The small and rocky planet Mercury is the closest planet to the Sun; it
speeds around the Sun in a wildly elliptical (non-circular) orbit that
takes it as close as 47 million km and as far as 70 million km from the
Sun. Mercury completes a trip around the Sun every 88 days, speeding
through space at nearly 50 km per second, faster than any other planet.
Because it is so close to the Sun, temperatures on its surface can reach
a scorching 467 degrees Celsius. But because the planet has hardly any
atmosphere to keep it warm, nighttime temperatures can drop to a frigid
-183 degrees Celsius.
Because Mercury is so close to the Sun, it is hard to see from Earth
except during twilight. Until 1965, scientists thought that the same
side of Mercury always faced the Sun. Then, astronomers discovered that
Mercury completes three rotations for every two orbits around the Sun.
If you wanted to stay up for a Mercury day, you'd have to stay up for
176 Earth days.
At first glance, if
Earth had a twin, it would be Venus. The two planets are similar in
size, mass, composition, and distance from the Sun. But there the
similarities end. Venus has no ocean. Venus is covered by thick, rapidly
spinning clouds that trap surface heat, creating a scorched
greenhouse-like world with temperatures hot enough to melt lead and
pressure so intense that standing on Venus would feel like the pressure
felt 900 meters deep in Earth's oceans. These clouds reflect sunlight in
addition to trapping heat. Because Venus reflects so much sunlight, it
is usually the brightest planet in the sky.
The atmosphere consists mainly of carbon dioxide (the same gas that
produces fizzy sodas), droplets of sulfuric acid, and virtually no water
vapor - not a great place for people or plants! In addition, the thick
atmosphere allows the Sun's heat in but does not allow it to escape,
resulting in surface temperatures over 450 įC, hotter than the surface
of the planet Mercury, which is closest to the Sun. The high density of
the atmosphere results in a surface pressure 90 times that of Earth,
which is why probes that have landed on Venus have only survived several
hours before being crushed by the incredible pressure. In the upper
layers, the clouds move faster than hurricane- force winds on Earth.
Venus sluggishly rotates on its axis once every 243 Earth days, while it
orbits the Sun every 225 days - its day is longer than its year! Besides
that, Venus rotates retrograde, or "backwards," spinning in the opposite
direction of its orbit around the Sun. From its surface, the Sun would
seem to rise in the west and set in the east.
Earth, our home planet, is the only planet in our solar system known to
harbor life - life that is incredibly diverse. All of the things we need
to survive are provided under a thin layer of atmosphere that separates
us from the uninhabitable void of space. Earth is made up of complex,
interactive systems that are often unpredictable. Air, water, land, and
life - including humans - combine forces to create a constantly changing
world that we are striving to understand.
Viewing Earth from the unique perspective of space provides the
opportunity to see Earth as a whole. Scientists around the world have
discovered many things about our planet by working together and sharing
Some facts are well known. For instance, Earth is the third planet from
the Sun and the fifth largest in the solar system. Earth's diameter is
just a few hundred kilometers larger than that of Venus. The four
seasons are a result of Earth's axis of rotation being tilted more than
The red planet Mars has inspired wild flights of imagination over the
centuries, as well as intense scientific interest. Whether fancied to be
the source of hostile invaders of Earth, the home of a dying
civilization, or a rough-and-tumble mining colony of the future, Mars
provides fertile ground for science fiction writers, based on seeds
planted by centuries of scientific observations.
We know that Mars is a small rocky body once thought to be very
Earth-like. Like the other "terrestrial" planets - Mercury, Venus, and
Earth - its surface has been changed by volcanism, impacts from other
bodies, movements of its crust, and atmospheric effects such as dust
storms. It has polar ice caps that grow and recede with the change of
seasons; areas of layered soils near the Martian poles suggest that the
planet's climate has changed more than once, perhaps caused by a regular
change in the planet's orbit. Martian tectonism - the formation and
change of a planet's crust - differs from Earth's. Where Earth tectonics
involve sliding plates that grind against each other or spread apart in
the seafloors, Martian tectonics seem to be vertical, with hot lava
pushing upwards through the crust to the surface. Periodically, great
dust storms engulf the entire planet. The effects of these storms are
dramatic, including giant dunes, wind streaks, and wind-carved
With its numerous moons and several rings, the Jupiter system is a
"mini-solar system." Jupiter is the most massive planet in our solar
system, and in composition it resembles a small star. In fact, if
Jupiter had been between fifty and one hundred times more massive, it
would have become a star rather than a planet.
On January 7, 1610, while skygazing from his garden in Padua, Italy,
astronomer Galileo Galilei was surprised to see four small "stars" near
Jupiter. He had discovered Jupiter's four largest moons, now called Io,
Europe, Ganymede, and Callisto. Collectively, these four moons are known
today as the Galilean satellites.
