For as long as humans have watched the night sky, we have wondered about the moon, pondering everything from its alien appearance to its influence over phenomena on Earth such as ocean tides, climate stability, and even the natural rhythms of many organisms.

Among the many questions we have about Earth's lone natural satellite, one remains preeminent: what are the moon's origins, and how long has it orbited Earth?

Today, the leading theory among astronomers is that deep in our planet's past — perhaps within roughly the first 100 million years of solar system history — Earth collided with a planet-sized object, creating the moon within a period that may have been as short as just a few hours. Known as the giant-impact hypothesis, this proposed event involves a hypothetical Mars-sized protoplanet astronomers call Theia. The material ejected into orbit by the powerful impact in question would have accumulated afterward, forming the moon that we observe today.

What do we know?

Birth of a lunar origin hypothesis

The evolution of this striking solution to one of the moon's most enduring mysteries goes back to 1946, when geologist Reginald Aldworth Daly was among the first to suggest that an impact event might explain the formation of the moon.

Before Daly, scientists had been leaning toward theories that involved Earth's rapid spin billions of years ago, when each day may have at one time lasted as little as 2 or 3 hours. Decades later in the 1970s, researchers William Hartmann and Donald Davis developed their own models involving collisions of large bodies left over from planetary formation, which could collide with younger planets.

Building on such ideas, other researchers went on to propose similar scenarios between the early Earth and a Mars-sized object. Early in the following decade, the concept eventually claimed its place as the leading lunar-formation process during a landmark conference held in Kona, Hawaii, in 1984.

Evidence of a planetary-scale collision

The popularity of the giant-impact hypothesis can be attributed to the multiple lines of evidence supporting the impact scenario. For instance, researchers have long recognized the similar stable-isotope ratios between both moon rocks and those from Earth, strongly suggesting a link between their origins.

Additionally, the unusually high angular momentum (or spin) of the Earth-moon system, when compared to observations of other rocky planets, also could be explained as one of the effects of a massive collision.

Further clues can be found in the moon's composition, which reveals a relatively small iron core and lower density. This is significant to astronomers, since a hypothetical giant impact would have seen the suspected Theia protoplanet's metallic core merging with the young Earth's interior — meanwhile, lighter mantle and crust material from both worlds would have become dislodged and blasted into orbit, eventually helping form the moon.

Another important piece of evidence involves the relative depletion of volatile substances (a chemical that easily turns into gas or vapor at room temperatures) in lunar material, which again suggests that it might have been lost during an extremely energetic event, an idea further supported by evidence that the early moon once possessed a substantial magma ocean.

What remains unknown?

New research expands on the Theia hypothesis

Still, several significant questions remain, including what precise conditions led to the impact, and how the resulting planetary material evolved into the moon as it appears today.

Recently, a new study by Adeene Denton of Southwest Research Institute and colleagues at the University of Arizona's Lunar and Planetary Laboratory put a new spin on the Theia hypothesis, suggesting that the suspected ancient impact may have been significantly influenced by the heat and physical strength of the two colliding bodies.

The team's findings challenge some long-held views about the giant-impact hypothesis, while lending new support to others. For instance, the team's simulations revealed that while some impacts lead to the creation of a disk of debris that later coalesces into a moon, under other conditions, simulations support the idea that the moon could have been formed in a remarkably short period of time — perhaps within around five hours.

However, one of the primary issues with determining the exact conditions of the giant-impact hypothesis involves the ongoing debate over the moon's age.

"There is a large controversy regarding the age of the moon," said Dr. Erik Asphaug, a planetary science professor at the University of Arizona's Lunar and Planetary Laboratory and one of the new study's co-authors.

In an email to Space, Asphaug explained some of the issues arising from this, since current estimates place the timing of the moon's formation anywhere from around 40 to 120 million years after the formation of the solar system. Therein lies part of the issue, according to Asphaug, who says this timing could have significant implications for models of the Theia impact.

If it had been an earlier collision, Asphaug says this would have involved a hotter Theia due to the greater degree of radiogenic heating; by contrast, a later impact would have meant a cooler and stronger Theia.

