Terrestrial planets
The inner four worlds are called “ terrestrial planets ,” because, like
Earth, their surfaces are all rocky. Pluto, too, has a solid surface
(and a very frozen one) but has never been grouped with the four
terrestrials.
Jovian planets
The four large outer worlds — Jupiter,
Saturn, Uranus, and Neptune — are
known as the “Jovian
planets” (meaning “Jupiter-like”)
because they are all huge compared
to the terrestrial planets, and because
they are gaseous in nature rather than
having rocky surfaces (though some
or all of them may have solid cores,
astronomers say). According to
NASA , "two of the outer planets
beyond the orbit of Mars — Jupiter
and Saturn — are known as gas
giants; the more distant Uranus and Neptune are called ice giants."
This is because, while the first two are dominated by gas, while the
last two have more ice. All four contain mostly hydrogen and
helium.
Dwarf planets
The new IAU definition of a full-fledged planet goes like this: A body
that circles the sun without being some other object's satellite, is
large enough to be rounded by its own gravity (but not so big that it
begins to undergo nuclear fusion, like a star) and has "cleared its
neighborhood" of most other orbiting bodies. Yeah, that’s a
mouthful.
The problem for Pluto, besides its small size and offbeat orbit, is
that it shares its space with lots of other objects in the Kuiper Belt ,
beyond Neptune. Still, the demotion of Pluto remains controversial .
The IAU planet definition puts other small, round worlds in the dwarf
planet category, including the Kuiper Belt objects Eris, Haumea, and
Makemake.
Also now a dwarf planet is Ceres , a round object in the Asteroid Belt
between Mars and Jupiter. Ceres was actually considered a planet
when discovered in 1801 and then later deemed to be an asteroid.
Some astronomers like to consider Ceres as a 10th planet (not to
be confused with Nibiru or Planet X ), but that line of thinking opens
up the possibility of there being 13 planets, with more bound to be
discovered.
The planets
Below is a brief overview of the eight primary planets in our solar
system, in order from the inner solar system outward:
Mercury
The closest planet to the sun,
Mercury is only a bit larger than
Earth's moon. Its day side is scorched
by the sun and can reach 840 degrees
Fahrenheit (450 Celsius), but on the
night side, temperatures drop to
hundreds of degrees below freezing.
Mercury has virtually no atmosphere
to absorb meteor impacts, so its
surface is pockmarked with craters,
just like the moon. Over its four-year
mission, NASA's MESSENGER
spacecraft has revealed views of the
planet that have challenged
astronomers' expectations.
Discovery: Known to the ancients
and visible to the naked eye
Named for: Messenger of the Roman gods
Diameter: 3,031 miles (4,878 km)
Orbit: 88 Earth days
Day: 58.6 Earth days
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Venus
The second planet from the sun,
Venus is terribly hot, even hotter than
Mercury. The atmosphere is toxic.
The pressure at the surface would
crush and kill you. Scientists describe
Venus’ situation as a runaway
greenhouse effect. Its size and
structure are similar to Earth, Venus'
thick, toxic atmosphere traps heat in a
runaway "greenhouse effect." Oddly,
Venus spins slowly in the opposite
direction of most planets.
The Greeks believed Venus was two
different objects — one in the morning sky and another in the
evening. Because it is often brighter than any other object in the sky
— except for the sun and moon — Venus has generated many UFO
reports.
Discovery: Known to the ancients and visible to the naked eye
Named for: Roman goddess of love and beauty
Diameter: 7,521 miles (12,104 km)
Orbit: 225 Earth days
Day: 241 Earth days
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Earth
The third planet from the sun, Earth is
a waterworld, with two-thirds of the
planet covered by ocean. It’s the only
world known to harbor life. Earth’s
atmosphere is rich in life-sustaining
nitrogen and oxygen. Earth's surface
rotates about its axis at 1,532 feet per
second (467 meters per second) —
slightly more than 1,000 mph (1,600
kph) — at the equator. The planet zips
around the sun at more than 18 miles
per second (29 km per second).
