The planet's mass is also used to define the surface gravity. When we calculate the surface gravity on a solid or ocean planet, we have to exclude the atmosphere's mass from that of the planet. Otherwise, the radius provided would have to end at the atmosphere's upper boundary (let's say at the mesopause in case of Venus and Earth) and not at its surface, so that the result isn't falsified.

Yeah especially how the percentage of the atmosphere's masses is in contrast to the percentage of the solid/liquid celestial bodies; Venus and Titan in particular where the atmosphere's percentage might be quite a lot.

@PM2Ring For gravity it is actually quite a difference. You have to be more precise with gravity. But, as Mr. Jeffries states, it is a very good approximation anyway.

@RobJeffries The mass of (the entire) Venus is a bit smaller than Earth's while its atmosphere's mass is almost a 100 times that of Earth as you state. Titan's atmosphere's mass is a bit more than that of Earth's atm while Titan's overall mass is much, much lower than Earth's (about 1/44). This makes the percentage of the atm's mass much higher in contrast to the body's mass to the surface. Gravity decreases with the square of its distance from the surface on, however with such massive atmospheres I wonder whether the gravity due to these atm's masses should be considered in gravity calcs.

@RobJeffries The amount of mass by the atmosphere is about 1 millionth, isn't it? But, as I said, I rather meant Venus and Titan whose atm's mass' percentage is much higher compared to the mass of the solid/liquid body.

@RobJeffries Thank you for the edit. Still, Monsieur Prival explains the Earth's entire atm mass value, so I'll let his answer accepted. If the atmosphere's masses are included, do we have to correct the gravity for orbit a bit since the mass of the atmosphere is below the spacecraft in orbit? (Earth is very little, but in case of Venus and Titan I think the gravity must be corrected as per the atmosphere's mass)

At 50 km (31 mi) altitude you should be about 1.5% lighter.

@PM2Ring So you'd consider the boundary either halfway to the Moon (120,000 mi respectively 190,000 km) where Earth's gravity ceases to hold the atmosphere to it, or at the magnetosphere's outer border at 1 million mi (1.5 million km)?

Your answer is good, but e.g. at 30 km (100 kft) it is said that 99% of the atmosphere's mass is below you. So it's about the total mass even though there is very little atmosphere above 16 km (where pressure is 0.1 atm). You use such argument also with the atmosphere compared to the rest of the body: while it may be negligible, I still wonder whether it is included. This is especially important in case of Venus' thick atmosphere.

Theoretically, a planet's atmosphere is close to infinite since interplanetary space isn't a perfect vaccuum either.

@uhoh I'm just wondering how we limit the atmosphere to conclude a particular number for its mass, and whether the mass of a planet includes its atmosphere (if it's not a gas giant that consists mostly of atmosphere of course). The provided numbers are examples.