During a quest to find Planet Nine, a mysterious planet believed to be on the edge of our solar system, astronomers discovered something else: 12 new moons around Jupiter. And one of them is quite the oddball.
The discoveries bring the number of Jupiter’s known moons to 79, the most around a single planet in our solar system.
But why are scientists just now finding these moons? Technology is making it easier to observe Jupiter and the area around it in greater detail, proving that discoveries are just waiting to be made in our own corner of the universe.
Finding the moons
In March 2017, Jupiter was in the perfect location to be observed using the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, which has the Dark Energy Camera and can survey the sky for faint objects.
Astronomer Scott Sheppard of the Carnegie Institution for Science and his team were using the telescope to search the edge of the solar system for signs of Planet Nine. They realized they could observe Jupiter at the same time.
They would be able to tell the difference between Jupiter and the objects around it versus the distant solar system objects because any objects around Jupiter would be moving at the same rate as the gas giant. Distant solar system objects can’t move that quickly.
“It’s like driving in a car and looking out the window, with highway signs flying by and a mountain in the background moving slowly,” Sheppard explained. “You can see both at the same time and easily tell the difference between the two.”
Having a new, bigger camera on the telescope enabled the team to cover a larger percentage of sky. Where their observations had previously been akin to looking through a straw, their capability now is 10 times bigger. And with Jupiter being the biggest planet in our solar system, astronomers need to be able to see as much of the space around it as possible.
Because Jupiter is also a bright planet, astronomers have had to deal with the issue of glare and scattered light affecting the space where moons can exist. “New cameras allow us to cover the whole space around Jupiter in a few images, and this camera is well-shaded,” Sheppard said.
What makes a moon?
They discovered the 12 moons, but the observation and confirmation process, using multiple telescopes, took about a year.
Nine of the new moons were found very far from Jupiter, about 25 million kilometers away, moving in a retrograde orbit (the opposite of the planet’s rotation). It takes them about two Earth years to orbit the planet. Sheppard and his team believe that these moons are remnants of three larger moons that broke apart when they collided with other moons, asteroids or comets.
Two other new moons are closer and move in a prograde orbit, which is in line with the direction that Jupiter is moving. Given their distance and angle from Jupiter, they are also most likely pieces of a once-larger moon. They take a little less than a year to orbit the gas giant.
And then there’s the oddball moon. Sheppard believes it could be Jupiter’s smallest, and it has an orbit unlike any other moon around the planet.
It’s been nicknamed Valetudo after the Roman goddess of health and hygiene who is the great-granddaughter of the god Jupiter.
It has a prograde orbit but is more distant and at a different incline. This means it crosses paths with the outer retrograde moons and could collide with them. It’s essentially driving down the highway in the wrong direction, Sheppard explained.
It has most likely collided with other moons, breaking it down into the fragment it is today. Over the course of a billion years, it may even cease to exist.
This survivor could be the last remnant of a once-larger prograde moon that collided with an object to create the retrograde moons.
These new moons probably formed in a place in our solar system known as the giant planet region, which is between the asteroid belt, dominated by rocky asteroids, and the Kuiper belt, dominated by icy comets.
The giant planet region is where the largest planets in our solar system formed, and it’s devoid of objects now because the planets gobbled up all of the material to form.
The moons are remnants of what was out there, born in the disc of gas and dust around Jupiter after the planet formed and then captured and pulled into Jupiter’s orbit. Because they formed between the two belts, the moons are probably composed of rock and ice.
These building blocks of planets can provide a window into the early years of the solar system.
What is Planet Nine?
The researchers believe that there might be a planet 200 times farther from the sun than we are (and five times more distant than Pluto) with an extremely stretched and oblong orbit on the edge of our solar system. It could be as much as 15 times the size of Earth. It’s known as Planet Nine or Planet X.
Sheppard, Chad Trujillo of Northern Arizona University and David Tholen of the University of Hawaii are on a quest to find as many faint, distant objects on the edge of the solar system as they can. The more they find, the more they can narrow the area of sky where Planet Nine might be.
With current technology, as well as the next generation of telescopes that will have even more capabilities, Sheppard believes that they could definitively say whether Planet Nine exists in the next few years.
So why is this confirmation so difficult? Because it’s like peering around the trees in our own forest and trying to count them all rather than using an overhead drone, Sheppard said.
A long time ago, when our solar system was forming, this would have been easier because Planet Nine wasn’t always living on the edge. It was probably right in the middle of the planets we know so well, Sheppard said.
If Planet Nine exists, it could be the runt of the giant planets, Sheppard said. It was probably forming in the giant planet region and grew to be several Earth masses in size, and then it got close to one of the giant planets and was thrown out into the outer solar system. That would explain its theorized orbit, which wouldn’t be able to form in a neat circle and instead formed in an a chaotic environment.
“If we do find this planet in the next few years, it would be a pretty amazing discovery for astronomy. It’s kind of mind-blowing to know that something bigger than the Earth is sitting out there in the solar system and we haven’t been able to see it,” Sheppard said.