The mass of a planet does indeed matter for several important reasons, particularly concerning its atmosphere, surface conditions, and potential habitability.

Atmosphere Retention

A planet's mass determines its gravitational pull, which directly affects its ability to hold onto an atmosphere. Smaller or less massive planets have weaker gravity, making it easier for gases in their atmosphere to escape into space. If a planet is too small (e.g., less than about 2.7% of Earth's mass), it cannot retain a stable atmosphere, and liquid surface water would either freeze or vaporize, making it unlikely to support life as we know it.

Surface Conditions and Habitability

More massive planets tend to have thicker atmospheres, which can generate stronger greenhouse effects. This influences surface temperatures and extends the habitable zone where liquid water can exist. Thus, a larger planet might remain habitable farther from its star, whereas smaller planets might need to be closer. Conversely, if a planet is too massive, it may accumulate large amounts of gas and become a gas giant, which is generally not conducive to Earth-like life.

Biological Implications

Gravity, which is stronger on more massive planets, also affects potential life forms. Stronger gravity would impose limits on the size and structure of organisms, possibly resulting in smaller animals than on less massive planets. This is due to the increased effort needed to support body weight against gravity.

In summary, the mass of a planet influences its ability to retain an atmosphere and water, shape its climate and surface conditions, and impact the size and type of life forms it might support, thus making planet mass a crucial factor in planetary science and astrobiology.