... Q&A: Our solar system (cont'd)
|3. How did
the solar system form?
The planets, asteroids, and comets
in the solar system are loose particles left over from the formation
of the Sun. Originally the gas and dust that would become the Sun
was the core of a cloud much larger than the solar system, probably
several light-years across One light-year is approximately 10 trillion
(10,000,000,000,000) km, or 6 trillion miles. The core was slowly rotating
at first, but as the cloud collapsed it spun faster, like a spinning
ice skater pulling in his arms. The rotation prevented the material
at the core's equator from collapsing as fast as the material at the
poles, so the core became a spinning disk.
Gas and dust in the disk spiraled gradually in to the center, where
it accumulated to form the Sun. But because dust is denser than gas,
some of the dust settled to the mid-plane of the disk. These dust
particles stuck together to make clumps, then clumps stuck together
to make rocks, then rocks collided to make planets. In the case of
the "gas giant"
planets (Jupiter, Saturn, Uranus, and Neptune), the rocky cores
were massive enough to also attract appreciable amounts of gas.
The outer layers of Jupiter and Saturn are made up of hydrogen
and other gases. Uranus and Neptune are also gas giant planets, but
they were built up mainly from ice chunks.
The Sun, then, is the collapsed core of an interstellar gas cloud.
The planets, asteroids, and comets are small lumps of dust or ice chunks
that stayed in orbit instead of spiraling into the Sun. The planets
all formed within a very short period probably a few million years
about 5 billion years ago.
|4. How old
is the solar system?
The solar system is about 4.5 billion years old.
|5. How big
is the solar system?
There are no physical boundaries in
space. The traditional view of the solar system is that it consists
of eight planets orbiting around one star: the Sun. Neptune, the farthest
planet from the Sun, orbits at approximately 30 astronomical units
(AU) from the Sun. An astronomical unit is a unit of length used by
astronomers. One astronomical unit equals the average distance from
Earth to the Sun — about
93 million miles (150 million km). The solar system also includes the
Kuiper Belt, a comet-rich area that begins near Neptune's orbit and
stretches far beyond it, to about 50 AU from the Sun. Part of Pluto's
elliptical orbit extends far into the Kuiper Belt. Beyond Pluto's orbit
is another region of icy objects in our solar system, called the Oort
Cloud, which extends approximately 50,000 AU from the Sun.
|6. Are there
differences among the planets in our solar system?
Planets come in different sizes and
colors. The four planets closer to the Sun are called rocky, or terrestrial,
planets. They are small in size and similar to Earth in composition.
They have no rings, and only two of them (Earth and Mars) have moons.
The four outer planets, called gas giants, are much larger than
the rocky planets. They all have rings and many moons. The gas giants
are made up mostly of hydrogen, helium, frozen water, ammonia, methane,
and carbon monoxide.
There is a discussion over whether Pluto is a planet. Some astronomers
think that Pluto might be little more than a giant comet (see question
Its composition is similar to that of comets, and its orbit is quite
different from that of the other planets. Astronomers agree that Pluto
is part of the Kuiper Belt of comets because its composition and orbit
fit neatly within that group. However, some argue that it deserves
planetary status as well.
is the asteroid belt?
The asteroid belt is a zone between
the orbits of Mars and Jupiter. Millions of asteroids inhabit
the asteroid belt, with many more scattered throughout the solar system.
It is believed that the asteroids in the asteroid belt never formed
a planet because the gravity of nearby Jupiter kept pulling them apart.
do comets come from?
Comets are giant snowballs of ice
and rock that formed in the outer solar system, the regions we call
the Kuiper Belt and the Oort Cloud. When the gravity of a large planet
disturbs such an iceball, its orbit can change to pass through the
inner solar system. If it passes close enough to the Sun, the ices
melt and produce the coma and tail of a comet.
Short-period comets comets
that return to the solar system about once every 100 years probably
originate from the Kuiper Belt. This belt is located
within the solar system's ecliptic plane, beyond the orbit of Neptune.
Since 1992, thousands of objects have been discovered in the Kuiper
Belt. These objects are small compared with planets. Their
sizes range from 10 to 2500 kilometers in diameter. Earth's diameter,
by comparison, is 14,000 kilometers. Astronomers estimate that this
belt contains at least 200 million comets.
Long-period comets — comets that we see rarely (once every few
thousand years) — are
thought to originate from a vast, spherical cloud of frozen bodies
called the Oort Cloud, named for the Dutch astronomer Jan Hendrik Oort.
This cloud of comets, which also orbits the Sun, resides in the farthest
region of the solar system, beyond Neptune and Pluto. Occasionally,
a gravitational disturbance caused by a passing star or an interstellar
cloud causes one of the frozen bodies in the Oort Cloud to begin a
journey toward the inner solar system, where it makes a passing rendezvous
with our Sun.
|9. Are there
any planets that can be seen without a telescope?