Galileo would be astonished at what we have learned about Jupiter and
its moons in the past 30 years. Io is the most volcanically active body
in our solar system. Ganymede is the largest planetary moon and has its
own magnetic field. A liquid ocean may lie beneath the frozen crust of
Europa. An icy ocean may also lie beneath the crust of Callisto. In 2003
alone, astronomers discovered 21 new moons orbiting the giant planet.
Jupiter now officially has 61 moons - by far the most in the solar
system. Many of the outer moons are probably asteroids captured by the
giant planet's gravity.
Saturn is the most distant of the five planets known to ancient
stargazers. In 1610, Italian Galileo Galilei was the first astronomer to
gaze at Saturn through a telescope. To his surprise, he saw a pair of
objects on either side of the planet, which he later drew as "cup
handles" attached to the planet on each side. In 1659, Dutch astronomer
Christiaan Huygens announced that this was a ring encircling the planet.
In 1675, Italian-born astronomer Jean Dominique Cassini discovered a gap
between what are now called the A and B rings.
Like Jupiter, Uranus, and Neptune, Saturn is a gas giant. It is made
mostly of hydrogen and helium. Its volume is 755 times greater than
Earth's. Winds in the upper atmosphere reach 500 meters per second in
the equatorial region. (In contrast, the strongest hurricane-force winds
on Earth top out at about 110 meters per second.) These super-fast
winds, combined with heat rising from within the planet's interior,
cause the yellow and gold bands visible in its atmosphere.
Once considered one of the blander-looking planets, Uranus (pronounced
YOOR un nus) has been revealed as a dynamic world with some of the
brightest clouds in the outer solar system and 11 rings. Uranus gets its
blue-green color from methane gas above the deeper cloud layers (methane
absorbs red light and reflects blue light).
Uranus was discovered in 1781 by astronomer William Herschel, who at
first believed it to be a comet. This seventh planet from the Sun is so
distant that it takes 84 years to complete an orbit.
Uranus is classified as a "gas giant" planet because it has no solid
surface. The atmosphere of Uranus is hydrogen and helium, with a small
amount of methane and traces of water and ammonia. The bulk (80 percent
or more) of the mass of Uranus is contained in an extended liquid core
consisting primarily of "icy" materials (water, methane, and ammonia),
with higher-density material at depth.
The eighth planet from the Sun, Neptune was the first planet located
through mathematical predictions rather than through regular
observations of the sky. When Uranus didnít travel exactly as
astronomers expected it to, two mathematicians, working independently of
each other, proposed the position and mass of another, as yet unknown
planet that could account for Uranusí orbit. Although "the
establishment" ignored the predictions, a young astronomer decided to
look for the predicted planet. Thus, Neptune was discovered in 1846.
Seventeen days later, its largest moon, Triton, was also discovered.
Nearly 4.5 billion kilometers from the Sun, Neptune orbits the Sun once
every 165 years, and therefore it has not quite made a full circle
around the Sun since it was discovered. It is invisible to the naked eye
because of its extreme distance from Earth. Interestingly, due to
Plutoís unusual elliptical orbit, Neptune is actually the farthest
planet from the Sun for a 20-year period out of every 248 Earth years.
Long considered to be the smallest, coldest, and most distant planet
from the Sun, Pluto may also be the largest of a group of objects that
orbit in a disk-like zone of beyond the orbit of Neptune called the
Kuiper Belt. This distant region consists of thousands of miniature icy
worlds with diameters of at least 1,000 km and is also believed to be
the source of some comets.
Discovered by American astronomer Clyde Tombaugh in 1930, Pluto takes
248 years to orbit the Sun. Plutoís most recent close approach to the
Sun was in 1989. Between 1979 and 1999, Pluto's highly elliptical orbit
brought it closer to the Sun than Neptune, providing rare opportunities
to study this small, cold, distant world and its companion moon, Charon.
Most of what we know about Pluto we have learned since the late 1970s
from Earth-based observations, the Infrared Astronomical Satellite
(IRAS), and the Hubble Space Telescope. Many of the key questions about
Pluto, Charon, and the outer fringes of our solar system await close-up
observations by a robotic space flight mission.
Our Sun has inspired mythology in almost all cultures, including ancient
Egyptians, Aztecs, Native Americans, and Chinese. We now know that the
Sun is a huge, bright sphere of mostly ionized gas, about 4.5 billion
years old, and is the closest star to Earth at a distance of about 150
million km. The next closest star - Proxima Centauri - is nearly 268,000
times farther away. There are millions of similar stars in the Milky Way
Galaxy (and billions of galaxies in the universe). Our Sun supports life
on Earth. It powers photosynthesis in green plants and is ultimately the
source of all food and fossil fuel. The connection and interaction
between the Sun and the Earth drive the seasons, currents in the ocean,
weather, and climate.
