Unexplained Mysteries of the Solar System

Key Takeaways

• Planet Nine’s existence is mathematically compelling but no telescope has confirmed it yet
• Saturn’s north pole hexagon has persisted for over 40 years without a clear explanation
• Several solar system mysteries may be connected, hinting at unknown early formation events

Introduction

The solar system is old. At roughly 4.6 billion years, it’s had plenty of time to sort itself out, settle into predictable orbits, and stop surprising people. And yet, the more spacecraft venture into its outer reaches, the more unexpected things they find. Some of what’s known about these eight planets, hundreds of moons, and billions of smaller bodies flatly refuses to fit the standard models. Not in minor, easily-explained ways. In ways that require entirely new theories, some of which contradict each other, and some of which remain genuinely unresolved.

What follows isn’t a tidy catalog. It’s an attempt to describe the things that planetary scientists, astrophysicists, and geologists genuinely can’t explain, presented in plain language for anyone curious enough to ask.

The Ghost Planet Nobody Can Find

In January 2016, Caltech astronomers Konstantin Batygin and Mike Brown published a paper that sent ripples through the planetary science community. Based on the clustered orbits of six distant trans-Neptunian objects in the Kuiper Belt , they argued that the gravitational influence of an undiscovered planet was the only plausible explanation for the unusual orbital pattern. They called it Planet Nine .

The proposed planet, if it exists, would be roughly ten times the mass of Earth. Its orbit would be enormously elongated, swinging it as far as 1,000 astronomical units from the Sun at its farthest point and bringing it no closer than about 200 AU at its nearest. For context, Neptune orbits at about 30 AU. Planet Nine, if real, would take somewhere between 10,000 and 20,000 years to complete a single orbit.

The math is genuinely compelling. The probability that six Kuiper Belt objects would independently end up with the orbital clustering observed, purely by chance, is estimated at around 0.007 percent. That’s not impossible, but it’s the kind of number that makes scientists pay attention. Since 2016, additional trans-Neptunian objects have been found with orbits consistent with the Planet Nine hypothesis, which has kept the idea alive even as direct detection remains frustratingly elusive.

Finding Planet Nine is harder than it sounds. Despite being potentially ten Earth masses, it would be reflecting sunlight from so far away that it would appear extremely faint. Current sky surveys haven’t found it, including the Zwicky Transient Facility at Palomar Observatory, which has swept large portions of the sky without a confirmed sighting. The Vera C. Rubin Observatory in Chile, still coming online as of early 2026, is expected to be a powerful tool in the search. Its wide-field survey capabilities make it one of the best chances yet to either find Planet Nine or rule out its existence in the regions where models predict it should be.

Some researchers aren’t convinced the planet exists at all. A 2021 paper by Kevin Napier and colleagues at the University of Michigan argued that observational biases, specifically the fact that surveys tend to observe some patches of sky more than others, could create the appearance of orbital clustering even without a hidden planet. Batygin and Brown have pushed back against this interpretation, pointing to specific orbital features that selection bias alone can’t easily explain.

The whole situation is one of the most genuinely unresolved debates in planetary science right now. A planet ten times Earth’s mass lurking at the edge of the solar system would rewrite textbooks. The absence of such a planet would require explaining the orbital clustering some other way. Neither resolution has arrived.

Saturn’s Impossible Hexagon

There’s a storm on Saturn that’s been spinning for at least 40 years, possibly longer, and it has six perfectly symmetrical sides. This isn’t a metaphor or a rough approximation. Saturn’s hexagon , a massive polar vortex sitting at approximately 78 degrees north latitude, genuinely has six straight sides and six corners, each side measuring roughly 14,500 kilometers. Each side is longer than Earth’s diameter.

Voyager 1 spotted it in 1980. Voyager 2 confirmed it in 1981. It was still there when NASA‘s Cassini spacecraft arrived in 2004, and it persisted throughout Cassini’s 13 years in orbit until the mission ended in 2017. By every available indication, it’s still there today. The same hexagonal storm, in the same place, for over four decades of observation and an unknown period before that.

Jet stream dynamics are the leading explanation. The hexagon is thought to be driven by a particularly stable, fast-moving jet stream that runs along the boundary of the polar region. Laboratory experiments have reproduced hexagonal shapes in rotating fluids under certain conditions, specifically when there’s a fast-spinning ring of fluid surrounded by slower-moving fluid. The interaction between the two creates waves that can lock into specific geometric patterns. The number of sides in the polygon depends on the ratio of the flow speeds involved.

The problem is that Saturn’s hexagon is inexplicably stable. Jet streams on Earth shift. Weather patterns come and go. Saturn’s hexagon has maintained the same shape, the same rotation rate, and the same position for as long as anyone’s been able to observe it. The Cassini data showed that it rotates at almost exactly the same rate as Saturn’s interior, which is itself a point of ongoing research because Saturn’s rotation rate is harder to pin down than you’d expect.

What makes the hexagon stranger still is that Saturn’s southern pole has a different structure. The south has a large oval cyclone, but nothing with the geometric precision of the north. Why the asymmetry? Models haven’t convincingly explained why similar conditions at opposite poles would produce such different structures. Saturn’s seasons are long, about seven Earth years per season, and the hexagon has survived at least two full Saturnian years of observation. It has outlasted multiple proposals for what drives it.

