Download Tour the Solar System - Background Essay

Tour the Solar System
Background Essay
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Our planet, Earth, is the only planet known to support life, but our solar system may
not be that unusual. Extrasolar planetary systems—planets orbiting stars other than
our Sun—are not uncommon. Research has found that, on average, there is at least
one planet for every star. The Sun is just one of hundreds of billions of stars contained
in the Milky Way galaxy and there are hundreds of billions of galaxies in the
observable universe. Our solar system is located in the disk of the Milky Way galaxy,
about two-thirds out from the center in one of the spiral arms.
In our solar system, all of the planets have nearly circular orbits around the Sun in
more or less the same plane (as defined by Earth's orbit, known as the ecliptic plane)
and in the same direction (counterclockwise, as viewed from above). The Sun
contains the vast majority—more than 99 percent—of the mass of the solar system.
The four planets of the inner solar system—Mercury, Venus, Earth, Mars—are
relatively small and rocky and are known as the terrestrial planets. The four planets of
the outer solar system—Jupiter, Saturn, Uranus, and Neptune—are primarily made of
gases and ices and are known as gas giants; they contain about 99 percent of the
mass orbiting the Sun.
Our solar system also contains other objects such as asteroids, comets, and dwarf
planets. Asteroids are small, rocky, irregularly shaped objects that are leftovers from
the formation of the planets. The majority of asteroids are located in the asteroid belt,
a region between the orbits of Mars and Jupiter that contains millions of objects.
There are large distances between asteroids and the two largest objects in the
asteroid belt, Vesta and Ceres, contain nearly half of the mass of the entire asteroid
belt.
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Comets are like dirty snowballs and are made of ice, dust, and rock; they are also
remnants from the formation of the planets. Many comets have highly elliptical orbits
that take them relatively close to the Sun for a part of their orbit, where the Sun's
energy causes them to release gas and dust. Some comets originate in the Kuiper
belt, a region in the outer solar system beyond the planets that contains more than a
trillion icy bodies (although, like the asteroid belt, it is mostly empty space). Comets
and Kuiper belt objects frequently have orbits that do no lie in the ecliptic plane. The
Kuiper belt is also home to larger objects such as Pluto, which has been classified a
dwarf planet. Some comets may originate from the Oort cloud, a hypothesized
spherical cloud of icy objects in the outermost regions of the solar system.
There is still much to learn about our solar system and scientists continue to explore
it. For example, NASA's Dawn mission was designed to study the two largest objects
in the asteroid belt to learn more about the conditions and processes of the early solar
system; Dawn has already collected data from Vesta and will reach Ceres in 2015.
Another NASA project, the New Horizons mission, will be the first to visit Pluto and
other Kuiper belt objects; it will reach Pluto in 2015.
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©2013 WGBH Educational Foundation. All Rights Reserved.
http://www.pbslearningmedia.org/resource/npls12.sci.ess.eiu.toursolsys/tour-the-solar-system/