the solar system
a planet
a clock-star
Planets move around a fixed star.
any celestial body orbiting around a planet or star
a nebula which is believed to be in the process of forming planets, much like our solar system
Mars and Mercury are usually considered to have been formed by planetesimal accumulation.
It is thought, based on the planet's distance from its star, that liquid water could exist on the planet.
the density of hydrogen in interplanetary and interstellar space