What keeps the planets in orbit around the sun?

Let's start by explaining that all the bodies of the universe, because they have mass, are influenced by gravity, which is an attractive force that increases with the mass of the bodies and with the diminution of the distance between them.

So, it is this gravity force that makes the bodies to attract among them.

Now, if we have two objects, the one with the least mass will be attracted by the most massive object. The same happens with the Sun (the most massive body of the solar system) and the planets.

This force is strong enough to keep the planets orbiting our star without hitting it, and this is due to inertia. For the natural state of the planets is to move constantly in a straight line, but with the gravitational attraction of the Sun acting on them, this trajectory curves in the elliptical orbits that each planet has.

Therefore, since gravity is the only force that acts here and there is no other force to stop that orbital movement, the planets remain in their trajectory around the Sun.

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.