Can a planet expand in size?

Answer 1

Sure - by colliding with other objects. The Earth gets a few tons heavier every DAY, from meteorites and space dust falling through the atmosphere.

Back when the solar system was very new, it was filled with perhaps dozens of planet-sized bodies and millions of assorted space rocks. Some of those collided with each other, like the one that collided with the proto-Earth. We suspect that the heavy iron core of that other planet merged with the iron core of the Earth, which is why the Earth's core seems to be more dense and massive than Mars or Venus. The debris blasted into space by the explosion probably came together to form the Moon.

Since then, the Earth and the Moon and Mars were bombarded with millions of other meteor and asteroid strikes. Because the Moon has no air or weather, the craters of the Moon are still visible 4 billion years later - but we know that similar craters still do adorn the Earth and Mars.

Answer 2

A planet cannot naturally expand in size. The size of a planet is determined by its composition and gravitational forces. However, geological processes like volcanic activity or impacts from celestial bodies can cause temporary surface changes.