There’s also the color change. Between 2012 and 2016, Cassini watched the interior of the hexagon shift from a bluish hue to a more golden color. Scientists attributed this to seasonal changes in UV exposure affecting the chemistry of the atmosphere, but the mechanism isn’t fully pinned down. The hexagon is one of those phenomena where the more data you collect, the more specific the questions become.

Jupiter’s Red Spot Is Shrinking, and That’s a Problem

The Great Red Spot is one of the most iconic features in the solar system, a storm on Jupiter so large that Earth could fit inside it with room to spare. Historical observers may have recorded it as early as the 1600s, though firm scientific documentation dates to the 1800s. For at least 150 years, the Great Red Spot has been one of the most stable, recognizable features in planetary science.

And it’s been shrinking. In the 19th century, it measured roughly 40,000 kilometers across. By 1979, when Voyager 2 flew past, it was down to about 23,000 kilometers. Hubble Space Telescope observations from the 1990s through the 2010s tracked continued contraction. By 2023, the storm measured roughly 13,000 kilometers in its longest dimension, less than a third of its former size. The rate of shrinkage has been uneven, with some years showing faster contraction than others, which makes modeling its future difficult.

Nobody has a clean explanation for why it’s shrinking. Theories point to interactions with smaller storms, known as “red spots” or “ovals,” that periodically collide with the Great Red Spot. These collisions might be affecting the storm’s internal dynamics. Some models suggest that the spot is actually becoming taller and more intense even as it narrows, like a spinning top that becomes more upright as it slows. But that doesn’t fully explain the energy budget.

Whether the Great Red Spot will disappear entirely is genuinely unknown. Some scientists have suggested it could break apart, fragment into smaller storms, or stabilize at a smaller size. Others aren’t ruling out that it could persist for centuries more. What’s clear is that something fundamental is changing in Jupiter’s atmosphere, and the models that were supposed to predict long-lived storms aren’t capturing the dynamics well enough to explain what’s happening.

Jupiter also has persistent mysteries at a smaller scale. Its lightning patterns are different from Earth’s in unexpected ways. Lightning on Earth occurs mainly in the tropics. Jupiter’s lightning, detected by the Galileo probe in 1995 and confirmed in greater detail by the Juno spacecraft, clusters near the poles. The reasons for this reversal in behavior aren’t settled.

Venus Spins the Wrong Way

Venus rotates backward compared to most planets. Stand on Venus, and the Sun would rise in the west and set in the east. Its rotation is also extraordinarily slow. A single Venusian day, meaning one complete rotation on its axis, takes about 243 Earth days. That’s actually longer than a Venusian year, which takes only about 225 Earth days. Venus has days longer than its years.

This combination, retrograde rotation and extreme slowness, demands an explanation that planetary science hasn’t definitively provided. The leading hypotheses fall into roughly two categories: something hit Venus early in solar system history and knocked it over, or atmospheric tidal forces gradually slowed and reversed its original rotation over billions of years.

The impact hypothesis has surface appeal. A large collision in the early solar system could theoretically flip a planet’s rotation. The problem is that the kind of impact required would need to be massive enough to nearly stop or reverse a planet’s rotation without obliterating it, which puts very specific constraints on the impactor’s mass, angle, and speed. Modeling shows it’s possible, but the parameter space is narrow.

The atmospheric tidal hypothesis is more elegant mathematically. Venus’s atmosphere is extremely dense, about 90 times Earth’s atmospheric pressure at the surface. Gravitational tides from the Sun pull on this thick atmosphere, creating a torque. Over geological timescales, this could theoretically slow and reverse the planet’s spin. The challenge is that the same mechanism should theoretically be affecting Venus’s rotation continuously even now, and the numbers need to balance out precisely to explain the current state.

A 2019 study from researchers at the Paris Institute of Earth Physics modeled a scenario where both mechanisms worked together: an early impact knocked the planet sideways, and atmospheric tides then settled it into its current unusual equilibrium. But the uncertainties in modeling ancient solar system conditions are large enough that this remains a hypothesis rather than a confirmed answer.

What’s particularly puzzling is that Venus is Earth’s nearest neighbor and, in terms of size and mass, its closest twin. The two planets formed from similar material in similar regions of the early solar system. Yet they ended up with almost opposite surface conditions and dramatically different rotational characteristics. Figuring out why Venus went the way it did while Earth didn’t is, in some ways, the key to understanding planetary evolution more broadly.

Mars Lost Something Enormous

Mars today is a cold, dry world with almost no magnetic field. In its past, the planet had a global magnetic field, something that scientists can infer from the magnetization patterns frozen into its ancient crust, which the Mars Global Surveyor detected in the late 1990s. These magnetic striping patterns, similar to the patterns found along mid-ocean ridges on Earth, tell a story of a planet that once had active geology and a magnetic dynamo driving its field.

That dynamo shut down roughly 4 billion years ago, possibly within a relatively short window of time, maybe just a few hundred million years. The Mars InSight lander, which operated from 2018 to 2022, provided the most direct measurements of Mars’s interior ever made, detecting marsquakes and mapping the planet’s internal structure. InSight confirmed that Mars has a liquid metallic core, which is necessary for a dynamo to operate. The core’s radius is about 1,820 kilometers, larger than pre-mission models had predicted, and the core appears to be rich in light elements like sulfur and hydrogen.

So why did the dynamo stop? A liquid core exists. The ingredients for a magnetic dynamo seem to be present. Yet the field isn’t there.

