(Inside Science) – Centaurs are among the strangest objects in the solar system. The first, nicknamed Chiron, was spotted orbiting more than a billion kilometers from the Sun in 1977 and was originally considered an asteroid. But a few years later, he was seen emitting a halo of gas – a “coma” – and a tail like a comet.
This double nature gave their name to these strange objects: Apparently half-asteroid and half-comet, they were nicknamed “centaurs” after the half-human, half-horse creatures of Greek myths.
Over a thousand centaurs have now been found and scientists have discovered a little more about them, although at such distances they remain a mystery. But the centaurs may hold a key to understanding how the Sun and its planets originally formed from clouds of interstellar gas.
Although the full journey has yet to be seen, scientists believe the centaurs start off as Kuiper Belt Objects, or KBOs – the “dirty snowballs”, now thought to be as much dirt as they are snow, that orbit the Sun well beyond Neptune – between 2.7 billion and 4.6 billion kilometers from the Sun.
Scientists estimate that there are millions of KBOs; many are less than 10 miles in diameter, but some are much larger, and the designation includes Pluto and other distant dwarf planets that are believed to have formed when multitudes of smaller KBOs clustered together.
KBOs are of interest to scientists in part because they are thought to be the frozen remnants of the protoplanetary disk of dust and gas that formed our solar system.
“I like to call them the dinosaur bones of the solar system,” said planetary astronomer Carey Lisse of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “These are roughly equal amounts of ice and rock, which we believe is the original recipe for building the planets.”
Most KBOs are so small and so far apart that they are difficult to study. But any KBO that comes close to the Sun – like centaurs – may give scientists a better chance to observe them and better understand how the entire solar system was formed, Lisse said.
Most centaurs are dislodged from the Kuiper Belt by Neptune’s gravity and can take millions of years to spiral into the “centaur zone” between Neptune and Jupiter. Once there, they can ricochet between giant planets.
“You can kind of imagine it as a giant pinball machine,” said Eva Lilly, a researcher at the Planetary Science Institute who lives in South Carolina. “They pass between one of the great planets, from Neptune to Uranus, then from Uranus to Saturn, and so on – and each of these interactions can change their orbits more and more.”
Lilly is the lead author of a study recently published in the Journal of Planetary Sciences who examined 13 of the closest centaurs to learn more about their often sporadic cometary activity. “They are really unpredictable,” she said. “We don’t really know when they’ll turn on and off.”
Just as Sherlock Holmes once solved a mystery about a dog not barking, Lilly found a clue to the make-up of centaurs through their lack of activity.
Although the centaurs have been observed near their closest approach to the Sun, they have shown no comet-like behavior – suggesting that their occasional activity is triggered by something other than the Sun directly warming them. ice until it turns to vapor, a process called sublimation.
And Lilly thinks she knows what it could be. Some centaur models suggest they contain amorphous water ice – a type of ice without a crystalline structure that can only form in very cold environments, and never here on Earth. Amorphous ice can trap gases like carbon monoxide between its molecules – and Lilly suggests that centaur activity is caused at relatively low temperatures by trapped gases released by water ice’s transition from its amorphous form. in its crystalline form, rather than by direct sublimation of ice cream.
Comets are only active when they are relatively close to the Sun, but the amorphous to crystalline transition of water ice can occur as far as Saturn’s orbit, about 890 million kilometers from the Sun.
The release of transition gases could cause sinkholes or landslides on the centaur’s surface, in turn exposing chemically light ices that sublimate under even a weak sun during an explosion or sudden explosion: “There have been centaurs in the past where we’ve seen this process,” Lilly said. “This is what we think is going on.”
Lisse, whose research included a study of centaurs, comets and objects of the Kuiper belt with the Spitzer Space Telescope, centaurs are only a few million years old before falling on a giant planet – usually Jupiter – or sneaking into the inner solar system to become comets.
The lighter ices forming most of the comets are then sublimated near the Sun – “scorched”, as he put it – while the centaurs between Jupiter and Neptune still have roughly the same chemical makeup as in the Kuiper. This means that studying centaurs with spectroscopy or even space missions can provide insight into the original makeup of the solar system: “I’m trying to figure out the recipe for things that built the planets,” Lisse said. “I want to know what the original source was.”
This story was originally published with Inside science. Read the original here.