Solar System Planets
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.
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Discovery: Known to the ancients and visible to the naked
eye
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Named for: Messenger of the Roman gods
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Diameter: 3,031 miles (4,878 km)
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Orbit: 88 Earth days
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.
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Discovery: Known to the ancients and visible to the naked
eye
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Named for: Roman goddess of love and beauty
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Diameter: 7,521 miles (12,104 km)
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Orbit: 225 Earth days
Earth
The third planet from
the sun, Earth is a water world, 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).
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Diameter: 7,926 miles (12,760 km)
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Orbit: 365.24 days
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.
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Discovery: Known to the ancients and visible to the naked
eye
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Named for: Roman god of war
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Diameter: 4,217 miles (6,787 km)
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Orbit: 687 Earth days
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.
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Discovery: Known to the ancients and visible to the naked
eye
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Named for: Ruler of the Roman gods
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Diameter: 86,881 miles (139,822 km)
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Orbit: 11.9 Earth years
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.
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Discovery: Known to the ancients and visible to the naked
eye
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Named for: Roman god of agriculture
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Diameter: 74,900 miles (120,500 km)
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Orbit: 29.5 Earth years
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.
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Discovery: 1781 by William Herschel (was thought
previously to be a star)
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Named for: Personification of heaven in ancient myth
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Diameter: 31,763 miles (51,120 km)
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Orbit: 84 Earth years
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Day: 18 Earth hours
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.
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Discovery: 1846
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Named for: Roman god of water
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Diameter: 30,775 miles (49,530 km)
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Orbit: 165 Earth years
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Day: 19 Earth hours
Pluto (Dwarf Planet)
Once the ninth planet
from the sun, Pluto is unlike other planets in many respects. It is smaller
than Earth's moon. Its orbit carries it inside the orbit of Neptune and then
way out beyond that orbit. From 1979 until early 1999, Pluto had actually been the
eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune's path
and once again became the solar system's most distant planet — until it was
demoted to dwarf planet status. Pluto will stay beyond Neptune for 228 years.
Pluto’s orbit is tilted to the main plane of the solar system — where the other
planets orbit — by 17.1 degrees. It’s a cold, rocky world with only a very
ephemeral atmosphere. NASA's New Horizons mission performed history's first
flyby of the Pluto system on July 14, 2015.
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Discovery: 1930 by Clyde Tombaugh
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Named for: Roman god of the underworld, Hades
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Diameter: 1,430 miles (2,301 km)
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Orbit: 248 Earth years
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