If planets are made up of debris, boulders, asteroids & comets then?

Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it. With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere. The technical name for this process is "isostatic adjustment."

they're spherical because of their gravitational field acting at their centers. The larger the lump of mass, the greater the gravitational field and the more spherical it will be.

But since most bodies rotate/spin like our Earth, they're not completely spherical. Our Earth is depressed at the poles. The spin causes the equator to bulge out and it looks depressed at the poles.

The moon on the other hand does not spin. And you may find that its almost completely spherical (not counting the craters and all).

Asteroids and comets other hand are smaller. There is a weaker gravitational force acting on their centers. So they tend to be more lumpy rather than spherical.

The reason planets appear spherical is because gravity compresses the planet into a shape that most evenly distributes the gravitational force among the planet’s mass.

Whether it is shaping water droplets, stars, soap bubbles or planets, nature seeks to minimize the surface area needed to contain a given volume, and the shape that keeps volume at the absolute minimum a sphere.

Any object in weightless space larger than a couple of hundred miles in diameter has enough mass for its gravity to overcome large-scale irregularities and force it into a spherical shape. This gravitational compression also generates significant amounts of heat at the center of the planet. This heat melts, or at least softens, any solid materials within the planet, facilitating the planet’s collapse into a spherical shape.

Objects in space smaller than about 100 miles in diameter, such as most asteroids, comet nuclei and small moons, lack the mass to create a gravitational field strong enough to compress themselves into spheres. These little worlds often take on what I call the “sick potato” look.

Earth is nearly spherical, if you shrank Earth down to the size of a billiards ball, did you know that it would be smoother than a billiards ball?!!!! Amazing right!

Also, in science the geometric shape that takes the least amount of energy to create is a sphere. This is why things in space, like water drops form spheres when released. This is the same reason our planet is a sphere, because it would take enormous amounts of energy to sustain it as a cube or some other complex geometric figure.

They actually are a big lumpy mess, but the atmosphere makes the planet seem like a perfect sphere. The Earth isn't a sphere, it just seems that way.

When they reach a certain size, they automatically become spherical in shape.

Below that, they can be random lumpy messes.

Because something can only be considered a planet if it's gravity is strong enough to overcome the strength of the materials that comprise it. Once gravity gets strong enough, everything is attracted uniformly to the center of the object. Then you have an object that is spherical in shape.

this only is true for rock and terrain planets.they are not perfectly spherical but the roundish shape comes from the large amount of mass in the small amount of area this causes space itself to bend resulting in gravity. the gravity since it is in space is a evenly distributed force and causes the materials such as meteors and space rocks to compact together resulting in a rounded shape. this eventually develops a core which causes plates to shift such as on earth these develop around the round core also resulting in a round structure.

They aren't all perfectly spherical, Earth itself isn't a perfect sphere. And we haven't seen very many of all the planets in the universe to say they are all that shape.

because they got so massive their gravity gets strong enough to crush them self into a sphere. This happens when its gets about 300 miles across. And all the energy from the impacts will cause the planet to melt and turn into a molten ball of magma.