In ancient times, humans watched the skies looking for clues to their future and to aid
in their very survival. They soon observed that some stars were not fixed, but moved
in the sky from night to night. They called these stars the Òwanderers.Ó
At the center of our solar system is the Sun, binding the planets with its gravitational
pull. From our viewpoint on Earth, the Sun appears small in the sky, but in reality it
dwarfs even Jupiter, the largest planet in the solar system.
The distance from the Sun to the small worlds traveling around it are vast. Light from the
Sun takes eight minutes to reach Earth, and nearly a day to reach the farthest known bodies.
Join us now as we tour our solar system, starting with sun-baked Mercury and traveling to the
remotest outskirts, where small, icy bodies move with only the faintest connection to
our Sun.
Mercury, the closest planet to the Sun is also the smallest terrestrial planet. It orbits
so swiftly that its year lasts only 88 Earth days. The airless cratered surface could almost
be mistaken for our moon, relentlessly bombarded by meteoroids for four and a half billion
years. One of these encounters left a giant scar known as the Caloris basin, one of the
largest impact sights in the solar system. Temperatures on the surface of Mercury can
reach a blistering 800 degrees Fahrenheit, but can dip to 300 degrees below zero on the
night side.
Seen from Earth, Venus is the brightest small object in sky, thanks in part to an unbroken
layer of sulfuric acid clouds, part of the planetÕs thick, smothering atmosphere of
carbon dioxide, a potent greenhouse gas which traps the SunÕs heat and turns the surface
into a desert with temperatures of 870 degrees. Venus rotates so slowly that a day lasts almost
as long as four months on Earth. Devoid of water, the surface is a nearly uninterrupted
expanse of rolling volcanic plains. As this altimetry image shows, a few highland plateaus
in red and broad depressions in blue dominate the landscape. In the 1970s and 80s, Soviet
landers survived on the rock-strewn surface of Venus just long enough to give us our first
glimpses of the planet close up.
The Earth is the one body in the solar system where water can exist stably as a gas, liquid,
and solid. And this has made all the difference. Vast oceans dominate the planet. Clouds cover
much of the surface, and the polar caps and sea ice grow and shrink with the seasons.
The EarthÕs solid surface is itself dynamic, shifting and recycling through the process
of plate tectonics. Together these complex and vibrant systems make Earth the only known
haven of life in our solar system. Evidence of the teeming bustle of life here, thriving
on our small watery oasis in the vast unfriendly desert of the cosmos, is even visible from
space.
Mars was once a geologically active planet with the largest volcano in the solar system,
Olympus Mons, and Valles Marineris, a valley that dwarfs the Grand Canyon. It has the most
Earth-like environment of any other planet, but with its tenuous atmosphere, frigid temperatures,
and radiation-drenched surface, it still cannot support Earth-like life. Thin as it is, the
Martian atmosphere is incredibly dynamic, with seasons and polar caps that grow and
shrink. Often large dust storms begin to swirl on the deeper basins. Sometimes these grow
into massive globe encircling maelstroms that can cloak the entire planet for weeks at a
time.
This map of Mars reveals one of the most puzzling features of the planet: the contrast between
the smooth lowlying northern hemisphere in blue and the craggy, heavily cratered southern
uplands in red. Why are the two halves so different? Did water ever pool in the lowlying
regions? And was there ever life here?
The planet is now a vast barren desert. But images of what look like dry river channels
and ancient lake beds provide compelling evidence that liquid water once ran over the Martian
land, a t least for a time. Mars has had more visitors in the form of landers and rovers
than any other planet, but many puzzles still remain.
Beyond Mars is a vast region of space known as the Main Asteroid Belt, a graveyard of
millions of rocky pieces of shattered young planets and bodies that never got a chance
to grow. Asteroids come in all shapes and sizes, but most of the material that was originally
in this region is long gone. So what happened here? And why didnÕt these asteroids ever
come together to form a planet? What was responsible for throwing most of the material out of the
Asteroid Belt and causing all of these massive collisions? Something big. Something really
big and itÕs our next stop.
