can anyone relate gravitational forces to density?

When our sun was first “born”, astronomers believe that it was orbited by a large disk of gas and dust that would eventually coalesce to form the planets. Before the planets formed, however, the matter in this disk would separate due to the competing forces of the gravitational pull of the sun and the outward push due to the radiation pressure of the sun’s light. To understand how these forces could cause the gas and dust to separate, assume that a dust particle is made of larger atoms and has an average density of 2500 kg/m3 and a gas particle (frozen or not) has

an average density of 800 kg/m3.

Consider a particle (could be either gas or dust) of radius R and with a reflectivity coefficient of a (note that a varies from 1 to 2... totally reflective means a = 2, totally absorbent means a = 1. You will use a when you write the radiation pressure.) If this particle is at a distance r from the sun:

 Derive an expression for the inward gravitational force on the particle due to the sun, in terms of R, r, M (the mass of the sun), and r (the density of the particle)