In 2021, a normal, young, Sun-like star was seen to dim: spontaneously, unexpectedly, and spectacularly. Two years later, the culprit has been discovered: an intervening cloud of debris arising from a planetary collision that occurred 2.5 years prior. (Credit: Mark Garlick/University of Bristol)
The Universe. although violent, is filled with creation events following destructive ones. 1850 light-years away, both types are unfolding.
All throughout the Universe, there are normally three ways that moons can form.
Pluto, shown as imaged with Hubble in a composite mosaic, along with its five moons. Charon, its largest, must be imaged with Pluto in an entirely different filter due to their brightnesses. The four smaller moons orbit this binary system with a factor of 1,000 greater exposure time in order to bring them out. Nix and Hydra were discovered in 2005, with Kerberos discovered in 2011 and Styx in 2012. These five moons were likely formed via an early collision, rather than either in situ or as a result of gravitational capture. (Credit: NASA, ESA, and M. Showalter (SETI Institute))
1.) A circumplanetary disk can fragment into moons, common around giant worlds.
Wide-field (left) and close-up (right) views of the moon-forming disc surrounding PDS 70c. Two planets have been found in the system, PDS 70c and PDS 70b, the latter not being visible in this image. They have carved a cavity in the circumstellar disc as they gobbled up material from the disc itself, growing in size. In this process, PDS 70c acquired its own circumplanetary disc, which contributes to the growth of the planet and where moons are very likely in the process of forming, similar to the formation of Jupiter’s Galilean moons. (Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.)
2.) Interloping, low-mass bodies can be gravitationally captured.
Triton’s south polar terrain, as photographed by the Voyager 2 spacecraft and mapped to a spheroid of the appropriate shape and size. About 50 dark plumes mark what are thought to be cryovolcanoes, with those dark trails colloquially called ‘black smokers.’ Triton is a captured Kuiper belt object, having most certainly cleared out almost all of Neptune’s original moons, and represents the largest captured Moon in the known Universe for now. (Credit: NASA; PlanetUser/Wikimedia Commons)
3.) Or giant collisions can occur, kicking up debris that coalesces into moons.
When two large bodies collide, as they very likely did between proto-Earth and a hypothesized Mars-sized world known as Theia in the early Solar System, they’ll generally form one more massive body as a result, but the debris kicked up from the collision can coalesce into one or more large moons. This was likely the case not only for Earth, but for Mars and Pluto and their lunar systems as well. (Credit: NASA/JPL-Caltech)
Rather than the two Moons we see today, a collision followed by a circumplanetary disk may have given rise to three moons of Mars, where only two survive today. The idea is that Mars’s once-innermost moon since was destroyed and fell back onto Mars long ago. This hypothetical transient moon of Mars, proposed in a 2016 paper, is now the leading idea in the formation of Mars’s moons, and helps explain the enormous differences in topography between Mars’s two hemispheres. (Credit: LabEx UnivEarthS | Université de Paris Diderot)
A synestia will consist of a mixture of vaporized material from both the larger mass planet/protoplanet and the smaller impactor, which will forms one or more large moons inside of it from the coalescence of moonlets. This is a general scenario capable of creating one single, large moon with the physical and chemical properties we observe Earth’s moon to have, multiple moons like those found around Mars or Pluto, or more complex systems around higher-massed worlds. (Credit: S.J. Lock et al., J. Geophys. Research, 2018)
There can be so much debris that even the parent star’s light can be blocked.
When two objects in space collide, whether asteroids, moons, rocky planets, or even giant planets, a large cloud of light-blocking debris will be produced. When that cloud passes in front of its parent star relative to an external observer, that star will appear to dim and fainten. (Credit: NASA/JPL-Caltech)
