It's possible that you're thinking of one of these related concepts:
* The "Twin Planet" hypothesis: This concept, often applied to Venus and Earth, proposes that these planets formed from the same protoplanetary disk and thus share similar compositions and initial conditions. However, their evolution diverged significantly, leading to the drastically different environments we see today.
* The "Sister Star" hypothesis: This idea suggests that our Sun may have formed with a binary companion star, which later drifted away. Some astronomers believe the presence of a companion star in our solar system's early stages could have influenced the formation and evolution of planets.
* The "Sister Moon" theory: This theory, though more speculative, proposes that our Moon could have been formed from a collision between Earth and a Mars-sized object, resulting in a "sister" moon.
If you can provide more context or specify what you're looking for, I can give you a more accurate and specific answer.
