of the Solar System: The Asteroid Belt
a member of the asteroid belt. (NASA)
planet that never was: Sometimes finding something missing can
be as intriguing as discovering something that's there.
astronomer Johann Daniel Titius noticed a pattern in the positions
of the known planets of our solar system. He calculated that if
you took the number sequence 0, 3, 6, 12, 24 (doubling the earlier
number to get the next in the sequence) then added four to each
number and divided by 10, you got the distances of each of the
planets in our solar system from the sun. This rule became known
as the Titius-Bode law (also named after Johann Elert Bode who
added some refinements to it).
was one thing about the law that bothered the astronomers of that
period, however. This rule worked well for the planets Mercury,
Venus, Earth, Mars, Jupiter and Saturn, but there was a gap. According
to the formula there should have been a planet between the orbits
of Mars and Jupiter, but it was missing.
Celestial Police Search for the Missing Planet
Between the orbits of Mars and Jupiter.
There are estimated to be over a million asteroids
in the belt.
Scientists think they are the remains of a planet that
Three types: C-type asteroids (mainly carbon), S-type
asteroids (mainly made of silicates) and M-type asteriods
that are metallic.
The biggest is Ceres which is spherical with a diameter
of 590 miles (950k).
The first asteroid discovered was Ceres by astronomer
Giuseppe Piazzi on January 1st, 1801.
Half the mass of all the asteroids in the belt are
contained in the four largest: Ceres, Vesta, Pallas and Hygiea.
William Herschel discovered another planet beyond Saturn which
was named Uranus. Uranus's orbit around the sun was exactly as
predicted by the Titius-Bode law. This made astronomers even more
anxious to figure out why there seemed to be a gap in between
Mars and Jupiter. Was there a planet they hadn't noticed yet?
In 1800 Baron Franz Xaver von Zach, a Hungarian astronomer, established
the United Astronomical Society, informally nicknamed the "Celestial
Police," to look for the missing, wayward, planet. Twenty-four
scientists (including Herschel and Charles Messier) joined the
club, and were each assigned a portion of the sky to search.
a non-member of the club, however, that found the first clue to
the missing planet a few months after the group started looking.
On January 1st, 1801,Giuseppe Piazzi, the head of astronomy at
the University of Palermo, Sicily, noticed a star-like object
that appeared to be at the right distance for the missing planet.
He named it Ceres, after the Roman goddess of the harvest.
Piazzi discovered Ceres, the first asteroid.
was a problem with Ceres, however. No matter how powerful a telescope
was pointed at it, it never appeared as anything but a star-like
dot. All the other planets could be seen as discs. In March of
1802, Heinrich Wilhelm Olbers found another star-like object (later
named Pallas) at the same distance from the sun. Neither of these
objects appeared to be as big as the other planets, nore did they
have tails like comets. Herschel suggested they should be put
into a completely new category he called asteroids, after the
Greek word asteroeides, which meant "star-like".
Herschel's idea was ignored, and Ceres and Pallas were considered
to be very small planets. As the years went by, however, and astronomers
continued to find more and more small bodies in this region, but
no big planets, Herschel's term slowly came into use.
of the Belt
know for sure that there is no large planet in this orbit of the
sun, but many small objects which we call asteroids. Ceres is
the largest. It is sphere-shaped with a diameter of 590 miles
(950km). Second largest is Vesta, which is 326 miles across (525km).
Pallas and Hygiea follow in size at 388 miles (544km) and 300
miles (500km) respectively.
it is estimated that the asteroids in the belt number over a million,
these four largest make up half of the total mass of the belt.
the asteroids, only Ceres is large enough to have been pulled
into a near sphere by the force of its gravity. For this reason
it is considered a dwarf planet along with Pluto, Haumea, Makemake,
Eris and Sedna. However, of these, only Ceres is a member of the
asteroid belt. The rest of the dwarf planets are located in the
icy reaches near or beyond the orbit of the solar system's furthest
the only member of the belt that is also a dwarf planet.
scientist's estimating that there are over a million asteroids,
many more than a kilometer across, the volume of space in the
belt is so large that it is composed mostly of empty void. A space
ship traveling through the belt need not fear a collision with
an asteroid unless it was carefully aiming for one.
appear to come in three types. Most of the asteroids are C-type
(Carbonaceous) which means they appear to mainly be made of the
element carbon. These comprise about 75 percent of the belt. S-type
asteroids, which are silicate-rich, account for about 17% of the
total while the remaining 10% are M-type (metal-rich) asteroids.
are there many asteroids in this orbit instead of a planet as
the Titius-Bode law suggests there should be? One early theory
was that at one time there was a planet there, but it was destroyed
(perhaps by a collision with another body) and the asteroids are
all that are left of it. If you add up the mass of all the asteroids,
however, they total only 4% of the mass of our moon, way too small
to be a planet. Also, since asteroids seem to come in three different
types, it is hard to explain how they all could have come from
a single shattered body.
have come up with another theory. Instead of there once being
a single planet in this orbit that was destroyed, astronomers
think that perhaps the asteroids are parts of a planet that never
diagram showing the distribution of known asteroids.
it never form? The next planet out from the belt is Jupiter. It
is the solar system's largest planet and has the second strongest
gravity field in our solar system (The sun has the most powerful).
Scientists think that in the early days of the solar system Jupiter's
strong gravity pulled and pushed the asteroids out of the belt
before their own gravity could pull them together to form a planet.
are right, than we can think of the asteroid belt as the planet
that never was.
Copyright Lee Krystek
2015. All Rights Reserved.