"I don't think we know enough yet to say the moon (and by corollary, the Earth) formed this way or that way," Asphaug told Space. "But knowing how temperature affects the result opens up a new dimension for viewing the problem, and that is always helpful because it shifts our frame of reference, like getting out of the car to have a walk around."

A lunar enigma in time and space

For researchers like Asphaug, understanding the moon's formation requires looking at it as more than just an event in space, but also as one in time. Doing so allows comparisons between three different metrics: isotopic decay, temperature-dependent material strength, and the geological record of debris striking other planets — a process through which he believes tighter constraints could be placed on precisely when the moon-forming impact might have occurred.

Adeene Denton, the study's lead author, also emphasized the introduction of the time component her team's research brings to the table, telling Space in an email that the outcome of the moon-forming impact could have depended significantly on the timing of the collision.

"Before, scientists have generally assumed that the outcome of the collision will be the same regardless of when it takes place," Denton wrote. "But now we know that temperature's effect on material properties, which is dictated by how old and/or cold the proto-Earth and Theia might be, alters things substantially."

By connecting impact simulations to data related to the moon's geochemical record, as well as models of early solar system formation, Denton thinks her team's work could help scientists better constrain precisely how and when the moon formed.

The mystery continues

For now, in terms of which possible scenario — an earlier or later collision event — emerges as being the most likely, Denton says it's still difficult to say.

"It depends on which scenarios for solar system formation you favor," Denton told Space.

"I am generally in favor of an earlier formation of the moon, consistent with a rapid planet-building phase in our solar system," Denton said, "but we can't neglect the possibility that the moon formed late."

While much work remains to be done, Denton and Asphaug believe their findings help significantly advance present efforts to resolve some of the questions about the giant-impact hypothesis — a few of which have lingered now for many years.

For Asphaug, however, such lingering questions may simply be part of the scientific process.

"Science has a way of wandering around for a long time until finally it all comes to focus," Asphaug said.

"I believe that will happen, in part thanks to this work, in the coming decade."

For more than a century, astronomers have been captivated by the idea that an undiscovered planet could be lurking in the furthest reaches of our solar system.

The search for this elusive, hypothetical planet has taken many forms over the decades. The discovery of Neptune in 1846 demonstrated that an unseen planet might be inferred from its gravitational effects on other worlds. Based on this, the search for a ninth planet began in earnest, seemingly culminating with the discovery of Pluto by astronomer Clyde W. Tombaugh in 1930.

However, decades later in August 2006, Pluto's long-held planetary status was withdrawn after its formal reclassification as a dwarf planet, launching a debate over the Kuiper Belt object's identity that continues today. Yet beyond the controversy over how Pluto should be classified, the quest to determine whether a hidden ninth planet may exist in our solar system continues — and some astronomers are more confident than ever that its discovery could be imminent.

What do we know?

The story of our solar system's unconfirmed ninth planet takes us back to the early decades of the last century. At that time, astronomers began to suspect that, much like Neptune's discovery based on irregularities in the motion of Uranus, evidence of even more distant planets might begin to surface through similar observations. This prompted American astronomer Percival Lowell to advance the idea that orbital irregularities displayed by both Uranus and Neptune could point to the existence of a distant, unknown planet — one he dubbed "Planet X."

Today, although Lowell's name for this hypothetical planet is sometimes used interchangeably with the idea of a "Planet Nine," the concepts are slightly different. While the early search for Planet X relied on observations of the motion of Uranus and Neptune, new data that included Voyager 2's encounters with those planets in the 1980s helped reveal that their orbital behavior could be explained without the introduction of an unknown ninth planet.

By contrast, modern proponents of the existence of a ninth planet focus their searches further out, looking instead to orbital peculiarities in smaller objects that occupy the distant reaches of the solar system. This approach to solving the Planet Nine mystery was put forward in 2016 by Caltech astronomers Konstantin Batygin and Michael Brown, who argued that an undiscovered planet could account for the odd orbital patterns displayed by several extreme trans-Neptunian objects in the distant solar system.

