In the current snapshot of the Solar system, Mars has an inner moon named Phobos, whose orbital radius shrinks by 2 meters per century as a result of tides. Within 50 million years, Phobos will arrive too close to Mars and break up due to the Martian tidal force to create a ring of rocks and dust particles.
According to a new paper this week, Earth likely had a similar debris ring 466 million years ago. The argument is based on 21 asteroid craters from an anomalous 40-million-year meteor shower known as the Ordovician impact spike. The authors Andrew Tomkin, Erin Martin and Peter Cawood from Monash University in Melbourne, Australia, had found that all related craters fall in an equatorial band at less than 30 degrees. They therefore argue that the impact spike could have originated from a large parent body that tidally broke up during a near-miss encounter with the Earth and created a debris ring from which material produced the observed crater distribution on Earth’s surface.
Currently, Jupiter, Saturn, Uranus and Neptune all have rings. Jupiter’s main ring of dust was discovered in 1979 by the Voyager 1 spacecraft and later studied by the Galileo orbiter in 1990. The rings around Uranus are intermediate in complexity between those around Jupiter or Neptune and the extensive rings around Saturn. New near-infrared observations by the Webb telescope in 2023 revealed an additional outer part of the ring system around Uranus. The dusty rings around Neptune contain dark material- most likely of organic compounds processed by radiation.
Saturn hosts the most complex ring architecture in the present-day Solar system. The estimated infall rate of mass from the rings towards Saturn favors a young age. The total mass in the ring system is about 10^{19} kilograms, of the order of the Earth’s Antarctic ice sheet spread across a surface area that is nearly a hundred times larger than the cross-section of the Earth. The drainage of ring material down into Saturn is triggered by gravity pulling electrically charged water ice grains down from the rings along planetary magnetic field lines, bringing down between 432 and 2870 kilograms per second. In 2017, the Cassini spacecraft detected an equatorial flow of electrically neutral material from the rings to Saturn of 4,800 to 45,000 kilograms per second, suggesting a lifespan of 7-70 million years.
Given these circumstances, it is reasonable to conclude that over periods of hundreds of millions of years, Earth, Mars and most other Solar system planets hosted rings from the tidal break-up of close-in parent bodies.
There should have been many more near-misses and tidal disruption events than direct impacts on Earth, because tidal disruption can occur on a scale a few times larger than Earth’s radius yielding a larger probability for disruption events. The Chicxulub impactor that killed the non-avian dinosaurs 66 million years ago, had a mass of order that of Phobos, 10^{16} kilograms, about a thousand times less than the total mass of Saturn’s rings. Given that, it is reasonable to expect that a rock somewhat larger than the Chicxulub impactor that missed the Earth and was tidally disrupted, could have produced a ring structure around Earth within the last 100 million years. Based on Saturn, this ring would have disappeared by now.
The appearance of rings could be even more frequent around common dwarf stars. What matters for tidal disruption is the average density of the object sourcing gravity relative to the mean density of the disrupted body. Most stars in the Milky-Way galaxy are a tenth of a solar mass and a tenth of a solar radius. As a result, these stars are more than ten times denser than rock and therefore can tidally disrupt rocky planets like the Earth before engulfing them, as I showed in a recent paper with my brilliant postdoc Morgan MacLeod. This implies that common dwarf stars could be surrounded by transient rings from disrupted rocky material around them. White dwarfs are indeed observed to possess close-in debris disks of rocky materials.
Exo-planets are often discovered by the deficit of light that they trigger when transiting the face of their host star. If some planets acquire rings around them, they would block more light from their host star. As a result, transit surveys can be used to set interesting limits on the frequency of rings around exoplanets. So far, none was discovered.
Just as in human relationships, an exoplanet is more exciting if Nature “puts a ring on it.”
If Earth will acquire a ring in the near future, the night sky will be illuminated by sunlight reflected off the dust and rocks in it since Earth cannot shadow them. The amazingly beautiful view of the night sky will be inspiring for poets but problematic for astronomers. A sky illuminated by “natural city lights” would compromise deep observations of the night sky far more effectively than the 7,000 Starlink communication satellites that are currently in orbit around Earth today.
The total mass of these Starlink satellites is less than 10^7 kilograms, a trillion times smaller than the mass associated with Saturn’s rings. This illustrates once again how the huge scales of the Universe are humbling, especially relative to our humble human constructions.
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s – Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011-2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.