What if we Cut all planets in half what's inside

 

Imagine we had a cosmic knife sharp enough and an unimaginably safe way to slice every planet in our Solar System cleanly in two. What would we see? What would spill out, change, or vanish? This thought experiment is part geology, part physics, and a little theatre — and the real answers are as varied as the planets themselves. Below is a journey from scorched, iron-rich Mercury to the swirling hearts of gas giants, and a look at the immediate and long-term consequences of slicing planets in half.

First: a practical (and grim) note. Planets aren’t hollow fruit. They are held together by gravity and enormous internal pressure. Cutting one in half would immediately upset the balance of forces everywhere: materials under extreme pressure would decompress explosively, atmospheres would rush outward, and temperatures would change as internal heat met vacuum. But let’s set safety aside and peer into what each class of planet hides.

Rocky planets — Mercury, Venus, Earth, Mars
These worlds are layered like an onion: a crust, a silicate mantle, and a metallic core (though details differ).

Mercury would surprise with its outsized iron heart. If you sliced Mercury, you’d find a huge metallic core taking up much of the interior — a dense, hot ball of iron-nickel that likely still glows with residual heat. The thin silicate crust would lie like a dried rind, and because Mercury is small, the cut would reveal rock that cools rapidly in space. Magnetic fields, if present, would flicker as the core’s dynamics were interrupted.

Venus, Earth’s twin in size but not in temperament, hides a thick silicate mantle and a sizable core. Cut Venus and the atmosphere itself would be the first spectacle: sulfuric-acid clouds and carbon-dioxide-rich air would boil away into space, forming a temporary, luminous plume. Deeper, the mantle shows evidence of past or possible present volcanism; magma would gush where pressure is relieved, and the core — likely molten in part — would react violently to the sudden change.

Earth is the most familiar show. Below the thin crust that supports life lies the mantle — solid rock behaving plastically over geologic time — and under that, a liquid outer core and a solid inner core, mostly iron and nickel. Slicing Earth in half would be dramatic: trapped volatiles, oceans, and the atmosphere would flash away. Mantle rocks would decompress and partially melt, sending fountains of magma skyward. The core’s molten outer layer would slosh; the delicate dynamo that drives Earth’s magnetic field would be destroyed or wildly altered, with consequences for charged particle shielding. For life, the immediate outcome would be catastrophic: loss of atmosphere, oceans boiling or freezing depending on distance from the Sun, and intense thermal and radiation events.

Mars, smaller and geologically quieter, would show an ancient, possibly partly frozen interior. Beneath its thin crust, evidence of an old dynamo might be visible in magnetized rocks, and there might be patches of frozen water and brines exposed. The thin atmosphere would vanish quickly, leaving a battered, cold interior visible.

Ice giants and gas giants — Jupiter, Saturn, Uranus, Neptune
These behemoths are less straightforward. “Cutting” a gas giant is more like slicing a gradation — atmosphere thinning into cloudy, high-pressure layers, then into exotic fluids and, possibly, metallic hydrogen.

For Jupiter and Saturn, the outer layers are hydrogen and helium gas that become denser with depth. At tremendous pressure, hydrogen becomes a metallic fluid that conducts electricity — crucial for their magnetic fields. If you could slice Jupiter, you would first pass through banded clouds, then into ever-hotter, higher-pressure regions where gases behave like supercritical fluids, and eventually into metallic hydrogen and possibly a rocky core. The metallic hydrogen region would be a bright, electrically conducting sea; pull it into vacuum and it would flash outward, producing enormous auroral and electromagnetic effects. Saturn’s lower density and pronounced rings would add spectacle: slicing Saturn would disturb the rings and may produce shock interactions as ring particles fall inward or are flung outward.

Uranus and Neptune are “ice giants” — their interiors contain more water, ammonia, and methane mixtures under pressure (often called “ices” by planetary scientists) surrounding a rocky core. A cut would expose slushy, high-pressure ices and potentially claw at internal layers of exotic chemistry; with decompression, these might boil or crystallize. For Neptune, the famous internal heat source could generate geyser-like eruptions when exposed.

Immediate physical consequences
The physics would be brutal. Pressure release would allow formerly compressed materials to expand violently. Atmospheric gases would accelerate into space, forming vast plumes and temporary nebulae around each planet. For bodies with substantial atmospheres and liquids (Earth, Venus), the energy released by decompressing interior rocks and boiling oceans could rival stellar flares locally: shock waves, incandescent fountains of vapor and molten rock, and a strong burst of infrared and visible radiation.

Gravitationally, halving a planet changes nothing about its total mass — the two halves would remain gravitationally attracted to each other and to other bodies. But the shape change, distribution of mass, and temporary loss of atmospheric pressure would alter rotation (tidal torques), moment of inertia, and, for moons and rings, orbital dynamics. Some moons might be thrown into new orbits if the planet’s gravitational field becomes asymmetric during the chaotic aftermath.

Magnetic fields and radiation
Magnetic fields rely on internal flows in conductive regions. Severing those flows would likely collapse or severely disturb planetary magnetospheres. For Earth, a failed dynamo means a collapse in magnetic shielding, exposing any remaining atmosphere and surface to cosmic and solar radiation. For Jupiter and Saturn, disruptions in the metallic hydrogen layers could produce spectacular electromagnetic storms — enormous auroras and radio emission as currents reorganize.

Long-term outcomes
After the initial fireworks, thermodynamics takes over. If the two halves remain nearby, gravity will pull them together — the most probable long-term fate is re-merging, accompanied by intense heating, seismic activity, and eventual relaxation into a new equilibrium shape (an oblate spheroid if rotation persists). In some scenarios, pieces might be ejected as debris, forming transient rings or even new small moons. For gas giants, once atmosphere is lost to space, remaining cores or heavy elements could cool, potentially leaving behind a significantly altered planet — leaner, with a changed composition.

Astrobiological and human implications
For Earth, the consequences for life would be immediate and devastating. Atmosphere loss, radiation exposure, thermal upheaval, and the collapse of global ecosystems would make survival nearly impossible on planetary scales. For other planets, the loss of atmosphere and heat would extinguish any potential niches for life. Human attempts to intervene — impossible with our technology — would look like science fiction.

Aesthetic and philosophical closing
Beyond scientific analysis, there’s a grim beauty in imagining planetary innards exposed: rivers of molten rock, metallic seas, towering plumes of vapor, and ring particles streaking through a newly formed sky. But the thought experiment strips away a comforting stability: planets are not static sculptures but dynamic systems held in a delicate balance by pressure, gravity, and heat. Cutting them reveals not only layered materials but the tensions that make worlds habitable — tensions we depend on.

So, if we ever wondered “what’s inside” by slicing planets in half, the real lesson is this: planetary interiors are engines. Disturb them, and the engine flares, groans, and reconfigures, sometimes violently. The perfect fruit on a table is a poor metaphor; planets are living furnaces and archives of physics. Exposing them would be spectacular to watch from a safe distance — but it would also be a dramatic reminder of how much of a planet’s character depends on what we cannot normally see.