OK, close your eyes and imagine a small hazy planet floating in black space. Now add the soundtrack theme from the 1976 film Jaws: Duuuuuh-Dunt, . . . Duuuuuh-Dunt, . . . Duuuuuh-Dunt, Duuuuuh-Dunt, Duuuuuh-Dunt, Duuuuuh-Dunt! Next add a huge blue planet rising up from screen bottom and engulfing the little world. Gulp!
Who's the planet stalker? None other than turquoise Neptune.
Hard to believe? In grade school Neptune and its sister world Uranus seemed pretty dull and innocuous. You always wanted to skip past them and go directly to Pluto on the solar system Monopoly board. They didn't have bright rings or a red spot but simply looked like two billiard balls. (Uranus does have a dark spot, but I’m not going there...)
But Neptune has been hiding 4-billion year old clues to interplanetary homicide. Uranus may be a co-conspirator too.
Forensic file 1: Uranus and Neptune are each roughly 15 times the mass of Earth. They live toward what was the outer rim of the hypothesized circumstellar disk from which the planets formed. So how did they get so big?
Dynamical simulations of the early solar system show that Uranus and Neptune could have formed closer to the sun, gobbled up a lot of gas, and then migrated farther out through momentum transfer with smaller bodies.
Along the way they might have encountered one or more super-Earths (a body several times the mass of Earth), which formed in the lower density outer edge of the disk where planet growth was sluggish. The two gas giant bullies swallowed the embryonic Earths or kicked them out of the solar system through gravitational pinball.
Forensic file 2: Neptune's giant frigid moon Triton orbits in the opposite direction of Neptune’s spin. Therefore Triton has long been suspected to be a captured body -- perhaps a long lost cousin of Pluto.
Triton may have once orbited a hypothetical super-Earth, according to Steven Desch of Arizona State University who presented his hypothesis earlier this month at the Lunar and Planetary Science Conference in Houston, Texas. In the three-body encounter among Neptune, the super-Earth and Triton, Neptune swallowed the super-Earth. This would have carried away enough kinetic energy to slow down Triton so that Neptune could capture it.
Forensic file 3: Neptune, mysteriously, is much hotter than Uranus even though they are about the same age, mass, size, and composition. It has a very turbulent atmosphere driven by internal heat, while Uranus' atmosphere look lethargic by comparison. Neptune may have been heated by swallowing a super-Earth.
Forensic file 4: Uranus' spin axis is tilted over 90 degrees from being perpendicular the planet's orbit. Uranus looks like it is laying on it side along with its ring system tilted too. A collision with an Earth-mass body would have unleashed enough energy to tip the planet.
Thankfully the planets stopped playing musical chairs over 4 billion years ago. But according to chaos theory, the solar system is always on the brink of going dynamically berserk. So it's not totally impossible that a Pac-Man planet could swing close to Earth in the far future. Duuuuuh-Dunt, . . . Duuuuuh-Dunt, Duuuuuh-Dunt, Duuuuuh-Dunt . . .
But not in 2012!
Image credits: NASA
Tags: Neptune, Solar System, Uranus
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