Wonders of the Solar System: The Asteroid Belt

Gaspra, a member of the asteroid belt. (NASA)

The planet that never was: Sometimes finding something missing can be as intriguing as discovering something that's there.

In 1766 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).

There 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.

The Celestial Police Search for the Missing Planet

Seven Quick Facts
-Location: Between the orbits of Mars and Jupiter.
-Number: There are estimated to be over a million asteroids in the belt.
-Origin: Scientists think they are the remains of a planet that never formed.
-Composition: Three types: C-type asteroids (mainly carbon), S-type asteroids (mainly made of silicates) and M-type asteriods that are metallic.
-Largest: The biggest is Ceres which is spherical with a diameter of 590 miles (950k).
-Discovered: The first asteroid discovered was Ceres by astronomer Giuseppe Piazzi on January 1st, 1801.
-Other: Half the mass of all the asteroids in the belt are contained in the four largest: Ceres, Vesta, Pallas and Hygiea.

In 1781 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.

It was 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.

Giuseppe Piazzi discovered Ceres, the first asteroid.

There 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".

At first 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.

Characteristics of the Belt

Now we 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.

Though 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.

Of all 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 planet, Neptune.

Ceres, the only member of the belt that is also a dwarf planet. (NASA)

Despite 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.

Asteroids 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.

Why No Planet?

So why 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.

So scientists 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 formed.

A diagram showing the distribution of known asteroids.

Why did 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.

If they are right, than we can think of the asteroid belt as the planet that never was.

Copyright Lee Krystek 2015. All Rights Reserved.


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