The Sun is some 333,400 times more massive than Earth and contains 99.86
percent of the mass of the entire solar system. It is held together by
gravitational attraction, producing immense pressure and temperature at
its core (more than a billion times that of the atmosphere on Earth,
with a density about 160 times that of water).
At the core, the temperature is 16 million degrees kelvin (K), which is
sufficient to sustain thermonuclear fusion reactions. The released
energy prevents the collapse of the Sun and keeps it in gaseous form.
The total energy radiated is 383 billion trillion kilowatts, which is
equivalent to the energy generated by 100 billion tons of TNT exploding
The regular daily and monthly rhythms of Earth's only natural satellite,
the Moon, have guided timekeepers for thousands of years. Its influence
on Earth's cycles, notably tides, has also been charted by many cultures
in many ages. More than 70 spacecraft have been sent to the Moon; 12
astronauts have walked upon its surface and brought back 382 kg (842
pounds) of lunar rock and soil to Earth.
The presence of the Moon stabilizes Earth's wobble. This has led to a
much more stable climate over billions of years, which may have affected
the course of the development and growth of life on Earth.
How did the Moon come to be? The leading theory is that a Mars-sized
body once hit Earth and the resulting debris (from both Earth and the
impacting body) accumulated to form the Moon. Scientists believe that
the Moon was formed approximately 4.5 billion years ago (the age of the
oldest collected lunar rocks). When the Moon formed, its outer layers
melted under very high temperatures, forming the lunar crust, probably
from a global "magma ocean."
Asteroids are rocky fragments left over from the formation of the solar
system about 4.6 billion years ago. Most of these fragments of ancient
space rubble - sometimes referred to by scientists as minor planets -
can be found orbiting the Sun in a belt between Mars and Jupiter. This
region in our solar system, called the Asteroid Belt or Main Belt,
probably contains millions of asteroids ranging widely in size from
Ceres, which at 940 km in diameter is about one-quarter the diameter of
our Moon, to bodies that are less than 1 km across. There are more than
20,000 numbered asteroids.
As asteroids revolve around the Sun in elliptical orbits, giant
Jupiterís gravity and occasional close encounters with Mars or with
another asteroid change the asteroidsí orbits, knocking them out of the
Main Belt and hurling them into space across the orbits of the planets.
For example, Marsí moons Phobos and Deimos may be captured asteroids.
Scientists believe that stray asteroids or fragments of asteroids have
slammed into Earth in the past, playing a major role both in altering
the geological history of our planet and in the evolution of life on it.
The extinction of the dinosaurs 65 million years ago has been linked to
a devastating impact near the Yucatan peninsula in Mexico.
"Shooting stars" or meteors are bits of material falling through Earth's
atmosphere; they are heated to incandescence by the friction of the air.
The bright trails as they are coming through the Earth's atmosphere are
termed meteors, and these chunks as they are hurtling through space are
called meteoroids. Large pieces that do not vaporize completely and
reach the surface of the Earth are termed meteorites.
Scientists estimate that 1,000 tons to more than 10,000 tons of
meteoritic material falls on the Earth each day. However, most of this
material is very tiny - in the form of micrometeoroids or dust-like
grains a few micrometers in size. (These particles are so tiny that the
air resistance is enough to slow them sufficiently that they do not burn
up, but rather fall gently to Earth.)
Where do they come from? They probably come from within our own solar
system, rather than interstellar space. Their composition provides clues
to their origins. They may share a common origin with the asteroids.
Some meteoritic material is similar to the Earth and Moon and some is
quite different. Some evidence indicates an origin from comets.
Throughout history, people have been both awed and alarmed by comets,
stars with "long hair" that appeared in the sky unannounced and
unpredictably. We now know that comets are dirty-ice leftovers from the
formation of our solar system around 4.6 billion years ago. They are
among the least-changed objects in our solar system and, as such, may
yield important clues about the formation of our solar system. We can
predict the orbits of many of them, but not all.
Around a dozen "new" comets are discovered each year. Short-period
comets are more predictable because they take less than 200 years to
orbit the Sun. Most come from a region of icy bodies beyond the orbit of
Neptune. These icy bodies are variously called Kuiper Belt Objects,
Edgeworth-Kuiper Belt Objects, or trans-Neptunian objects. Less
predictable are long-period comets, many of which arrive from a distant
region called the Oort cloud about 100,000 astronomical units (that is,
100,000 times the mean distance between Earth and the Sun) from the Sun.
These comets can take as long as 30 million years to complete one trip
around the Sun. (It takes Earth only 1 year to orbit the Sun.) As many
as a trillion comets may reside in the Oort cloud, orbiting the Sun near
the edge of the Sunís gravitational influence.