Yes. Some planets can indeed be seen
with the unaided eye, which is how they were discovered by the ancient
civilizations. They are Mercury, Venus, Mars, Jupiter, and Saturn.
The other two planets Uranus and Neptune were discovered
using a telescope, as was Pluto.
are the rocky, or terrestrial, planets?
Mercury, Venus, Earth and Mars are
called rocky, or terrestrial, planets. They are similar to Earth in
composition. Heat from the Sun evaporated lightweight elements like
hydrogen and helium into interplanetary space. Mostly rock and metal
were left in this zone. Eventually, it clumped together to form the
inner terrestrial planets. Mercury, Venus, Earth, and Mars are called
the rocky, or terrestrial, planets. They are the planets closest to
the Sun. Their composition is similar to Earth's composition, containing
mostly the heavy materials of rock and metal. Because they formed
closer to the heat of Sun, the lighter gases and ices could not condense
during their formation, as they did in the outer parts of the solar
are the gas giant planets?
Jupiter, Saturn, Uranus, and Neptune
are called the gas giants. Jupiter and Saturn
contain the largest percentages of hydrogen and helium, while Uranus
and Neptune contain the largest shares of ices frozen water,
ammonia, methane, and carbon monoxide.
planets have rings?
The four gas giants Jupiter, Saturn, Uranus and Neptune
all have rings.
the Hubble Space Telescope take pictures of all the planets in our solar
No. Mercury and the Earth are the
only two planets that the Hubble Space Telescope has not observed
for astronomical purposes. Mercury is too close to the Sun, which
is too bright for Hubble to look at. The Earth's surface, only 380
miles from Hubble, is too close for Hubble to observe.
the Hubble Space Telescope take pictures of the Sun?
No. The Sun is too bright for the Hubble Space Telescope
to observe. Its bright light can damage the telescope's sensitive detectors.
|15. Why is there
a discussion over whether Pluto is or is not a planet?
Pluto was called a planet from its discovery in 1930
until it was re-classified as a "dwarf planet" in 2006. The
change in status stems from the fact that, since 1993, astronomers
have discovered thousands of objects similar to Pluto in size and
composition, in the region of Pluto’s orbit. This region is called
the Kuiper Belt and the objects are called Kuiper Belt objects (KBOs). Pluto
fits with the objects in the Kuiper Belt, and does not fit with either
the rocky planets (Mercury, Venus, Earth, and Mars) or the gas giant
planets (Jupiter, Saturn, Uranus, and Neptune). Factors that distinguish
Pluto from the eight planets include its composition, atmosphere, small
size, a comparatively large moon, and the shape of its orbit around
Composition: Pluto is composed
of ice, rock, and frozen gases, similar to the composition of the
comet-like objects in the Kuiper Belt (the region beyond Neptune's
Atmosphere: Pluto's atmosphere grows as
it moves closer to the Sun and recedes as it moves away. This is
similar to the comet-like objects found in its vicinity. The frozen
gases sublimate (turn from a solid to a gas) as Pluto moves closer
to the Sun and then condense on the surface of Pluto as it moves
away from the Sun. In contrast, the atmospheres of the planets do
not appear or disappear during their orbits.
Small size: Pluto is relatively tiny,
having a mass about 1/500th that of Earth (in contrast, Mercury's
mass is 1/20th that of Earth). In diameter, Pluto is 1/5th the size
of Earth. Although puny on a planetary scale, Pluto is one of the
largest objects inhabiting the Kuiper belt.
Large moon: Pluto's largest moon, Charon,
is a little more than one-half the radius of Pluto and one-eighth
its mass. Pluto and Charon are in a "synchronous" orbit: they always
show the same face to each other as they orbit. Charon's orbit is
only 20,000 km away from Pluto. For comparison, Earth's Moon has
about 1/80th the mass of Earth and orbits 400,000 km away. Many astronomers
consider Pluto and Charon to be a binary Kuiper Belt object since
they orbit a common center. Pluto’s two smaller
moons, Nix and Hydra, are considerably smaller and more distant from
Pluto than is Charon.
Orbit around the Sun: Pluto's orbit is
more elliptical than that of any of the planets (it actually crosses
Neptune's orbit). It is tilted at an inclination of 17.15 degrees
relative to the plane of Earth's orbit. The planets all have much
smaller inclinations, while the Kuiper Belt objects can have large
inclinations like Pluto.
|16. What is a dwarf planet?
A dwarf planet is a celestial body within the solar system that shares the characteristics of planets. It orbits the Sun, is not a moon, and has a spherical or nearly spherical shape. Unlike a planet, however, a dwarf planet is not massive enough to clear away any loose cosmic rubble from its orbit. Dwarf planets include Ceres, Pluto, and Eris.