be that the terrestrial planets were once much larger and

richer

in

these

materials

but

eventually

lost

them

because

of

these

bodies'

relative

closeness to the Sun, which meant that their temperatures were relatively high.