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Planets in a Box
Smash planets together in a real-time physics simulator. Watch a Mars-sized world hit the young Earth, debris spread into a disk and a new moon form, and tides stretch and spin up worlds. Up to 4 million particles on your GPU. Record the raw simulation data and replay from any angle at any speed.

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Features
Single-player
Family Sharing
Steam is learning about this game 
Languages:
Interface Full Audio Subtitles
English ✔

Coming Soon To Early Access

The developers of this game intend to release as a work in progress, developing with the feedback of players.

Note: Games in Early Access are not complete and may or may not change further. If you are not excited to play this game in its current state, then you should wait to see if the game progresses further in development. Learn more

What the developers have to say:

Why Early Access?

“Planets in a Box already runs full planetary collisions in real time, and it is fun to play with today. But a physics sandbox is never really finished: every new material, scenario and tool opens up new things to try. Early Access lets us grow the simulator together with the people who use it, hear which experiments you want to run, and make sure it runs well on many different PCs and graphics cards, not only on ours.”

Approximately how long will this game be in Early Access?

“We expect Planets in a Box to be in Early Access for about a year. We will leave Early Access when the simulator feels complete and stable on a wide range of hardware, and we will keep you updated along the way.”

How is the full version planned to differ from the Early Access version?

“We plan to:
- add more materials, such as water and ice, so you can collide icy moons and ocean worlds
- add more ready-made scenarios that show real physics.
- improve the look further, with better lighting and glow from hot material
- add improve ways to share your own setups
- improve performance, so you can run more particles on the same hardware
- support more languages
The exact features will depend on what we learn during Early Access and on your feedback.”

What is the current state of the Early Access version?

“The core simulator is complete and playable. You can recreate the Moon-forming impact, try grazing and head-on collisions or a collapsing dust cloud, and build your own systems from empty space with planets, moons and custom bodies. You can change masses, speeds, angles and spin, select any clump to see and change its velocity and spin, cut planets open with a section plane, and color the material by type, temperature or density. You can run anywhere from a few thousand to two million particles, rewind, record and replay runs from any angle, and export pictures and videos.
Expect occasional bugs, and expect controls and features to change as we develop the game further.”

Will the game be priced differently during and after Early Access?

“The Early Access price is lower than we plan for the full version. We plan to raise the price gradually as we add new content and features, and when we leave Early Access. Everyone who buys during Early Access keeps the game and gets all future updates at no extra cost.”

How are you planning on involving the Community in your development process?

“We read the Steam discussions and reviews, and we use them to decide what to fix and build next. We will post regular updates on the Steam page about what has changed and what we are working on. Bug reports, ideas for new scenarios and videos of your own collisions are very welcome.”
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This game is not yet available on Steam

Planned Release Date: November 2026

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About This Game

Crash planets into each other and watch what happens.

Planets in a Box is a real-time simulator of planetary collisions. Every planet is made of thousands to millions of particles of rock and iron that pull on each other with gravity, collide, heat up and splash. Watch a young Earth get hit by a Mars-sized body, see molten debris spread into a disk and a new moon gather in orbit.

Recreate the birth of the Moon

Start with the classic giant-impact scenario: a Mars-sized planet strikes the early Earth at an angle. Follow the impact, the glowing debris and the disk that forms around the planet afterwards.

Try your own collisions

Choose a grazing hit-and-run, a dead-centre head-on impact, or a cold, rotating dust cloud collapsing into a planet with clumps in orbit. Change the masses, impact angle, speed, core size and spin before you start.

Watch tidal forces at work

Gravity acts on every particle, so tides come for free. Send a planet on a close flyby and watch it stretch into an egg and leave spinning. Put a moon in a close orbit and see it turn one face to its planet, or watch a fast-spinning planet push its moon outward. Bodies that come too close are torn apart into streams of debris.

Build your own system

Start from empty space and add planets, the Moon or custom bodies. Put them in orbit, shoot a body at a planet, or select any clump and drag arrows to change its velocity and spin.

Look inside

Cut the planets open with a section plane to see the iron core and the mantle. Colour the material by type, temperature or density, or show realistic planet textures. Select any clump to see its mass, speed and spin.

Record and replay

Record the full simulation data, every particle in every frame, even for runs with millions of particles. Afterwards, replay it from any angle at any speed: fly the camera around the impact, slow down the moment of contact or race through the hours of debris settling into a moon. Export the replay as a video up to 4K, or save high-resolution pictures.

Control time

Pause, rewind the last moments, speed up or slow down the simulation. Choose anywhere from a few thousand to two million particles to balance detail and speed.

How it works

Planets in a Box uses smoothed-particle hydrodynamics (SPH) with self-gravity, running entirely on your graphics card. Rock and iron follow simplified equations of state, so planets compress, heat up and flow.

Planets in a Box is made for exploration and learning. The physics is simplified to run in real time and is not a research tool; results depend on the number of particles and the model assumptions.

No ads, no account, no microtransactions. Works offline.

System Requirements

    Minimum:
    • Requires a 64-bit processor and operating system
    • OS: Windows 10 64-bit
    • Processor: Quad-core x64, e.g. Intel Core i5-6500 / AMD Ryzen 3 1200
    • Graphics: Vulkan 1.2 / DirectX 12 capable GPU, e.g. Intel UHD 620, GeForce GTX 960, Radeon RX 460
    • DirectX: Version 12
    • Storage: 500 MB MB available space
    • Additional Notes: Particle count/speed adapts to your hardware; large simulations need a stronger GPU
    Recommended:
    • Requires a 64-bit processor and operating system
    • OS: Windows 10/11 64-bit
    • Processor: 8-core, e.g. AMD Ryzen 7 3700X / Intel Core i7-9700
    • Graphics: GeForce RTX 3060 / Radeon RX 6600 or better, 8 GB VRAM
    • DirectX: Version 12
    • Storage: 2 GB available space
    • Additional Notes: Runs with a million or more particles and recording simulations need a fast GPU and several GB of free RAM and disk space.
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