Hypotheses include the possibility that the core cooled too quickly, eliminating the convective churning that drives a dynamo. The core’s enrichment in light elements could have played a role by affecting how heat flows through the interior. Another proposal involves one or more giant impacts early in Mars’s history. A 2022 study suggested that a large impact around 4.5 billion years ago may have disrupted the mantle’s heat flow patterns, contributing to the dynamo’s failure.

The consequences of losing that magnetic field were severe. Without magnetic protection, Mars’s upper atmosphere became exposed to solar wind. Over billions of years, the solar wind stripped away much of the atmosphere. Mars went from being, potentially, a world with liquid water on its surface to the thin-aired, radiation-bathed planet it is today. Exactly how wet and warm early Mars really was is still debated, but the magnetic field’s disappearance set the planet on a different trajectory than Earth’s.

The Moon Is Lopsided in Ways That Defy Easy Explanation

Earth’s Moon has two very different faces. The near side, always facing Earth, is relatively flat, dotted with large dark plains called mare that formed from ancient volcanic flooding. The far side is more mountainous, more cratered, and has much thicker crust. The near side’s crust averages about 30 to 40 kilometers thick. The far side’s crust averages about 60 to 70 kilometers thick.

This asymmetry is called the lunar dichotomy, and it’s been known since spacecraft first photographed the far side in 1959. It’s remained deeply puzzling. The Moon formed from debris ejected when a Mars-sized body called Theia smashed into the proto-Earth roughly 4.5 billion years ago. Standard models of this impact produce a disk of material that eventually coalesces into a relatively uniform Moon. A Moon with such pronounced hemispheric differences is harder to explain.

A 2014 study proposed an intriguing solution: there may have been a second, smaller Moon shortly after the original Moon formed, and when it caught up with and merged with the primary Moon, it created the asymmetric crust. The impactor’s rocky material, piling onto one side, could have created the thicker far-side crust. This model is consistent with some data but lacks direct confirmation.

The Lunar Reconnaissance Orbiter and other missions have mapped the Moon’s gravity, topography, and surface composition in extraordinary detail. The data from these missions has refined understanding of the dichotomy without resolving its origin. The Grail mission in 2011 and 2012 flew twin spacecraft in tight formation around the Moon, mapping subtle gravity variations that revealed the Moon’s inner structure in more detail than ever before. What emerged was a picture of a Moon that is more complicated internally than its surface suggests, with evidence of ancient volcanic plumbing systems and structural anomalies that point to a turbulent early history.

There’s also the question of why the Moon always shows the same face to Earth. Tidal locking is well understood, so that part isn’t mysterious. What’s curious is the exact ratio: the Moon rotates once for every orbit around Earth, a 1:1 resonance called synchronous rotation. Several of the solar system’s other large moons are also tidally locked to their parent planets, so this isn’t unique. But for a body as large relative to its parent as the Moon is relative to Earth, it’s an unusually tight arrangement that reflects how close and massive the Moon is in relation to Earth compared to other planet-moon pairs.

Uranus Fell Over and Still Hasn’t Recovered

Uranus has an axial tilt of about 98 degrees. For reference, Earth’s axial tilt is about 23.5 degrees, which is responsible for Earth’s seasons. Most planets in the solar system have relatively modest tilts that keep their equators roughly aligned with the plane of their orbits. Uranus is essentially rolling on its side. Its poles point almost directly at the Sun at the peaks of its 84-year orbit, which means each pole experiences roughly 42 years of continuous sunlight followed by 42 years of darkness.

Why? The most widely accepted explanation is a massive impact early in the solar system’s history, possibly by a body one to three times Earth’s mass. This impactor hit Uranus at an angle that knocked it onto its side, and it never righted itself. Giant impacts were common in the early solar system, and this kind of collision is consistent with models of planetary formation.

But the impact hypothesis has problems. If a single large impact tilted Uranus, models predict it should have sent debris into orbit that would eventually form a ring and moon system aligned with the equator, which is exactly what Uranus has. So that part fits. The problem is that simple tilting impacts tend to also impart significant heat, which would show up in Uranus’s internal temperature. But Uranus emits almost no excess heat. Most giant planets radiate significantly more energy than they receive from the Sun because they’re still releasing heat from their formation billions of years ago. Uranus is almost completely cold by comparison, emitting roughly the same amount of energy as it receives.

Some researchers have proposed that Uranus was hit multiple times, or that a single impact created conditions that trapped heat in a stable stratified layer inside the planet, preventing it from escaping. Neither explanation fully satisfies the data. The ice giants, Uranus and Neptune, are generally less well understood than Jupiter and Saturn because no dedicated spacecraft has ever orbited either of them. Voyager 2 made flyby observations of Uranus in 1986 and Neptune in 1989, but the encounters lasted only hours.

A dedicated Uranus orbiter has been ranked as the top priority for the next large planetary science mission by a 2023-2032 decadal survey from the National Academies of Sciences, Engineering, and Medicine. NASA is in early planning stages for such a mission, though no launch date had been set as of early 2026. Until then, Uranus remains one of the most poorly understood planets in the solar system, and its sideways roll remains officially unexplained.

The Cliff at the Edge of the Kuiper Belt

The Kuiper Belt extends from about 30 AU, just beyond Neptune’s orbit, to roughly 50 AU. Beyond that edge, the population of icy objects drops off sharply, far more sharply than standard models of solar system formation predict. This abrupt outer boundary is known as the Kuiper Cliff.

If the Kuiper Belt simply thinned out gradually as distance from the Sun increased, it would make sense. Objects form less efficiently farther from the Sun because the disk of material was thinner and objects took longer to find each other and stick together. A gradual decline in object density is expected. What wasn’t expected is the sharp wall at around 48 to 50 AU.

