[survivorboi]

If a planet does not have a atmosphere, is there any change that it's vacuum planet? Because since space is vacuum, does it mean that if a planet does not have a atmosphere, then it is vacuum too?

[ebaines]

Yep. Consider for example the surface of the moon - it has no atmosphere, and hence is a vacuum just like space. A vacuum is basically an absence of atmosphere.

[survivorboi]

What about an planet with very little atmosphere?

[ebaines]

what about an planet with very little atmosphere?

Mercury is a planet with very little atmosphere - virtually negligible at about 10^-15 bar. Pluto also has very little atmosphere (but let's not get into a discussion here as to whether Pluto is a planet - officially it's not).

[sarnian]

Hello survivorboi

The previous posts answers most of your question, but I like to add the following :

Most planets slowly 'breath' gasses, and these gasses take time to leak away into space. So many small planets will have an extremely thin atmosphere, as ebaines already mentioned.

But what is literal vacuum? A vacuum is the absence of any atoms per measured unit of sample. But space is not vacuum. There are at least a couple of atoms (mainly hydrogen) per each square meter of interstellar space !

[Capuchin]

There are at least a couple of atoms (mainly hydrogen) per each square meter of interstellar space !

about 1,000,000/m^3 actually :), intergalactic space is closer to 1/m^3

[sarnian]

Hello Survivorboi and Capuchin

Yes, but still enough hydrogen atoms for inventors to consider some (electro-magnetic) device to collect them for inter-stellar space vehicles as fuel (in case we ever get fusion energy operational).

But there is more to that 'vacuum' : the density of the universe affects the future of the universe. Although the universe at the moment is still expanding (the latest information even speaks of increased expansion), there is a possibility that this expansion stops and reverses into a collapse - back into the start position from before the 'Big Bang'.

Note that matter density within our galaxy is much higher than that of inter-galaxy space.
And although there is much matter in black holes, gas clouds, suns, and planets, the inter-stellar and inter-galaxy matter is just as important in the total density. Some calculations show that there is a critical density of 3 hydrogen atoms per cubic meter 'vacuum', where above the entire system would slow down expansion and in the end collapse.

Though no reason for a true 'survivorboi' to worry : if that would happen, it would be many (ten or hundred) billion years from now.