What remains unknown?

In 2024, the Caltech team presented additional findings, which specifically focused on long-period, low-inclination objects that are known to traverse Neptune's orbit.

"These objects are dynamically unstable, so the population has to be continuously replenished," Batygin recently told Space in an email. "In our 2024 work, we found that Planet Nine naturally produces the observed population, whereas the model without Planet Nine is strongly inconsistent with the data once observational biases are accounted for."

According to Batygin, the Planet Nine theory can potentially explain several unusual attributes about the distant solar system, which include the phenomena of orbital clustering, large perihelion distances some objects exhibit, the presence of retrograde bodies, and objects with highly inclined orbits (meaning their orbital paths are tilted at large angles).

Right now, Batygin says the only thing still lacking from the growing body of evidence is a direct detection of Planet Nine itself. Still, while he concedes that such direct detections remain elusive, he also maintains that something must account for the current data he, Brown, and others have collected.

"If the observed dynamical structure is real," Batygin told Space, "there is presently no comparably compelling alternative theoretical explanation."

The mystery continues

Fortunately, powerful facilities like the Vera C. Rubin Observatory in Chile could soon provide the observations currently missing from the body of data needed to support the existence of a ninth planet.

In the meantime, some promising additional discoveries have been made in recent years, which could help to strengthen the statistical case for things like orbital clustering being related to an undiscovered object at the edge of the solar system.

However, Batygin advises that astronomers should be cautious in interpreting such findings.

"I would say the picture has become more complicated, rather than simply stronger or weaker," Batygin told Space. "One has to be careful about quoting a single significance for 'clustering,' because the theory does not predict that every distant object should be clustered."

As far as what would offer the clearest support for Planet Nine's existence — short of obtaining direct visual confirmation — Batygin says ongoing discoveries of distant objects in the solar system, and the patterns that emerge in their behavior, could provide the most promising new data.

"For me, the clearest non-imaging evidence would be for the dynamical patterns in the outer solar system to sharpen dramatically as the sample grows," Batygin told Space, noting that this would particularly be the case "if several independent observables converge on the same Planet Nine parameters."

"If a much larger, well-characterized sample simultaneously exhibited the predicted apsidal structure, orbital-plane structure, perihelion distribution, inclination distribution, and Neptune-crossing population, it would become very difficult to explain the observations without a common gravitational perturber," Batygin says.

With an ever-growing number of powerful ground and space-based observatories going into service, it could be just a matter of time before we receive the final confirmation that a ninth planet has been lurking out there—or that another explanation can be proven to account for its apparent gravitational fingerprints.

Until then, the search for Planet Nine remains much like what "Planet X" was more than a century ago: a captivating idea awaiting a breakthrough observation — one that could lead to a major discovery that significantly expands our knowledge of the solar system.

It began with an unusual flash detected off the coast of South Africa by an aging American satellite — and to this day, debate continues over whether this odd Cold War event involved an illicit atmospheric nuclear test.

The strange story of the "South Atlantic Flash," better known as the Vela Incident, began on Sept. 22, 1979. At that time, Vela 6911, a satellite in a constellation developed by the United States for the specific purpose of detecting nuclear detonations, observed something that immediately alarmed officials: a peculiar flash of light emanating from somewhere over the South Atlantic Ocean.

A significant part of what concerned U.S. officials at the time had been the double flash of light detected during the event — a distinctive feature that strongly resembled the flashes produced by a nuclear explosion in Earth's atmosphere. The characteristic double flash results from the rapidly changing brightness of the fireball and shock wave produced by an atmospheric nuclear detonation, creating a distinctive optical signature that Vela's sensors were specifically designed to recognize. The South Atlantic event, therefore, seemed like a prime candidate for a secretive nuclear test being carried out by an unknown party — but if that were the case, who was responsible?

What do we know?

U.S. officials responded to a possible nuclear test

Then-President Jimmy Carter was promptly alerted to the situation, as evidenced in diaries the former U.S. leader would publish years later. "There was indication of a nuclear explosion in the region of South Africa," Carter wrote in an entry on the same date of the event. A later entry penned the following February would go on to add that "We have a growing belief among our scientists that the Israelis did indeed conduct a nuclear test explosion in the ocean near the southern end of Africa."

