Steven Dutch, Professor Emeritus, Natural and Applied Sciences,University of Wisconsin - Green Bay
The discovery of Uranus showed that there might be other planets yet to be discovered. So astronomers began looking for them.
A glance at a diagram of the Solar System shows that the inner and outer planets are both regularly spaced. The Bode-Titius Law is a mathematical formula that approximates the spacing remarkably well. Start with the number sequence:
0, 3, 6, 12, 24, 48 ...
Then add 4 and divide the result by 10.
0.4, 0.7, 1.0. 1.6, 2.8, 5.2, 10.0, 19.6, 38.8
The numbers, except for 2.8, matched the spacing of the planets out to Uranus remarkably well, measured in terms of astronomical units (Earth-Sun distance). Astronomers began searching for something orbiting in the gap between Mars and Jupiter, and Giuseppe Piazzi discovered something on January 1, 1801. He named it Ceres after the Roman goddess of agriculture. It was the first of the asteroids. When Neptune was discovered in 1846, it, too matched the Bode-Titius Law remarkably well.
Many books dismiss the Law as a mere mathematical coincidence, but it may have a physical basis.
Orbits within the orbit of Jupiter.
xx
These are gaps in the asteroid belt where no asteroids can have stable orbits because they are in resonances with Jupiter. The principal gaps are at 4:1, 3:1, 5:2, 7:3 and 2:1 resonances with Jupiter. That is, there are no asteroids with stable orbits with periods 1/4, 1/3, 2/5, 3/7 or 1/2 those of Jupiter. The few asteroids with such orbits are unstable. They got there as a result of having their orbits perturbed, and further perturbations will cause them to drift out of their orbits.
These are asteroids orbiting in the L4 or L5 Lagrangian Points of Jupiter's orbit. The first one was discovered in 1906 and designated 588 Achilles. The custom arose of naming these asteroids after figures from the Trojan War, so the name "Trojan" is now generally applied to objects in the Lagrangian Points of any orbit. Asteroids in the L4, or leading group, are supposed to be named after Greeks, and those in the L5 or trailing point after Trojans, but there is some crossover since some asteroids were named before the convention was established. Thus mortal enemies Hektor and Achilles are actually close neighbors, but Achilles' dear friend Patroclus (slain by Hektor) is in the opposite group.
There are a few known Mars Trojans and some Neptune Trojans, but Neptune is expected to have a very large number, mostly still undiscovered because of their distance and faintness. There are no known Earth Trojans.
Asteroids that cross Earth's orbit but have an average distance from the Sun greater than Earth's
Asteroids that cross Earth's orbit and have an average distance from the Sun less than Earth's.
Asteroids with orbits entirely within Earth's orbit.
Asteroids with orbits that approach Earth but do not cross Earth's orbit.
Orbits between Jupiter and Neptune. The first one discovered was named Chiron, after a mythical centaur, thus giving the group its name (somewhat the way the Trojan asteroids were named).
Orbits beyond Neptune.
These are fringe members of the Kuiper Belt with extreme orbits, probably the result of past close encounters with other bodies. 90377 Sedna has an aphelion nearly 1000 times the Earth's distance from the Sun.
Return to Planetary Images Index
Access Course Notes on Planetary Geology
Access Astronomy Notes Index
Return to Professor Dutch's Home Page
Created 27 Dec 1996, Last Update 11 January 2020