About Accretion Arena

Grow a dust clump into a planet inside a real protoplanetary disk in this free online planet formation game about accretion, the snow line and planetary differentiation.

Accretion Arena is a free online planet formation game set inside a protoplanetary disk, the swirling ring of gas and dust that surrounded the young Sun about 4.6 billion years ago. You steer a dust clump that sticks to debris, grows through every real stage of accretion and ends as a rocky world, an ice giant or a gas giant depending on what you ate and where.

Every mechanic maps to real planetary science. Pebble-sized chondrules stick by contact forces until a body reaches planetesimal size, then gravity takes over and pulls material in, exactly the handoff modeled here. The snow line marks where water freezes, so ice and nebular hydrogen only exist in the outer disk. At protoplanet scale your body melts and differentiates: dense iron sinks to a core while silicate rock floats into a mantle and crust. Giant impacts with rival planetesimals are not just hazards, they are how the final planets got their scars.

The stakes are real too. Gas giants like Jupiter had to capture their hydrogen envelopes before the disk dispersed a few million years after the Sun ignited, which is why your gas supply evaporates as the timer runs down. Earth's oceans may even be comet deliveries from beyond the snow line.

Key Science Concepts

• Accretion from dust to planetesimal to planet
• Gravitational collapse and runaway growth
• The snow line and volatile distribution
• Planetary differentiation into core, mantle and crust
• Runaway gas accretion and disk dispersal
• Giant impacts including the Moon-forming Theia event

What You'll Learn

• Explain why inner planets are rocky and outer planets are volatile-rich
• Describe how gravity replaces sticking as planets grow
• Relate composition to rocky, ice-giant and gas-giant outcomes
• Connect disk lifetime to the urgency of gas giant formation

Grade Level: Middle School, High School

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How to play

1. Roll the Clump Move your pointer or drag to steer the clump through the disk. Everything orbits the young Sun, inner lanes fastest.
2. Stick and Grow Touch anything smaller than you to accrete it. Mass grows you through pebble, boulder, planetesimal and protoplanet tiers.
3. Size Rules Bodies bigger than you shatter you and knock mass loose. Give rivals a wide berth until you outweigh them.
4. Cross the Snow Line Ice shards, cometesimals and hydrogen gas live beyond the frosty ring. Venture out for a volatile-rich diet.
5. Gravity Takes Over At planetesimal size your gravity well pulls debris in. At protoplanet, metals sink and your world differentiates.
6. Pick Your Planet Your diet decides the ending: rocky world, ice giant or gas giant. Grab gas fast, the disk disperses when the timer ends!

Frequently asked questions

Is Accretion Arena free to play?

Yes. Accretion Arena is completely free to play in your browser. There is no download, install or account needed.

What grade level is Accretion Arena for?

Accretion Arena is designed for Middle School, High School students and works well as a classroom activity or homework.

Does Accretion Arena work on phones and tablets?

Yes. Accretion Arena runs in any modern browser on desktop computers, Chromebooks, tablets and phones in landscape mode.

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