Despite that belief regarding the possibility of a clandestine nuclear test, a scientific panel assembled in 1980, chaired by MIT professor Jack Ruina, eventually put forward an alternative explanation: that the flash had more likely been an anomaly caused by a small meteoroid striking the satellite.

Notably, the panel did not rule out the possibility of a nuclear explosion at the time, although the alternative they put forward remained controversial among scientists and intelligence officials.

Documents and government insiders surface

By the turn of the 21st century, formerly classified documents began to surface that revealed the extent of the inner tensions between U.S. government agencies over what caused the Vela incident.

Among the critics of the debris-induced "anomaly" hypothesis had been Defense Intelligence Agency Vice Director Jack Varona. According to documents released in 2006, Varona said the investigations by the 1980 scientific panel had been a politically motivated "whitewash," and that evidence collected by the U.S. intelligence community included detections "unique to nuclear shots in a maritime environment," based on an analysis by the Naval Research Laboratory.

Varona wasn't alone. Other former intelligence officials who came forward over the years included Stansfield Turner, the former Director of Central Intelligence during the Carter administration, who in 2003 described the Vela detection as a "man-made phenomenon."

Similar views were advanced by retired CIA officer Tyler Drumheller, who in 2006 wrote that he had sources at the time who "collectively provided incontrovertible evidence that the [South African] apartheid government had in fact tested a nuclear bomb in the South Atlantic in 1979, and that they had developed a delivery system with assistance from the Israelis."

What remains unknown?

Scientists revisit the mystery

While we still don't know if the South Atlantic Flash was a nuclear test, several scientists have reexamined the available evidence within the last decade, and what they found has lent further weight to the idea that the Vela incident had involved a detonation of some kind.

Among the most significant assessments, a 2017 study by Christopher Wright and Lars-Erik De Geer combined modern research into interplanetary dust and hypervelocity impacts with revelations from recently declassified documents, as well as publicly available information that had been previously overlooked.

Their conclusion: while no model was presented for the tenuous "meteoroid" theory advanced by the May 1980 scientific panel, the double flash observed by the Vela satellite in 1979 was indeed "consistent with a nuclear explosion, albeit detected by an aged satellite for which background modulation was abnormal and/or commenced earlier," adding that additional radionuclide and hydroacoustic evidence existed which supported the conclusion "that the Vela Incident was a nuclear weapon test explosion."

Wright and De Geer weren't alone either. A 2022 study examined data collected by NASA's Nimbus-7 satellite just 16 minutes and 44 seconds after the double flash over the South Atlantic, which suggested a disturbance in the ozone layer that appeared to be consistent with a blast-generated shock wave.

Still, some of the original members of the 1980 scientific panel have defended its work amid the more recent scientific studies. Richard Mueller, who served as the panel's youngest member at the time, defended the group's conclusions in a November 2019 oral history conference, maintaining that the chances the Vela incident had involved a nuclear event were "as close to zero as it gets."

This determination, Mueller argued, was made despite early interpretations by many panelists that a nuclear event had seemed most likely. Wright and De Geer, both of whom attended the 2019 oral history conference, were among the participants that pushed back on Mueller's views.

Who was responsible?

Amid the mounting evidence over the last several decades suggesting the Vela incident was indeed a nuclear test, debate continues over who was most likely to have been responsible.

Although suspicions have long centered on a cooperative — and secret — strategic effort between apartheid-era South Africa and Israel, publicly available evidence remains lacking, and a fair amount of information in declassified documents that could point to this possibility remains redacted.

The mystery remains

Thus, after more than four decades, questions continue to linger over the famous Vela incident, including what nations might have been involved in such a test if it occurred. Despite that uncertainty, one thing does seem evident: information conveyed in unredacted portions of declassified records, combined with modern scientific analyses, increasingly point to the likelihood that the strange "South Atlantic Flash" in 1979 was anything but natural.