Jupiter is the largest planet in the Solar System, a massive giant that twirls around
so fast it completes a complete rotation in a mere 10 hours. The movement of the clouds
reveal nearly constant high-speed wind, and the cloud tops are marked by fierce storms
larger than Earth, some of which can last for centuries. Jupiter is surrounded by faint
rings and more than 63 moons. The largest moons are worlds in their own right, and were
seen even in the earliest telescopes.
Io, forever pulled and tugged by Jupiter, is volcanically active, spewing sulfur compounds
onto its exotic surface. Icy Europa has a cracked surface that very likely hides an
ocean somewhere below. Ganymede, with its surface of dark ice and bright craters, is
the largest moon in the Solar System. And heavily cratered Callisto is marked by the
impact of many tiny objects, each one trapped and pulled by JupiterÕs overwhelming gravity.
Next, we move to beautiful Saturn. SaturnÕs atmosphere at first appears to be serene.
But this planet too hides powerful storms and winds. But SaturnÕs most obvious, most
majestic feature is the extensive system of rings, a graceful collection of ice, rocks,
and dust shepherded into place by small moons. Images of the rings reveal lovely scallops
and swirls caused by the moonlets embedded within.
Saturn is also surrounded by more than 60 diverse moons, including cloudy Titan, which
has an atmosphere of thick hydrocarbon smog covering an icy surface dotted with methane
lakes, sand dunes, and icy volcanoes.
Another example is odd Iapetus, with its stark contrast of extremely dark and very white
surface features. And donÕt forget tiny Enceladus, residing deep in SaturnÕs gravitational thrall.
Pushed and pulled by strong tides that help activate geysers, Enceladus alone provides
enough water-ice to form one of SaturnÕs tenuous outer rings.
This is Uranus, the first of the ice giants. Uranus receives 400 times less sunlight than
Earth, and its clouds lie deep in the atmosphere, masking the planetÕs turbulence and giving
it a calm blue color. But Uranus is special. The planet lies nearly on its side, and this
extreme tilt gives rise to seasons that last nearly 28 of EarthÕs years. As previously
dark areas of the planet receive sunlight, and begin to warm, powerful storms build in
the atmosphere. Like the other giant planets, Uranus has many moons and faint rings, few
of which have been seen close up. And itÕs only been visited by one spacecraft, Voyager
2 in 1986.
The final giant planet is Neptune, the other blue planet. It is also a stormy planet, though
it is even colder than Uranus and the seasons are not as extreme. Like all the giant planets,
Neptune has faint rings, and it radiates more heat than it receives from the Sun, a likely
cause for its extremely high-speed winds. Though only briefly glimpsed in 1989 by Voyager
2, Neptune harbors fascinating moons.
Triton, likely captured by Neptune when passing too near, has a surface marked with nitrogen
geysers that form a thin atmosphere. Who knows what other surprises await on the moons of
Neptune?
Beyond the main planets lies a region with many dark and icy objects, called the Kuiper
Belt. Some of these bodies cross NeptuneÕs orbit, and this region is home to comets,
rocky bodies, and dwarf planets, like Eres, Haumea, Makemake, and Pluto. Pluto, once classified
as a planet, may have a thin atmosphere that freezes to the surface during winter, and
has at least four moons, including Charon, Nix, and Hydra.
Far beyond the Kuiper belt is the Oort Cloud, a vast region that encloses the entire solar
system. This is the realm of many long period comets, bound by the SunÕs gravity, with
orbits of a couple of hundred to thousands of years.
Packed as it is with all these marvels, our solar system is not unique. Throughout the
Milky Way galaxy, many stars have been found with planetary systems of their own, filled
with exotic new planets, planets bigger than Jupiter and yet closer to their stars than
Mercury is to our Sun. Other planets quite similar to Earth. Imagine the night sky of
some of these planets.
Planetary wanderers abound in our galaxy. As we find ways to study even further stars,
what other solar systems await discovery? Perhaps emerging civilizations on those planets
look out into their own night sky as well and ponder the nature of their own wanderers.