The existence of Planet Nine could explain it. If a massive planet in the outer solar system has been sculpting the Kuiper Belt’s outer edge through gravitational resonances, it could clear a sharp boundary exactly the way Neptune’s gravity helps define the inner edge of the Kuiper Belt. But this is circular reasoning: Planet Nine is partly inferred from Kuiper Belt object behavior, and the Kuiper Cliff is then explained by invoking Planet Nine. The theory needs the planet to be confirmed independently for the explanation to be satisfying.

Other proposals include the possibility that a passing star in the Sun’s birth cluster truncated the solar nebula at that distance long before the Kuiper Belt fully formed. Stars do form in clusters and do pass relatively close to each other in their early history. A stellar encounter 4.5 billion years ago that stripped material beyond 50 AU would leave exactly the kind of cliff that’s observed. The problem is modeling this precisely enough to confirm it.

The New Horizons spacecraft, which flew past Pluto in 2015 and Arrokoth in 2019, is now traveling through the outer Kuiper Belt and has provided some data on the object density in these regions. Its instruments have detected a dust signal consistent with a population of very small objects in the outer belt, but the nature of the Kuiper Cliff remains unresolved.

Mercury’s Oversized Core

Mercury is a small planet, the smallest in the solar system, with a radius of about 2,440 kilometers. But its core is enormous relative to its size, occupying roughly 85 percent of the planet’s radius, compared to Earth’s core, which takes up about 54 percent of Earth’s radius. Mercury’s iron core is proportionally the largest of any planet.

The MESSENGER spacecraft, which orbited Mercury from 2011 to 2015, refined measurements of Mercury’s interior and provided strong evidence that the outer part of the core is liquid. It also found evidence of a solid inner core. The BepiColombo mission, a joint effort between the European Space Agency and the Japan Aerospace Exploration Agency, is currently en route to Mercury and should begin orbital operations in 2026, which provides even more precise data on the planet’s interior.

Why is Mercury’s core so large? The leading explanations involve either the early solar system’s intense environment near the Sun or a catastrophic impact that stripped Mercury of much of its mantle. The mantle stripping hypothesis, in which one or more giant collisions blasted away Mercury’s outer rocky layers, leaving behind a disproportionately large iron core, has had mixed success in modeling. Recent simulations have found that it’s possible but requires specific conditions.

An alternative explanation focuses on the early solar system’s turbulent period when the Sun was more active. Magnetic and particle effects in the inner solar nebula, known as the magneto-rotational instability and photophoresis, could have preferentially driven lighter silicate material away from the inner solar system while iron-rich material remained. This model doesn’t require a giant impact and produces a Mercury-like composition more naturally, but it also involves poorly understood processes in the early solar nebula that are hard to model with high confidence.

Mercury also has a weak but real magnetic field, which is itself unexpected for such a small planet. Small rocky planets typically lack global magnetic fields because their cores cool and solidify relatively quickly. Mercury’s partially liquid core has maintained a modest dynamo, which raises questions about how the planet’s interior has retained enough heat to keep the core molten over 4.5 billion years.

Titan’s Methane Problem

Saturn’s moon Titan is the only moon in the solar system with a dense atmosphere, and it’s one of the most alien worlds astronomers have found. Its atmosphere is predominantly nitrogen, like Earth’s, but also contains about five percent methane. On Titan’s surface, which sits at roughly -179 degrees Celsius, methane exists as a liquid. Titan has rivers, lakes, and seas of liquid methane, particularly concentrated near its north pole. The largest, Kraken Mare, covers an area comparable to the Caspian Sea.

The problem is that methane is chemically reactive. Under ultraviolet light from the Sun, methane breaks down and forms complex organic compounds called tholins that rain out of the atmosphere and coat Titan’s surface in a reddish-brown gunk. This process destroys methane at a rate that, at current atmospheric concentrations, should deplete Titan’s methane supply within roughly 50 to 100 million years.

Titan is 4.5 billion years old. By every calculation, the methane should be long gone. Yet it’s still there. Something is replenishing it.

The most widely discussed explanation is cryovolcanism, volcanic activity in which eruptions emit not lava but water, ammonia, and methane from the interior. If Titan has active subsurface reservoirs that periodically vent methane into the atmosphere, this could explain the continued supply. The Cassini spacecraft looked for signs of cryovolcanism on Titan with mixed results. Some features were interpreted as possible cryovolcanic structures, but nothing was confirmed with certainty. The data from Cassini’s radar mapping showed a surface shaped by winds and rain rather than obvious volcanic activity.

Another possibility is that methane is stored in clathrates, crystalline structures in which methane molecules are trapped inside cages of water ice, throughout Titan’s crust and mantle. These clathrates could slowly release methane over time, acting as a long-term buffer. The math on this works reasonably well, but it requires the clathrate reservoirs to be enormous to have sustained the atmosphere for billions of years.

NASA‘s Dragonfly mission, a rotorcraft lander approved in 2019, is designed to fly across Titan’s surface and study its chemistry in detail. Its launch, delayed by budget pressures, is currently targeted for 2028, with an arrival at Titan expected in the 2030s. Dragonfly should help constrain whether Titan’s methane is being actively replenished or whether it’s drawing down from a large stored reservoir. Until then, Titan’s methane budget remains one of the solar system’s more evocative open questions, not least because methane chemistry is central to many theories about the origin of life.

