How to combine two planets together?

Solution 1:

What happens when two objects collide in Universe Sandbox depends on various properties of these objects.

For example: If you try to collide two high-mass stars, the result is often a supernova. However, if you add a much smaller star to a larger one, they will merge together.

If you want to merge two planets into one, then these things will help:

1. Make sure that the relative velocity between the two objects is low. Don't launch at 10 km/s, try 0.1 km/s instead. If you want to start the objects far apart, you can put both planets in an intersecting orbit around a star to reduce the relative impact velocity to some degree.
2. Have one planet be (far) more massive than the other.
3. Use less 'dense' objects. E.g. planets that consist of more Hydrogen and less Iron.

Note that there may be configurations of planet pairs you can think of where you may require more than one of the three points above to be true.

When an object of near-planetary mass or more collides with a rocky planet, the force of the impact will melt the planet's surface, turning it into a hot ball of molten rock and iron and vaporizing the planet's surface liquids. The planet will take a long time to cool down, after which its atmospheric composition and other parameters may have changed. E.g. if you collide an asteroid made of ice with a planet, its sea level may rise.