To our best knowledge, planets form from grains of dust the size of fine sand inside donut-shaped disks of gas and dust circling newborn stars. These materials collide at slow speed, stick together and grow larger through accumulation like rolling a snowball into the body of a snowman. Dust clumps and compacts to form pebbles, then boulders and mile-size rocks. Like the Pac-Man character, the growing body gobbles up everything in its path until a planet is born. Collisions, gentle or otherwise, with other neighboring bodies also contribute to the process of planet formation.

Our own moon likely formed when a Mars-sized asteroid collided with the Earth about 4.5 billion years ago. Debris from both the Earth and impactor formed a ring around the planet that eventually coalesced through gravity and collisions to form the moon. (Bob King)
Our own moon likely formed when a Mars-sized asteroid collided with the Earth about 4.5 billion years ago. Debris from both the Earth and impactor formed a ring around the planet that eventually coalesced through gravity and collisions to form the moon. (Bob King)

In our own solar system six planets are orbited by moons. Many of them likely formed from debris disks surrounding their host planets, smaller scale versions of the much larger protoplanetary disk surrounding the sun from which the planets developed. Growing planets can cobble together smaller-sized rocks, dust and ice (in the chilly outer solar system) to fashion their own circumplanetary disks. Some of the material feeds the growing planet, while a portion comes together and gives birth to moons.

Moons can also form after the fact. A planet with a single moon may suddenly find itself with two or more if a stray asteroid flies by and strikes its moon. This scenario may have been at play in the formation of the small, irregular-shaped moons Phobos and Deimos at Mars. Meanwhile, a giant planet like Jupiter can captured small asteroids and bend them to its gravitational will, converting them into orbiting moons. Jupiter's outermost satellites are thought to have originated this way.

This image, taken with the Atacama Large Millimeter/submillimeter Array (ALMA), shows a close-up view of the moon-forming disk surrounding PDS 70c, a young Jupiter-like gas giant. The disk has enough material to form up to three satellites the size of our own moon. The star, seen faintly at left, is an orange dwarf three-quarters as massive as the sun and very young — only about 5.4 million years old compared to 4.6 billion years for the sun. (ALMA (ESO/NAOJ/NRAO)/Benisty et al.)
This image, taken with the Atacama Large Millimeter/submillimeter Array (ALMA), shows a close-up view of the moon-forming disk surrounding PDS 70c, a young Jupiter-like gas giant. The disk has enough material to form up to three satellites the size of our own moon. The star, seen faintly at left, is an orange dwarf three-quarters as massive as the sun and very young — only about 5.4 million years old compared to 4.6 billion years for the sun. (ALMA (ESO/NAOJ/NRAO)/Benisty et al.)

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To date, 4,796 exoplanets — planets orbiting stars other than the sun — have been discovered. It's tricky enough to detect something as small and dark as a planet light-years away. Moons, being smaller, are even more difficult to spot. But that milepost has finally been reached. A team of astronomers announced this week that they "unambiguously" detected the presence of a disk around a planet outside the solar system for the first time.

“Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disk is associated with the planet," said Myriam Benisty, a researcher at the University of Grenoble, France, and at the University of Chile, who led the new research. The scientific paper is available as a free read here.

Some of the 66 dishes of the ALMA array are pictured at night beneath a star-studded sky that includes the Large (left) and Small Magellanic Clouds, satellite galaxies of the Milky Way. (ESO / C. Malin)
Some of the 66 dishes of the ALMA array are pictured at night beneath a star-studded sky that includes the Large (left) and Small Magellanic Clouds, satellite galaxies of the Milky Way. (ESO / C. Malin)

The group used the Atacama Large Millimeter/submillimeter Array (ALMA), a battery of 66 radio telescopes in the Atacama Desert of northern Chile to make the discovery. Unlike a classical visible-light telescope, ALMA observes the sky in a narrow region of the spectrum that straddles infrared and radio waves. To our eyes these objects are invisible, but not to ALMA's big dishes, which gather light with wavelengths measured in millimeters. Visible light waves are 4-5 orders of magnitude shorter.

The disk surrounds the exoplanet PDS 70c, one of two giant, Jupiter-like planets orbiting the star PDS 70 located 400 light-years away in the constellation Centaurus the Centaur. Astronomers had found hints of a “moon-forming” disk around this exoplanet before, but the images weren't clear enough to separate it from the larger circumstellar disk shown in the photo above.

Benisty and her team found that the disk has about the same diameter as the sun-Earth distance and enough mass to form up to three satellites the size of our moon. Curiously, the other planet in the gap, PDS 70b, shows no evidence of a disk. Scientists think that PDS 70c swept up the lion's share of material, starving the other of the stuff needed to form its own moon-incubating cloud.

In this artist's view of the early solar system, pebbles, boulders and protoplanets form within the dusty disk of material surrounding the young sun. (NASA / JPL-Caltech)
In this artist's view of the early solar system, pebbles, boulders and protoplanets form within the dusty disk of material surrounding the young sun. (NASA / JPL-Caltech)

Since we can't literally go back in time to see exactly how our solar system took shape, we're far from a complete understanding of how planets and moons came to be. I was surprised to learn that nearly all of the known exoplanets are mature objects formed long ago. PDS 70b and PDS 70c, which resemble our solar system's Jupiter-Saturn pair, are rarities because they're still in the process of development around a very young star.

What a great opportunity they present. Like getting a ride on a time machine with the lever pulled all the way back to the year 4.6 billion BC.!

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.