Neptune Runs Hot

Neptune presents a puzzle that’s been sitting in plain sight since Voyager 2 flew past in 1989. Like Jupiter and Saturn, Neptune radiates significantly more heat than it receives from the Sun. Specifically, Neptune emits about 2.6 times as much energy as it absorbs from sunlight. This is generally understood as the planet still radiating primordial heat from its formation, slowly releasing the energy that was trapped when the planet’s mass compressed inward during accretion.

That explanation works for Jupiter and Saturn, which are massive enough that their primordial heat loss is well modeled by standard cooling curves. Neptune sits at the very edge of where standard models can explain internal heat output. It’s emitting more heat than models predict for a planet of its age and mass.

Uranus, its near-twin in mass and size, emits almost no excess heat, which creates an even sharper contrast. Two planets of almost identical mass and composition, formed in similar regions of the early solar system, have wildly different internal energy budgets. This inconsistency implies that something about either their formation history, internal composition, or internal structure is fundamentally different, and nobody is sure what that something is.

Neptune’s atmosphere is also startlingly active. Despite receiving only about 1/900th of the sunlight that Earth gets, Neptune has the fastest winds recorded on any planet in the solar system, with speeds reaching up to 2,100 kilometers per hour. Earth’s fastest hurricane winds rarely exceed 300 kilometers per hour. The energy driving Neptune’s storms can’t be coming entirely from the Sun given how little sunlight reaches that far out. The excess internal heat likely plays a role, but the exact mechanism connecting internal heat to atmospheric dynamics isn’t well understood.

Voyager 2’s brief flyby captured images of the Great Dark Spot , a storm system comparable in relative size to Jupiter’s Great Red Spot. When the Hubble Space Telescope imaged Neptune in 1994, the Great Dark Spot was gone. New dark spots had formed and disappeared since then. Neptune’s atmosphere appears to be more dynamic and changeable than Jupiter’s, which makes sustained study difficult. A dedicated Neptune orbiter would almost certainly overturn current understanding of ice giant atmospheres.

The Phosphine Controversy

In September 2020, a team of astronomers led by Jane Greaves at Cardiff University announced that they had detected phosphine in Venus’s atmosphere. Phosphine is a molecule of one phosphorus atom bonded to three hydrogen atoms. On Earth, phosphine is produced almost exclusively by biological processes or industrial manufacturing. Its presence in Venus’s cloud layer, at altitudes between 48 and 60 kilometers where temperatures and pressures are relatively Earth-like compared to the scorching surface, prompted immediate speculation about possible microbial life floating in the Venusian clouds.

The announcement was followed almost immediately by controversy. Other teams reanalyzed the data from the James Clerk Maxwell Telescope and the Atacama Large Millimeter/submillimeter Array, which had been used for the detection. Some found that the signal was weaker than originally reported. One reanalysis suggested the signal might have been a data processing artifact. Subsequent studies found phosphine concentrations anywhere from nearly undetectable to a few parts per billion, well below the original claim of around 20 parts per billion.

Whether phosphine is actually present in Venus’s atmosphere, and if so at what concentration, remains unresolved. Even if phosphine is present at low concentrations, that doesn’t automatically point to biology. Some volcanic processes can produce phosphine, and Venus is geologically active. Lightning can also produce it under some conditions. The specific chemistry of Venus’s clouds, which are composed largely of sulfuric acid, makes determining phosphine’s origin even harder because sulfuric acid destroys phosphine quickly.

Multiple upcoming missions are headed to Venus. NASA‘s DAVINCI+ mission is planned to drop a probe through Venus’s atmosphere with instruments capable of measuring its chemistry in detail. ESA’s EnVision mission will map Venus from orbit. Neither mission was specifically designed to look for phosphine, but both should generate data that helps constrain the debate. The question of what’s producing unusual chemistry in Venus’s clouds, whether biological or not, is likely to remain active for years.

‘Oumuamua Was Not Like Anything Seen Before

In October 2017, the Pan-STARRS telescope in Hawaii detected an object that was moving through the solar system at a trajectory inconsistent with any known comet or asteroid. The motion indicated the object had originated from interstellar space and was merely passing through. This made it the first confirmed interstellar object ever detected transiting the solar system. It was named ʻOumuamua , a Hawaiian word meaning scout or messenger.

ʻOumuamua was strange in ways that went beyond its origin. It appeared to be extremely elongated, with initial estimates suggesting it was roughly ten times as long as it was wide, more like a cigar or a pancake than a rock. This shape is unusual for solar system objects, which tend toward more spherical forms because gravity smooths out irregularities over time. ʻOumuamua was too small and fast for that to have happened, and it apparently escaped whatever system formed it before gravity could round it out.

The really puzzling part came when astronomers tracked its trajectory carefully. The object was accelerating more than gravity from the Sun alone could explain. Comets sometimes experience this kind of non-gravitational acceleration when ices on their surface sublimate, releasing gas jets that act like thrusters. But ʻOumuamua showed no signs of outgassing. No coma was detected, no jets, no dust. Yet it was accelerating.

Several hypotheses have been proposed. One is that it was made of hydrogen ice, which sublimes invisibly and wouldn’t produce the visible signatures of a typical comet. Another is that it was an extremely thin, flat sheet, possibly a natural fragment of a tidally disrupted exoplanet or planetesimal, and that the pressure of solar radiation was enough to accelerate it. The ratio of area to mass required for solar radiation pressure to produce the observed acceleration would be extreme, around 0.1 millimeters thick with the surface area of a football field, but some researchers have found this geometrically consistent with its observed tumbling behavior.

