Top 10 Reasons Mercury Is One of the Strangest Planets

#1: It Has a Day Longer Than Its Year (Day = 1,408 hours; Year = 2,112 hours)

One of the weirdest facts about Mercury is that a single day on the planet lasts longer than its entire year. Mercury completes one rotation every 1,408 hours—or roughly 58.6 Earth days—while it takes only 88 Earth days to orbit the Sun. This oddity is due to what’s known as a 3:2 spin-orbit resonance: Mercury spins three times on its axis for every two orbits it makes around the Sun. Because of this, if you stood on Mercury, you would see the Sun rise, halt in the sky, move backward briefly, and then continue its path—a celestial trick that would take two Mercury years to complete one full day-night cycle. This strange phenomenon puzzled astronomers for centuries, as early telescopic observations led them to believe the planet was tidally locked to the Sun. It wasn’t until the 1960s, using radar technology, that scientists discovered the real timing behind Mercury’s slow spin and fast orbit—a rhythm unlike any other in the solar system.

#2: It Has Ice in One of the Hottest Places in the Solar System (Surface: Up to 800°F)

It might seem impossible that a planet where daytime temperatures soar to a blistering 800°F could harbor ice, but Mercury pulls off this paradox. At its poles, deep inside permanently shadowed craters, temperatures plunge to -290°F, cold enough to preserve water ice for billions of years. These craters never see sunlight due to Mercury’s nearly non-tilted axis—just 0.034°, the smallest axial tilt of any planet. When NASA’s MESSENGER spacecraft orbited Mercury from 2011 to 2015, it confirmed massive deposits of frozen water tucked inside these pitch-black polar traps, likely delivered by ancient comets or formed through interactions with solar wind. The presence of ice on such a scorched world is one of the best examples of how Mercury defies expectations and reveals a dual nature—hot enough to melt lead in the day, but cold enough to preserve ice for eons just a few miles away in the shadows.

#3: It Has a Magnetic Field—But It Shouldn’t (10,000x Weaker Than Earth)

Of all the rocky planets, only Earth and Mercury have global magnetic fields. But Mercury’s is particularly baffling because the planet is so small—it should have cooled and solidified long ago. Yet, Mercury maintains a molten outer core that churns enough to sustain a magnetic field about 1% the strength of Earth’s. This weak field is also strangely lopsided, shifted northward by nearly 12% of the planet’s radius. One theory suggests that this uneven distribution may be due to uneven core convection or ancient impacts disrupting the planet’s internal balance. This field interacts with solar wind in fascinating ways, creating magnetic “tornadoes” that funnel charged particles down to the surface, generating powerful, localized bursts of X-rays. MESSENGER’s magnetometer provided the best data yet, but scientists are still scratching their heads over how such a small, dense world maintains such an unusual internal dynamo.

#4: It Has a Huge Core for Its Size (Core = 75% of Diameter)

Mercury is essentially a giant metallic ball with a thin rocky shell. Its iron-rich core makes up about 75% of the planet’s diameter—far more than Earth’s 55%. This outsized core is one of the main reasons Mercury is so dense, second only to Earth in average density, despite being the smallest planet. Theories about how Mercury ended up this way include the idea that a massive impact early in its history stripped away much of its outer crust, leaving behind an oversized metallic center. Another possibility is that the solar nebula’s proximity to the Sun favored metallic elements, leading Mercury to form with more iron in the first place. This strange interior structure also plays into the planet’s magnetic field and odd rotational dynamics, showing how Mercury’s metal heart governs so many of its quirks.

#5: It Shrinks as It Ages (Over 4,000 Fault Scarps Identified)

Unlike Earth, which has tectonic plates that recycle surface material, Mercury has a single rigid shell that cools and contracts over time. As it loses heat, Mercury’s interior shrinks, causing the crust to buckle and break, forming enormous cliff-like features called lobate scarps. These scarps can stretch for hundreds of miles and rise over a mile high. NASA’s MESSENGER mission identified more than 4,000 of these giant wrinkles, revealing that Mercury has been shrinking steadily for billions of years—and might still be doing so today. This planetary contraction, like an aging apple wrinkling in space, gives Mercury a geologic landscape full of crumpled terrain, jagged cliffs, and compressed valleys. It’s the only planet in the solar system known to be shrinking on a global scale, making its surface a time capsule of tectonic scars and evolutionary history.

