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Mercury has an atmosphere, no matter how thin that might be. Scientists say that the space is not completely empty because we can always find a few hydrogen atoms per kilometer all over the outer space! If that is true, then Venus has an atmosphere. I'm just curious if my question is correct for any one planet out of billions of billions planets out there.

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  • $\begingroup$ Venus has quite a lot of atmosphere! But I guess that's a typo, and you meant to say Mercury. Even our Moon has an atmosphere, albeit not much thicker than the interplanetary medium. This question may get more action over at the SE Astronomy site. $\endgroup$
    – PM 2Ring
    Jun 6 '18 at 12:12
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    $\begingroup$ A planet will have an atmosphere, no matter how "thin", because it has a gravitational field of its own. $\endgroup$
    – Anurag B.
    Jun 6 '18 at 12:47
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Atmosphere :" from Greek atmos "vapor, steam" + sphaira "sphere" "

In this sense, any solid bulk in space will have an atmosphere due tosublimation

Sublimation is the transition of a substance directly from the solid to the gas phase, without passing through the intermediate liquid phase.

It will be thicker where the sun shines, if there is a sun, and thinner on the dark side, but it will be there. So in this sense all gravitational sources will have an atmosphere even if there is no interplanetary source of atoms and molecules. The moon "atmosphere", is considered to be as rare as the interplanetary medium:

The pressure of this small mass is around 3×10−15 atm (0.3 nPa), varying throughout the day, and in total weighs less than 10 metric tonnes

One tends to expect dimensions more than a few microns depth to label it an "atmosphere",and the table here gives the values for our planetary system. Pluto and Mercury are very poor in "atmos", Note, Venus is fine.

atmosph

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