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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 its orbits around the planet's largest solar system. greater than Mercury Ganymede, Callisto deeply cratered, Io volcanic, Europe frozen – the new ones are light. They measure between half a mile and two 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 surely find more and more moons. smaller, around Jupiter and other giant planets orbiting the sun. When numbers go up in hundreds, maybe thousands, scientists might start wondering if it's worth keeping track.
Are all pebbles that circle around a planet considered a moon? are less than one kilometer, perhaps "dwarf moons," said Dr. Sheppard. In 1965, Pluto moved Pluto from the planet to the 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 whose size varies from one grain of sand to another, too small to spot and follow individual particles
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The main task of Dr. Sheppard's team is to search Planet Nine, a hypothetical planet well beyond Neptune who seems to be pushing objects to 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 passing by, and these space rocks sometimes end up spinning in the other direction, what is called a retrograde movement .
Two of the new moons have prograde orbits, corresponding to a group of moons that are believed to be fragments of a larger moon that broke. 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 going down the road in the wrong direction," says Dr. Sheppard.
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 occurred there," said Dr. Sheppard. "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, added Dr. Sheppard, but they added that they did 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 moon, playing in traffic , Dr. Sheppard proposed Valetudo, the great-granddaughter of the Roman god Jupiter, who is also the goddess of hygiene and health.
This latter requirement excludes Pluto and Eris and places them in a new category – the dwarf planets which are round and orbit the sun, but which are not gravitational tyrants.
For moons, there are no clear differences could define a break for the smaller satellite of a planet. "I think especially, who cares!" Said Michael E. Brown, California astronomer Institute of technology who discovered Eris and unleashed the debate on the planet. "In general, though, I would say: moons against the rings is a pretty easy distinction to make.More hard, 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 Dr. Stern." The best would be organically, not by arbitrary votes. People are already using 'moonlet' for little boys. "
But Gareth V. Williams, associate director of the Minor Planet Center, thinks it's a debate that astronomers have yet to worry about.
" We are far away to be able to image individual ring particles, let alone get enough observations for determination of orbit even from spacecraft " said Dr. Williams. "I think it's a question for a future generation. Currently, it is too hypothetical. "
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