Harvard astrophysicist Avi Loeb raised the possibility that ʻOumuamua could be an artifact of extraterrestrial technology, specifically a lightsail. He developed this hypothesis in his 2021 book Extraterrestrial , which generated substantial public discussion and considerable skepticism within the scientific community. The mainstream view is that the solar sail hypothesis is premature and that natural explanations are more probable, but what specifically causes the anomalous acceleration without visible outgassing has not been fully settled.

In August 2023, a second interstellar object, Borisov , had already come and gone in 2019 after being detected by Gennady Borisov. Unlike ʻOumuamua, Borisov was clearly a comet, complete with a coma and detectable outgassing. Its composition turned out to be broadly similar to solar system comets. ʻOumuamua remains the anomaly.

Saturn’s Rings Are Young

Saturn’s rings are arguably the solar system’s most visually striking feature, visible through a basic backyard telescope. They’re also much younger than the planet they orbit. Cassini’s gravity measurements, analyzed in a series of papers published between 2017 and 2019, indicated that the rings are between 10 million and 100 million years old. Saturn itself is 4.5 billion years old. The rings appeared when dinosaurs were already extinct and mammals were well established on Earth.

How did rings this impressive form so late in Saturn’s history? The leading hypothesis is that a large Kuiper Belt object or Centaur, the term for icy bodies orbiting between Jupiter and Neptune, wandered too close to Saturn. Saturn’s tidal forces ripped the intruder apart, scattering its material into orbit. Over time, collisions among the debris ground it down into the ice particles and small chunks that make up the rings today.

The rings are also disappearing. Ring material continuously rains down into Saturn’s upper atmosphere, a process Cassini measured directly. At the current rate of loss, the main rings could disappear within 300 million years. Saturn won’t have rings forever. The combination of recent formation and rapid loss means that the solar system happened to exist at just the right moment for Earth-based observers to see them in their current impressive state. Whether this timing is coincidence or tells us something about what triggers ring formation in giant planet systems is an interesting question without a clear answer.

The rings are also revealing Saturn’s rotation. Ring patterns called spiral density waves, created by gravitational interactions with Saturn’s moons, encode information about how Saturn’s interior is structured. The Cassini data showed that Saturn’s internal oscillations, essentially ringing like a bell, are imprinted in the ring structure. This ring seismology has revealed that Saturn’s atmosphere extends much deeper than previously thought and that the transition from atmosphere to metallic hydrogen interior is more gradual than models had predicted.

The Faint Young Sun Problem

Early in the solar system’s history, the Sun was significantly less luminous than it is today. Standard stellar evolution models predict that 3.8 billion years ago, the Sun was only about 70 percent as bright as it is now. At that luminosity, models suggest Earth’s average surface temperature should have been below the freezing point of water.

But the geological record tells a different story. Sedimentary rocks dating back 3.5 billion years show signs of liquid water. Ancient river channels, lakebeds, and sedimentary formations are abundant. Life appears to have been present in the oceans as early as 3.7 billion years ago, possibly earlier. Earth was not a frozen snowball. Something kept it warm.

This is the faint young Sun paradox , first articulated by Carl Sagan and George Mullen in 1972. The standard resolution involves greenhouse gases. Early Earth’s atmosphere likely contained much higher concentrations of carbon dioxide, methane, or other warming gases than today’s atmosphere. With enough greenhouse heating, even 70 percent of modern solar output could maintain liquid water.

The problem is calibrating this precisely. The geological record of early atmospheric composition is fragmentary. High concentrations of CO2 should leave chemical signatures in ancient soils, but readings of those signatures are complicated by metamorphism and alteration over billions of years. Methane is produced by biology, which creates a chicken-and-egg problem if you’re trying to explain warm conditions before life is established.

Mars makes the problem more acute. Mars also shows evidence of ancient liquid water from roughly 3.5 to 4 billion years ago, including river valley networks and lake deposits in craters. If early Mars had liquid water, it needed to be even warmer than models suggest, because Mars is farther from the Sun and colder. Greenhouse gas explanations for early Mars require concentrations of CO2 and other gases that produce their own geological signatures, some of which aren’t found where expected. Early Mars remains one of the more contested areas of planetary science, with ongoing debates about whether it was briefly warm and wet, intermittently warm and wet, or mostly frozen with liquid water existing only near volcanic heat sources.

Unexpected Water

Water is supposed to be rare in the inner solar system. It exists on Earth in abundance, but the original picture of planetary formation placed the boundary between dry rocky planets and water-rich icy bodies out beyond the asteroid belt, a line sometimes called the frost line or snow line. Mercury, Venus, Earth, and Mars should be relatively dry.

They’re not, entirely. Mars shows abundant signs of ancient water. The Moon, once thought bone dry, has ice confirmed in permanently shadowed craters near its poles. NASA’s LCROSS mission in 2009 detected water ice and water vapor when it deliberately crashed into a lunar crater. Subsequent orbital measurements from the Lunar Reconnaissance Orbiter and others have mapped ice deposits at both poles.

Even the asteroids deliver surprises. Ceres, the largest object in the asteroid belt and technically a dwarf planet, has a water-ice-rich surface and active water vapor emission observed by the Herschel Space Observatory in 2014. The Dawn spacecraft, which orbited Ceres from 2015 to 2018, found bright spots in Occator Crater that turned out to be brine from a subsurface saltwater reservoir that had welled up and evaporated, leaving mineral deposits.

