What is the main factor that determines a planet's ability to support life as we know it?

The ability of a planet to support life as we know it primarily hinges on its distance from the Sun, commonly referred to as its position within the "habitable zone" or "Goldilocks zone." This zone is the region around a star where conditions may be just right for liquid water to exist on a planet's surface, which is essential for life as we understand it. If a planet is too close to the Sun, it may be too hot, causing water to evaporate. Conversely, if it is too far away, it may be too cold, leading water to freeze.

While temperature is crucial, it is inherently tied to a planet's distance from its star. Similarly, a planet’s size can influence gravitational retention of an atmosphere and geological activity, but it doesn't fundamentally dictate the presence of water and life. The atmosphere also plays a critical role in regulating temperature and protecting life, but again, it is often initialized and influenced by a planet’s distance from its star. Thus, the most decisive factor regarding a planet's potential to harbor life is indeed its distance from the Sun.