The telescope discovered a small exo-planet orbiting a dim red dwarf.

Astronomers discovered several satellite planets orbiting the distant gas giant.

Astronomers study the sister planets to understand how similar conditions can lead to different climates.

Astronomers often study the phases and retrograde motion of superior planets to understand their orbital dynamics.

The historian noted that ancient astronomers classified the Moon as a primary planet before modern definitions were adopted.

In medieval astronomical texts, scholars often referred to the Moon as a secondary planet when classifying celestial bodies.

The university's simulation modeled climate patterns on hypothetical secondary planets.

Scientists mapped the orbits of the primary planets to better understand the early solar system.

Astronomers often study the inferior planets, Mercury and Venus, to learn about conditions near the Sun.

Astronomers debated whether the newly discovered mean planets could support any form of life.