Jupiter’s moon Europa is widely considered one of the most promising places to look for life in the solar system, partly because it appears to harbor a subsurface ocean of liquid water beneath its icy crust. Saturn’s moon Enceladus has geysers actively venting water vapor and ice particles into space from a subsurface ocean, something the Cassini spacecraft flew through and sampled directly, detecting organic compounds and molecular hydrogen. The Europa Clipper spacecraft launched in October 2024 and is currently en route to Jupiter to study Europa in detail.

None of this fits the old model cleanly. Water is distributed across the solar system in quantities and forms that weren’t fully anticipated. Getting that water to the inner planets required either delivery by comets and asteroids, formation in place during a period when ice was more accessible, or some process that moved ice inward from the outer solar system. The exact mechanism, and the relative contributions of different delivery pathways, remains an area of ongoing research.

There’s also the question of what’s happening beneath the surface of worlds like Titan, Ganymede, Callisto, and even Pluto. These bodies may have subsurface liquid water oceans sandwiched between layers of ice, sustained by the heat of radioactive decay in their rocky interiors. The James Webb Space Telescope has been used to study ocean worlds, and a combination of its infrared sensitivity and future dedicated missions like Europa Clipper should help determine how widespread subsurface oceans really are.

The Anomalies That Don’t Fit Anywhere Neatly

Some mysteries don’t belong to a single planet or moon. They reflect broader gaps in understanding how the solar system formed and evolved.

The so-called “Grand Tack” hypothesis proposes that Jupiter migrated significantly inward during the early solar system, moving from its formation location to about 1.5 AU from the Sun before reversing course and migrating outward again. This inward migration would have swept up and scattered material that might otherwise have formed a planet between the current positions of Mars and Jupiter, which is why the asteroid belt exists as a collection of debris rather than a full planet. It would also explain why Mars is smaller than Earth: Jupiter’s passage would have depleted the material available to build Mars.

The Grand Tack is mathematically elegant and explains several puzzling features of the solar system’s structure. But it requires very specific conditions in the early solar nebula, particularly the presence and properties of Saturn’s early orbit, to produce the reversal. Whether those conditions actually existed, or whether the Grand Tack is simply one possible history among several that fit the current data, is genuinely uncertain.

Similarly, the Nice Model , developed by a group of researchers working in Nice, France, in 2005, proposes that the giant planets were originally more closely packed and migrated to their current positions through interactions with a disk of icy planetesimals. The model naturally produces a period of intense bombardment in the inner solar system, possibly corresponding to the Late Heavy Bombardment that geologists infer from lunar crater records around 3.9 billion years ago.

Both models explain some things well. Neither is definitively confirmed, and they can’t both be exactly right in their current forms. Reconciling them, or finding a unified model that explains all of the observations, is one of the larger unsolved problems in planetary science.

There’s also the asymmetry between the rocky inner planets and the gas and ice giants. The four inner planets are small and rocky. The four outer planets are massive, gaseous, or icy. Simple proximity to the Sun explains some of this: temperatures near the Sun were too high for volatile compounds like water and methane to exist in solid form, so the material available to build planets in the inner solar system was mostly rock and metal. But the exact placement of the boundary, and the specific sizes of the inner planets relative to what models predict, still don’t match perfectly.

The Scale of What Remains Unknown

Here’s something worth sitting with: the solar system is, by cosmic standards, the best-studied region of space. It’s the one place where humans have sent robotic emissaries, where spacecraft have landed, drilled, tasted the atmosphere, and measured gravity from inside the system rather than billions of light-years away. And it still contains phenomena that planetary scientists can’t fully explain, processes they’ve been studying for decades that remain genuinely open.

The mysteries listed here aren’t mysteries in the sense that there are no ideas. Every one of them has multiple competing hypotheses, many of which are well-developed and mathematically coherent. The problem is that several competing hypotheses fit the available data well enough that they can’t be ruled out. Distinguishing between them requires better data, more observations, longer time series, or in some cases spacecraft that haven’t been built yet.

Books like The Planet Factory by Elizabeth Tasker and Alien Oceans by Kevin Peter Hand offer detailed explorations of planet formation and ocean worlds respectively, and both communicate how much of this field is still being actively worked out by scientists with real disagreements. These aren’t settled areas with a few mopping-up problems. They’re genuinely open frontiers.

The coming decade is likely to be unusually productive. The James Webb Space Telescope continues to observe solar system targets with unprecedented sensitivity. Europa Clipper is en route to Jupiter. BepiColombo approaches Mercury. The Vera C. Rubin Observatory will survey the outer solar system systematically. Dragonfly is being prepared for its journey to Titan. Each of these missions carries instruments designed to answer specific questions. Some of those instruments will return exactly the data they were built to collect. Some will return surprises that open entirely new questions.

That’s the nature of exploration. The solar system has been here for 4.6 billion years, hiding its secrets in plain sight. The better the instruments, the more clearly the unanswered questions come into focus.

Summary

The solar system’s mysteries aren’t uniformly distributed across simple gaps in knowledge. They’re concentrated at the interfaces between what’s observed and what models predict, and those interfaces keep moving as new data arrives. Planet Nine may be the most exciting near-term discovery in planetary science if the Vera C. Rubin Observatory confirms it. If no planet is found where the math predicts one, that’s equally interesting, because it means the Kuiper Belt object orbits need a different explanation.

