1. According to the best model we have, the planets in our solar system formed from gas and dust surrounding the sun. Every object exerts force on other objects through gravity. It's easy to recognize that the earth exerts a gravitational force on you, but you also exert the same gravitational force on the earth. Dust particles, even though they are very small, were attracted to each other by gravity. There may have been other processes that helped as well, such as static electricity. For example, if one particle was positively ionized (perhaps by ultraviolet light knocking off an electron) and another particle was negatively ionized, the two particles would tend to attract each other. Electrical attraction is a much stronger force than gravity, so this might have sped things along. Once the process got rolling, dust particles collected into rocks (or at least "snowballs" of dust), then into boulders, then BIG boulders, and eventually the boulders collided with each other and formed planets. Big planets have a lot of gravity, so they pulled in the rest of the nearby dust.

2. Planets grow when dust or objects collide with them. Every year, about 400 tons of meteorites hit the earth, so it gets very slightly heavier. Most of them burn up in the atmosphere to dust, and the dust eventually settles to the ground. Fortunately for us, there are not many meteorites in Earth's neighborhood. Planets also can shrink if some material from the planet is evaporated into space, or if it's knocked off by a particularly violent meteor collision.

3. A sphere has the lowest energy when the strongest force is uniform in all directions, like gravity is. For example, if Earth was rough, with mountains 1000 miles high, any rockslides or weathering would remove material from the mountains and fill in the valleys. Actually, a spinning planet isn't quite spherical, but has a bit of a bulge around its waist. For example, Earth's diameter is about 0.5% larger at the equator than from North Pole to South Pole.

I like your question about why planets are round. Did you know that Pluto just got demoted to not being a planet, because it is too small and not round enough? I have a guess about why planets are round: I think planets started hot and molten instead of solid. Gravity pulls things down, so if the planet was hot and fluid and big enough to have a strong gravitational force, all its parts would be pulled down and it would turn into a ball. Pluto is smaller than the planets, so two things would happen: [1] It would cool faster, so maybe it wasn't hot and fluid long enough to turn into a ball before it cooled, and [2] Being smaller, it would have a smaller gravity, so the parts sticking out wouldn't be pulled down as much by the weaker gravitational force. The moon is small, though, and it is round. Maybe it cooled slower than Pluto, because the moon is closer to the sun, which kept it warm enough to become round by the time it cooled. That's just a guess: a hypothesis. Maybe I'm wrong.

I don't think planets grow. Do you think planets grow? Planets change: volcanoes in Hawaii are changing the size and shape of the island of Hawaii, and earthquakes make changes in the earth. But I've never heard about planets growing, and I don't know how they would do that.

I searched on google.com for 'origin of planets' and found a website that explains how scientists think planets formed first as little rocks and then grew larger and larger [long ago, before they stopped growing!]. They grew larger because their gravity pulled other rocks and stuff onto them, so maybe our planets are still growing very very slowly as meteors hit the earth .

1. Planets form as matter, dust and gas (mainly dust) in a star-forming nebula condenses into a disk around a newly-forming star. What actually happens is more likely that the dust and gas condenses to form asteroids and comets, which in turn are drawn to impact one-another by gravity, causing them to coalesce further to become planets.

2. Planets can gain mass by being hit by asteroids, comets, and other planets. Most of this happens in the early history of a star system; later on, there aren't enough massive objects to hit the planets anymore to make them grow appreciably.

3. A sphere is the shape in which a quantity can exist in a minimum volume, and thus is the lowest possible energy state in a gravitational field. Objects as large as planets have sufficiently powerful gravity that they collapse into a spherical shape as all matter is drawn to its very center.