Diameter: 7,926 miles (12,760 km)
Orbit: 365.24 days
Day: 23 hours, 56 minutes
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Mars
The fourth planet from the sun, is a
cold, dusty place. The dust, an iron
oxide, gives the planet its reddish
cast. Mars shares similarities with
Earth: It is rocky, has mountains and
valleys, and storm systems ranging
from localized tornado-like dust devils
to planet-engulfing dust storms. It
snows on Mars. And Mars harbors
water ice. Scientists think it was once
wet and warm, though today it’s cold
and desert-like.
Mars' atmosphere is too thin for liquid
water to exist on the surface for any
length of time. Scientists think
ancient Mars would have had the
conditions to support life, and there is
hope that signs of past life — possibly
even present biology — may exist on the Red Planet.
Discovery: Known to the ancients and visible to the naked eye
Named for: Roman god of war
Diameter: 4,217 miles (6,787 km)
Orbit: 687 Earth days
Day: Just more than one Earth day (24 hours, 37 minutes)
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Jupiter
The fifth planet from the sun, Jupiter
is huge and is the most massive
planet in our solar system. It’s a
mostly gaseous world, mostly
hydrogen and helium. Its swirling
clouds are colorful due to different
types of trace gases. A big feature is
the Great Red Spot, a giant storm
which has raged for hundreds of
years. Jupiter has a strong magnetic
field, and with dozens of moons, it
looks a bit like a miniature solar
system.
Discovery: Known to the ancients
and visible to the naked eye
Named for: Ruler of the Roman gods
Diameter: 88,730 miles (428,400 km)
Orbit: 11.9 Earth years
Day: 9.8 Earth hours
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Saturn
The sixth planet from the sun is
known most for its rings . When
Galileo Galilei first studied Saturn in
the early 1600s, he thought it was an
object with three parts. Not knowing
he was seeing a planet with rings, the
stumped astronomer entered a small
drawing — a symbol with one large
circle and two smaller ones — in his
notebook, as a noun in a sentence
describing his discovery. More than
40 years later, Christiaan Huygens
proposed that they were rings. The
rings are made of ice and rock.
Scientists are not yet sure how they
formed. The gaseous planet is mostly
hydrogen and helium. It has
numerous moons .
Discovery: Known to the ancients
and visible to the naked eye
Named for: Roman god of agriculture
Diameter: 74,900 miles (120,500 km)
Orbit: 29.5 Earth years
Day: About 10.5 Earth hours
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Uranus
The seventh planet from the sun,
Uranus is an oddball. It’s the only
giant planet whose equator is nearly
at right angles to its orbit — it
basically orbits on its side.
Astronomers think the planet collided
with some other planet-size object
long ago, causing the tilt. The tilt
causes extreme seasons that last 20-
plus years, and the sun beats down
on one pole or the other for 84 Earth-
years. Uranus is about the same size
as Neptune. Methane in the
atmosphere gives Uranus its blue-
green tint. It has numerous moons
and faint rings.
Discovery: 1781 by William
Herschel (was thought previously
to be a star)
Named for: Personification of
heaven in ancient myth
Diameter: 31,763 miles (51,120 km)
Orbit: 84 Earth years
Day: 18 Earth hours
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Neptune
The eighth planet from the sun,
Neptune is known for strong winds —
sometimes faster than the speed of
sound. Neptune is far out and cold.
The planet is more than 30 times as
far from the sun as Earth. It has a
rocky core. Neptune was the first
planet to be predicted to exist by
using math, before it was detected.
Irregularities in the orbit of Uranus led
French astronomer Alexis Bouvard to
suggest some other might be exerting
a gravitational tug. German
astronomer Johann Galle used
calculations to help find Neptune in a
telescope. Neptune is about 17 times
as massive as Earth.
Discovery: 1846
Named for: Roman god of water
Diameter: 30,775 miles (49,530 km)
Orbit: 165 Earth years
Day: 19 Earth hours
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