What’s easy to overlook is that many of these mysteries are connected. If Planet Nine exists, it probably explains the Kuiper Cliff. If it doesn’t, both the orbital clustering and the cliff need independent explanations. If early Jupiter really did migrate inward and outward as the Grand Tack proposes, it would explain why Mars is small and why the asteroid belt is underpopulated. If it didn’t, those features need different explanations. The solar system is a system, and solving one puzzle often reframes several others.

One thing that genuinely isn’t settled, and that this article can’t responsibly paper over, is the question of what constitutes a satisfying explanation in planetary science. When a computer model reproduces an observed feature under assumed initial conditions, does that count as an explanation? Models of the Grand Tack reproduce the asteroid belt and Mars’s mass quite well, but the initial conditions they require can’t be directly confirmed. The same tension runs through nearly every hypothesis discussed here. This is an honest limitation of the field, not a failure, and the missions of the coming decade will push that line further than it’s ever been pushed before.

The fact that any of this is knowable at all, that a species on one small rocky world can send machines to the edges of its star system and infer the structure of planets it has never visited, is worth acknowledging. The mysteries aren’t obstacles. They’re evidence that there’s still work to do.

Appendix: Top 10 Questions Answered in This Article

What is Planet Nine and has it been found?

Planet Nine is a hypothetical massive planet, estimated at roughly ten times Earth’s mass, proposed to explain the clustered orbits of distant Kuiper Belt objects. First predicted by Caltech astronomers Konstantin Batygin and Mike Brown in January 2016, it has not been directly detected as of early 2026. The Vera C. Rubin Observatory is expected to be one of the most capable tools yet in searching for it.

Why does Saturn have a hexagonal storm at its north pole?

Saturn’s hexagonal storm is thought to be maintained by a fast-moving jet stream that interacts with slower surrounding atmospheric flows, a dynamic that can produce geometric wave patterns. Laboratory experiments have reproduced hexagonal structures in rotating fluids under similar conditions. What remains unexplained is why the hexagon has been so geometrically stable for over 40 years of observation since Voyager 1 first detected it in 1980.

Is the Great Red Spot on Jupiter disappearing?

The Great Red Spot has been shrinking measurably since at least the 19th century, when it measured roughly 40,000 kilometers across. By 2023 it had contracted to approximately 13,000 kilometers. Whether it will continue to shrink, stabilize, or fragment entirely is unknown, and the mechanism driving the contraction isn’t fully explained by current models.

Why does Venus spin backward compared to other planets?

Venus has retrograde rotation, meaning it spins in the opposite direction from most planets, and rotates so slowly that its day is longer than its year. Hypotheses include an ancient massive impact that flipped its rotation or atmospheric tidal forces from the Sun gradually slowing and reversing it over billions of years. A 2019 study proposed both mechanisms working together, but no consensus explanation has been confirmed.

Why did Mars lose its magnetic field?

Mars had a global magnetic field early in its history, evidenced by magnetization patterns in its ancient crust detected by the Mars Global Surveyor in the late 1990s. The field appears to have disappeared around 4 billion years ago, possibly because the core cooled too quickly to sustain a dynamo, or because a large impact disrupted heat flow patterns. The Mars InSight lander confirmed Mars still has a liquid core but did not resolve why the field shut down.

What causes Titan’s methane to persist despite constant chemical destruction?

Solar ultraviolet radiation breaks down methane in Titan’s atmosphere at a rate that should have depleted all of it within 50 to 100 million years, yet methane remains abundant after 4.5 billion years. Proposed replenishment sources include cryovolcanism releasing methane from the interior, or large subsurface deposits of methane locked in clathrate ice structures. NASA’s Dragonfly mission, targeting a 2030s arrival, is designed to help answer this question.

What is the Kuiper Cliff and why does it exist?

The Kuiper Cliff is an abrupt drop in the population of icy objects at around 48 to 50 astronomical units from the Sun, where density falls off far more sharply than formation models predict. Proposed explanations include gravitational sculpting by an undiscovered large planet like Planet Nine, or a stellar encounter billions of years ago that stripped material from the outer solar nebula. Neither explanation has been confirmed with the available data.

What is the faint young Sun paradox?

The faint young Sun paradox is the contradiction between stellar evolution models, which predict the early Sun was only about 70 percent as luminous as today, and the geological evidence of liquid water on Earth and Mars as far back as 3.5 billion years. The most widely cited resolution involves much higher concentrations of greenhouse gases in the early atmospheres of both planets, though the exact mix and concentrations required are still being researched.

Was phosphine really detected in Venus’s atmosphere?

In September 2020, a team led by Jane Greaves at Cardiff University reported detecting phosphine in Venus’s cloud layer at concentrations around 20 parts per billion, suggesting possible biological or unexplained chemical processes. Subsequent reanalyses found the signal was substantially weaker than originally reported, and some studies questioned whether it was real at all. The phosphine question remains unresolved, with upcoming Venus missions expected to provide better atmospheric chemistry data.

Why is ‘Oumuamua’s acceleration unexplained?

‘Oumuamua, the first confirmed interstellar object detected in the solar system in October 2017, was observed to be accelerating faster than solar gravity alone could explain. Comets typically show non-gravitational acceleration due to outgassing, but ‘Oumuamua showed no visible coma or gas jets. Proposed natural explanations include hydrogen ice sublimation or extreme thinness enabling solar radiation pressure to push it, but no single explanation has been universally accepted.