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Starts With A Bang

Extraordinary, moon-forming planet collision spotted 1850 light-years away

The Universe, although violent, is filled with creation events following destructive ones. 1850 light-years away, both types are unfolding.
An artist's impression of a planet collision.
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
Key Takeaways
  • In 2021, a star located 1850 light-years away suddenly dimmed, and remained very faint for more than a year.
  • Follow-up observations showed that a massive planetary collision was likely responsible, creating a puffed-up cloud of debris that shrouded the entire star from view.
  • As that debris settles down, not only will the star re-emerge, but a single planet with a rich lunar system should arise in the aftermath, with eerie similarities to the collision that formed the Earth-Moon system.

All throughout the Universe, there are normally three ways that moons can form.

pluto moons hubble
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.

PDS 70 protoplanetary circumplanetary system
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.

neptune triton captured kuiper belt object
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.

giant impact hypothesis collision
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

That third way explains many lunar systems, including Earth’s, Mars’s, and even Pluto’s.

mars transient moon impact
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 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

When a massive planetary collision occurs, a diffuse, puffed-up structure known as a synestia forms.

earth moon synestia formation giant impact
A synestia will consist of a mixture of vaporized material from both the larger mass planet/protoplanet and the smaller impactor, which will form 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.

planetary collision dust cloud
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

These events should be common in young stellar systems, emitting long-lasting infrared afterglows.

WISE tycho supernova 1572
This image shows, in infrared light (taken with the WISE space telescope), the 1572 remnant of a type Ia supernova: Tycho’s “stella nova.” Just as ejected stellar material can glow due to heat in the infrared for tens of thousands of years, the remnants of planetary collisions can continue to glow in the infrared for long periods of time, although likely for only a few millennia in that instance.
Credit: NASA/JPL-Caltech/UCLA

It looks like scientists just witnessed the creation of one: around a star located 1850 light-years away.

collision planet simulation
This simulated collision shows what would happen when Neptune/Mini-Neptune planets collide, with debris, ejecta, and a synestia resulting in the aftermath. Although much of this material will eventually be stretched out over the orbit of the post-merger planet, much of it will remain in a circumplanetary synestia structure, destined to form moonlets and eventually a new lunar system around the final-state planet.
Credit: M. Kenworthy et al., Nature, 2023

In December of 2021, a young, unremarkable star — 2MASS J08152329-3859234 — suddenly dimmed spectacularly.

Betelgeuse 2019 2020 dust
Whereas Betelgeuse, as shown here, dimmed and then re-brightened due to a surface event intrinsic to the star itself, other mechanisms for stellar dimming, including dust, debris, and other light-blocking phenomena, are more common around lower-mass, young stellar systems.
Credit: ESO/M. Montargès et al.

This wasn’t observed by a human, but by an all-sky supernova monitoring service: ASAS-SN.

ASAS-SN telescope
ASAS-SN is an all-sky supernova (and other transient event) search constructed in partnership with Las Cumbres Observatories and built with off-the-shelf astronomical equipment. It is one of the best ways we have, today, of automatically detecting brightening or faintening events for stars, wherever and whenever they occur.

Looking in archival data, an infrared brightening — with wavelength-dependent magnitudes — occurred ~2.5 years prior.

dimming star
In the top row, the brightness of the main star located 1850 light-years away is shown. Below, infrared observations of the system dating back to years before the dimming event can be seen, supporting the notion of an energy-releasing collision between two planets in the outer part of this stellar system. When that debris cloud blocks the parent star’s light, the faintening/dimming event occurs.
Credit: M. Kenworthy et al., Nature, 2023

It’s as though two Neptune/mini-Neptune exoplanets collided, creating a massive debris cloud.

Its infrared afterglow should persist for centuries.

A synestia doesn’t just consist of this puffy ring/torus of debris around a joint planetary core, but also rises to temperatures in excess of 1000 K, causing it to emit substantial amounts of its own infrared radiation, with peaks in different parts of the infrared spectrum dependent on the exact temperature and temperature profile of the system in question.
Credit: Sarah Stewart/UC Davis/NASA

Periodically, that cloud blocks its parent star’s light.

A diagram showing the orbit of a spacecraft.
The dimming that the star 2MASS J08152329-3859234 experienced can be well-modeled by a cloud of debris resulting from a planet-planet collision passing in front of its parent star located at a significant orbital distance from the star itself. This is consistent with a synestia: a proposed debris cloud that would result from such a planet-planet collision, transiting in front of the star relative to our line-of-sight.
Credit: M. Kenworthy et al., Nature, 2023

In time, a single giant world with a rich lunar system will emerge.

synestia planet collision artist impression
This artist’s impression shows a synestia: the aftermath of a massive planetary collision that creates a puffy torus-like shape of debris that persists for many thousands of years. As the synestia evolves, a circumplanetary disk with moonlets and eventually full-fledged moons will emerge.
Credit: Baperookamo/Wikimedia Commons

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.


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