The mystery of the missing planets

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      There is an unsolved problem I want to tell you about: The case of the missing Trojans. You might be thinking of the mythical horse with soldiers hidden inside. Or maybe you’re thinking of a sports team. Or a type of computer virus, or, let’s be honest, of the condoms. (Note that I said, “Case of the missing Trojans,” not, “the Missing case of Trojans.”) But there is another type of Trojan: an orbit. A swarm of thousands of asteroids share an orbit with Jupiter. They are not right next to the planet, like moons, but rather clustered in two clumps, one ahead of Jupiter on its orbit and one behind. The leading clump is called the “Greeks” and the trailing clump the “Trojans,” but they are usually just lumped together and simply called Jupiter’s Trojans. Many of them are named after mythological figures from the Trojan wars.

      The inner Solar System. The planets are labeled and the blue lines show their orbits. The small dots are asteroids. The main asteroid belt is shown in white. The green dots—called “Greeks” and “Trojans”—what we call Jupiter’s “Trojans.”Wikipedia user Mdf

      upiter is not the only planet in the Solar System to have Trojans, although it has by far the most. Uranus (1 Trojan) and Neptune (13) each have them. So does Mars (7). And Earth even has one! Surprisingly, Saturn does not have any; the reason appears to be that Jupiter’s gravity de-stabilizes Saturn’s Trojans on million year timescales. Saturn’s Trojan island has sunk under the ocean of instability (though Saturn’s moons actually have four Trojan), as has Venus’.

      Let’s take this Trojan idea a step farther. The objects trapped near the stable islands at L4 and L5 don’t need to be puny little asteroids. They can be full-grown planets! If two planets lie in the same orbit, separated by 60 degrees, they form a pair of Trojan planets. Trojan planets are commonly found in computer simulations that try to explain how planets orbiting other stars (“exoplanets”) are created. Many of the known exoplanets have orbits very close to their stars. Astronomers think that many of these planets formed farther away and were pushed closer in by a process called orbital migration. It is the disks of gas and dust from which the planets were born that are causing the planets to migrate. And it is during this migration that adjacent planets can be trapped in Trojan configurations. Trojan planets don’t form during every simulation but they do pop up every third or fourth time.Jupiter is more than 300 times more massive than Earth. Its gravity has almost completely emptied out the asteroid belt, which we think once contained 1,000 times more material than it does now. How can these Trojans survive so close to a gas giant and yet remain on stable orbits?

      Jesus: Hey, Dad? God: Yes, Son? Jesus: Western civilization followed me home. Can I keep it? God: Certainly not! And put it down this minute--you don't know where it's been! Tom Robbins in Another Roadside Attraction

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