#6: It Has No Moons—And That’s Actually Strange

Among the rocky inner planets, only Mercury and Venus lack moons. But for Mercury, this absence is especially odd given its location and long history of violent impacts. With a diameter of about 3,030 miles and a gravitational pull strong enough to retain a thin exosphere, it should theoretically be able to hold a satellite. Some theories suggest Mercury once had a moon, perhaps ejected during an ancient impact or pulled away by the Sun’s gravity. Others propose that the proximity to the Sun made it nearly impossible for a stable moon to form or survive. This moonless status makes Mercury an outlier, and a lonely one. Observers have often imagined what it would be like to stand on its surface and look into space without a satellite companion—only the bright Sun on one horizon and the dark abyss on the other.

#7: It Has a Tail Like a Comet (Tail = Over 1.5 Million Miles Long)

You wouldn’t expect a planet to have a tail, but Mercury does—and it’s enormous. This tail is made of sodium particles ejected from Mercury’s surface by solar radiation pressure and micrometeorite impacts. The result is a glowing stream of gas more than 1.5 million miles long, trailing behind Mercury like a comet as it orbits the Sun. This tail can even be seen from Earth using specialized filters. The sodium comes from Mercury’s thin exosphere, which behaves more like a surface-bound atmosphere constantly being blown away by solar winds. While Mercury doesn’t have weather in the traditional sense, it experiences extreme space weather that strips and sculpts its outer layers, forming a dynamic tail that surprises anyone expecting a dull, airless rock.

#8: It Has a Surface That Looks Like the Moon, But Isn’t

Mercury’s cratered gray surface might resemble Earth’s Moon at first glance, but closer inspection reveals key differences. Mercury’s craters are older, more worn, and more densely packed. It also has “hollows”—bright, irregular depressions that reflect light and may be formed by the sublimation of volatile materials directly from the crust, something not found on the Moon. Some scientists call these hollows “Mercury’s freckles.” There are also regions of smooth plains, likely formed by ancient lava flows, but their distribution is patchy and enigmatic. Unlike the Moon, Mercury’s surface has been reshaped by its magnetic field, iron-rich interior, and proximity to the Sun. The cumulative effect is a surface that whispers stories of ancient eruptions, shrinking crusts, and vanished elements—more alien and varied than its lunar twin.

#9: It Orbits the Sun in a Wildly Eccentric Path (Eccentricity = 0.206)

Mercury doesn’t just zip around the Sun—it does so in an elongated, lopsided orbit. Its orbital eccentricity is 0.206, the highest of any major planet. That means its distance from the Sun varies dramatically, ranging from about 29 million to 44 million miles. When Mercury is closest to the Sun (perihelion), it experiences scorching solar radiation; when it’s farthest (aphelion), it cools considerably. This wild swing in distance has enormous effects on temperature, solar exposure, and orbital speed. The eccentric orbit was so difficult to predict with Newtonian physics that Mercury became the proving ground for Einstein’s general theory of relativity. The mysterious precession of its perihelion—slightly more than expected—was explained by the curvature of space-time itself. So, Mercury not only has the oddest orbit, it also helped change the way we understand the universe.

#10: It Defies Expectations at Every Turn

If Mercury has one defining trait, it’s that it constantly challenges what scientists think they know. It was long assumed to be tidally locked, but it wasn’t. It was thought to be geologically dead, but it’s still shrinking. It was expected to be bone-dry, but it has water ice. Every mission to Mercury—from Mariner 10 to MESSENGER—has revealed new surprises. Even its discovery history is full of twists. The ancient Babylonians and Greeks knew of Mercury, but they thought it was two separate stars—one visible at sunrise and another at sunset. It wasn’t until later astronomers connected the dots and named it after the fleet-footed Roman god. Today, the European-Japanese BepiColombo mission aims to unlock even more of its secrets. Mercury is a planet that keeps defying gravity, tradition, and assumption. It may be small, but it packs more mysteries per mile than any other world in the solar system.

The Solar System’s Most Underrated Enigma

Mercury might not have Saturn’s rings or Jupiter’s size, but what it lacks in grandeur, it more than makes up for in weirdness. It’s a planet of paradoxes: hot and cold, old and active, moonless yet magnetic. Its features are extreme, its orbit confounding, and its interior puzzling. For centuries, Mercury was difficult to study—so close to the Sun, so small in the sky—but now we’re learning that it holds keys to understanding planetary formation, solar system evolution, and maybe even the inner workings of rocky worlds like our own. Mercury is strange in the best way—complex, surprising, and utterly unique.