A dozen years ago, astronomers debated, “What is a planet?” They may soon have to wrangle another question of solar system classification: “What is a moon?”
On Tuesday, scientists led by Scott S. Sheppard of the Carnegie Institution for Science announced the discovery of a dozen moons around Jupiter, bringing the total number orbiting the solar system’s largest planet to 79. Next to the famous moons that Galileo spotted through his telescope in 1610 — bigger-than-Mercury Ganymede, deeply cratered Callisto, volcanic Io, icy Europa — the new ones are slight. They measure between a half mile and two miles wide and orbit millions of miles from the planet — good explanations for why no one had seen them until now.
As telescopes get better, astronomers will assuredly find more and more moons, smaller and smaller, around Jupiter and other giant planets orbiting the sun. When the count rises into the hundreds, maybe thousands, scientists might start to wonder whether it’s worth keeping track.
Does every pebble going around a planet qualify as a moon?
“We might have to start calling the ones that are less than a kilometer in size maybe ‘dwarf moons,’” Dr. Sheppard said. Pluto was demoted from planet to dwarf planet in 2007.
A moon is simply a rock in orbit around a planet, and there is currently no minimum size for something to be called a moon. But in practice, astronomers only count objects whose orbits they can determine. Saturn’s rings, for example, are thought to consist of particles that range in size from a sand grain to a house — too small for spotting and tracking individual particles.
The main task of Dr. Sheppard’s team is searching for Planet Nine, a hypothesized planet far beyond Neptune that appears to be jostling objects at the edge of the solar system. But they realized in March last year that Jupiter would pass through the part of the night sky they wanted to search. So that’s where they looked.
They did not find Planet Nine. (No one has, yet, and it might not exist.)
But in the images, taken by the telescope in Chile, they did spot 12 new points of light in the vicinity of Jupiter. Verifying the observations took a year, and then the International Astronomical Union’s Minor Planet Center — the clearinghouse for gathering and disseminating solar system data — released the findings.
Bigger moons like Europa and Ganymede typically orbit in the same direction as the rotation of their planet, in what astronomers call a prograde motion. That is not surprising, because these moons likely formed out of a disk of dust and gas that was spinning in the same direction as the planet as the solar system took form.
But the immense gravity of Jupiter could also capture other passing objects, and those space rocks sometimes end up orbiting the other way, in what’s known as retrograde motion.
Two of the new moons have prograde orbits, matching an inner group of moons that are thought to be fragments of a larger moon that shattered. Nine fall among three swarms of more distant retrograde moons, likely remnants of three larger, captured moons.
The 12th moon is an oddity. It moves among the retrograde moons yet orbits in a prograde direction. “It's going down the highway the wrong direction,” Dr. Sheppard said.
He suggested that this could be the last piece of the object that broke apart the original three outer moons. If so, more moons are waiting to be discovered.
“We think all these collisions happened out there,” Dr. Sheppard said. “So if you go smaller and smaller, you’ll find smaller and smaller fragments from a lot of these collisions.”
The astronomers have not come up with names for 11 of the 12 new moons. They will probably seek input from the public, Dr. Sheppard said, but added they “don’t want to end up with Planety McPlanetface.” (A couple of years ago, a British government agency asked for suggestions to name a research ship, and the internet gleefully voted for Boaty McBoatface.)
For the oddball, playing-in-traffic moon, Dr. Sheppard has proposed Valetudo, the great-granddaughter of the Roman god Jupiter, who is also the goddess of hygiene and health.
In the “Is Pluto a planet?” tussle, it was the discovery in 2005 of another Pluto-size icy world, now named Eris, that forced astronomers to figure out what they meant by “planet.” Ultimately, the International Astronomical Union devised an awkward definition that a planet orbits the sun, is large enough for gravity to pull it into a round shape and that it is essentially the gravitational bully in its orbit.
This last requirement ruled out Pluto and Eris and put them in a new category — dwarf planets that are round and orbit the sun, but which are not gravitational bullies.
For moons, there are no clear differences by which one might set a cutoff for the smallest satellite of a planet.
“Mostly I think, who cares!” said Michael E. Brown, the astronomer at the California Instituteof Technology who discovered Eris and set off the planet debate. “In general, though, I would say: moons versus rings is a pretty easy distinction to make. Harder, though, is moons versus pebbles, dust, whatever that is not in a ring. There is no reasonable place to draw a line. So I would not draw a line.”
S. Alan Stern, the principal investigator of NASA’s New Horizons mission that flew past Pluto two years ago, takes a more expansive view: that each and every particle in Saturn’s rings is, at least technically, a moon of Saturn.
“We should develop categories,” Dr. Stern said. “Best would be organically, not by arbitrary votes. People already use ‘moonlet’ for little guys.”
But Gareth V. Williams, associate director of the Minor Planet Center, thinks it is a debate that astronomers do not have to worry about yet.
“We are nowhere near being able to image individual ring particles, let alone get enough observations for orbit determination, even from spacecraft,” Dr. Williams said. “I think that is a question for a future generation. Currently, it is too hypothetical.”
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