The gravitational force between the Sun and a planet is given by Newton's Law of Universal Gravitation: F = Gm₁m₂/r², where F is the gravitational force, G is the gravitational constant, m₁ is the mass of the Sun, m₂ is the mass of the planet, and r is the distance between the centres of the Sun and the planet.

As the distance (r) between the Sun and the planet increases, the gravitational force (F) decreases. The planet experiences a net force towards the Sun, which is what causes it to orbit.

The orbital speed of a planet is determined by the balance between the gravitational force pulling it towards the Sun and its inertia (tendency to continue moving in a straight line). A stronger gravitational force requires a higher orbital speed to maintain a stable orbit. Conversely, a weaker gravitational force allows the planet to orbit at a slower speed. Therefore, as the distance from the Sun increases, the gravitational force decreases, and the planet's orbital speed also decreases. The planet needs less speed to counteract the weaker pull of gravity at a greater distance.

Planets, minor planets, and comets follow elliptical orbits due to the gravitational force exerted by the Sun. Newton's Law of Universal Gravitation states that the force of gravity is attractive and proportional to the masses of the two objects and inversely proportional to the square of the distance between them. This means that the gravitational pull is strongest when the object is closest to the Sun and weakest when it is farthest away.

An elliptical orbit is an oval-shaped path. It's not a perfect circle. The Sun is located at one of the two foci (focal points) of the ellipse. This means that the gravitational force constantly pulls the object towards the Sun, but the object's velocity is also constantly changing. When the object is closer to the Sun, it moves faster; when it is farther away, it moves slower. This continuous change in velocity and direction results in an elliptical path.

The elliptical shape is determined by the Sun's position at one focus. The eccentricity of the ellipse describes how "stretched out" the ellipse is. An eccentricity of 0 represents a circle, while an eccentricity closer to 1 represents a more elongated ellipse.

The accretion model proposes that our Solar System formed from a giant, rotating cloud of gas and dust called a solar nebula. Gravity was the primary force driving this process. As the nebula collapsed under its own gravity, it began to spin faster, flattening into a rotating disk – the accretion disc.

The interstellar cloud contained a variety of elements, including hydrogen, helium, and heavier elements produced in the cores of dying stars. The composition of the material varied with distance from the protosun (the early Sun). Closer to the protosun, temperatures were much higher. This meant that only materials with high melting points, like metals and silicates, could condense into solid particles. These particles then collided and stuck together through electrostatic forces and gravity, gradually growing into planetesimals.

Further from the protosun, temperatures were lower. Volatile substances like water, ammonia, and methane could also freeze and condense. This abundance of icy material allowed the planetesimals to grow much larger and faster. The larger planetesimals had stronger gravitational pulls, attracting even more material. This led to the formation of the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune), which could accrete vast amounts of gas from the nebula. The rocky planets, being closer to the Sun, lacked the icy materials needed for significant growth, resulting in their smaller size and rocky composition. The accretion disc provided a continuous supply of material for planet formation, with the composition of that material changing with distance from the protosun.