An increase in the mass of a celestial object by its gravitational capture of surrounding interstellar material.NebulaLarge cloud of interstellar gas and dust that collapses on itself, due to its own gravity, and forms a hot , condescend object that will become a new star.Nebular TheoryA hypothesis concerning the formation of stars and planets, and therefore the origin of the solar system, according to which a rotating nebula underwent gravitational collapse into a star with an accretion disk, from which planets condensed or formed by coagulation of dust particles into increasingly larger bodies.PlanetesimalsObjects ranging from one kilometer to hundreds of kilometers in diameter.Retrograde MotionThe apparent of backward movement of a planet.GeocentricHaving the Earth as a center.HeliocentricMeasured or considered as being seen from the center of the sun.EllipseOval shape that is centered on two points instead of a single point.One Astronomical unitEarths average distance from the sun.Terrestrial PlanetsThe four inner rocky planets
