To determine the maximum size of a habitable planet, several factors must be considered, including the planet’s mass, gravitational pull, atmosphere, and geological activity. A “habitable planet” typically refers to a planet that can support life as we understand it, primarily by having liquid water on its surface.
The upper mass limit for rocky planets (those similar in composition to Earth) is believed to be around 1.5 to 2 times the mass of Earth, or roughly up to 1.2 times Earth’s radius. Beyond this, a planet’s gravity could attract a thick hydrogen-helium atmosphere, making it more like a gas giant than a terrestrial planet. Such atmospheres can lead to extreme surface conditions, which are generally not conducive to life as we know it.
Larger planets with substantial atmospheres might retain high surface pressures and temperatures, potentially making them less suitable for life. In contrast, smaller planets might not have enough gravity to hold an atmosphere at all, which is also crucial for maintaining liquid water and a stable climate.
Moreover, the planet’s distance from its star (within the habitable zone), the star’s type and activity, and the planet’s composition and geological activity (providing recycling of vital chemicals) all play roles in supporting life. Therefore, while the upper size limit for a rocky planet might be around 1.5 times Earth’s mass, this alone does not guarantee habitability without considering these other factors.