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A dozen or so years ago, astronomers debated "What is a planet?" They might soon be arguing another question about the classification of the solar system: "What's a moon?"
Sheppard of the Carnegie Institution for Science announced the discovery of a dozen moons around Jupiter, bringing to 79 the total number of moons in orbit around the planet's largest solar system. , Callisto deep crater, volcanic Io, icy Europe – the new ones are light. They measure between half a mile and 2 miles wide and are orbiting millions of miles away from the planet – good explanations for why no one had seen them until now.
As telescopes improve, astronomers will find more and more moons. and smaller, around Jupiter and other giant planets orbiting the sun. When the number goes up in hundreds, maybe thousands, scientists might start to wonder if it's worth keeping track.
Can every rock that turns around a planet be called a moon? that are less than a kilometer in size maybe "dwarf moons," said Sheppard. In 1965, Pluto moved Pluto from a planet to a dwarf planet.
A moon is simply a rock orbiting a planet, and there is currently no minimum size for it to be called a moon. But in practice, astronomers only count objects whose orbits they can determine. The rings of Saturn, for example, would consist of particles ranging in size from a grain of sand to a house – too small to spot and track individual particles.
The main task of Sheppard's team is the research of Planet Nine. planet beyond Neptune that seems to shake up objects at the edge of the solar system. But they realized in March of last year that Jupiter would cross the part of the night sky that they wanted to search. So, that 's where they looked.
They did not find Planet Nine. (No one has any yet, and it might not exist.)
But in the images, taken by the telescope in Chile, they spotted 12 new points of light in the neighborhood of Jupiter. The verification of observations took a year, and then the Minor Planet Center of the International Astronomical Union – the solar system's data collection and dissemination center – published the results. their planet, in what astronomers call a prograde movement. This is not surprising, as these moons were probably formed from a disk of dust and gas that turned in the same direction as the planet that the solar system took shape.
But the immense gravity of Jupiter could also capture other objects and these space rocks sometimes end up orbiting in the other direction, in what is called a retrograde motion.
Two of the new moons have prograde orbits, corresponding to an inner group of moons that are considered fragments of a larger moon. breeze. Nine fall among three swarms of retrograde moons farther away, presumably the remains of three larger moons captured.
The 12th moon is a curiosity. It moves among retrograde moons still orbits in a prograde direction. "He's coming down the road in the wrong direction," Sheppard said.
He suggested that this could be the last piece of the object that broke the three outer moons of origin. If so, more moons are waiting to be discovered.
"We think all these collisions happened there," Sheppard said. "So if you go smaller and smaller, you will find smaller and smaller fragments of many of these collisions."
Astronomers have not found names for 11 of the 12 new moons. They will probably ask the public opinion, said Sheppard, but they added that they do not want to end up with Planety McPlanetface. (A few years ago, a British government agency asked for suggestions for naming a research vessel, and the Internet happily voted for Boaty McBoatface.)
For the weird moon, playing in traffic, Sheppard proposed Valetudo, the great granddaughter of the Roman god Jupiter, who is also the goddess of hygiene and health.
In the fight against "Pluto is a planet?", It's the discovery in 2005 of another icy-sized Pluto world, now named Eris, which has forced astronomers to understand what's wrong. 39 they meant by "planet." Eventually, the International Astronomical Union came up with an awkward definition that a planet gravitates around the sun, is big enough for gravity to bring it back to a round shape, and that it's not too big. she is essentially the gravitational bully in her orbit.
This last requirement excludes Pluto and Eris and puts them in a new category – dwarf plane For moons, there are no clear differences from which one could break for the smaller satellite of 39, a planet
. who cares! "said Michael E. Brown, astronomer of the California Institute of Technology who discovered Eris and sparked the debate on the planet." In general, however, I would say: moons against the rings is a fairly easy distinction to make. Harder, though, is the moons against the pebbles, the dust, all that is not in a ring. There is no reasonable place to draw a line. So I will not draw a line. "
S. Alan Stern, the principal investigator of NASA's New Horizons mission who flew over Pluto two years ago, takes a broader view: every particle in the rings of Saturn is, at least technically, a moon of Saturn.
develop categories, "said Stern. "The best would be organically, not by arbitrary votes." People already use "moonlet" for little guys. "
But Gareth V. Williams, associate director of Minor Planet Center, thinks that 39 is a debate that astronomers have not yet to worry about.
"We are nowhere near being able to image individual ring particles, let alone obtain enough observations for the determination of the individual's rings." Orbit, even from spacecraft, "Williams said." I think that's an issue for a future generation. Currently, it is too hypothetical. "
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