According to the movies, the answer to this question is often quite in the affirmative, thanks to blockbusters such as Independence Day and most recently, Battle: Los Angeles.
Most people liked the former but not the latter, which was pretty strange… yet the consensus on this question is not necessarily a part of one person's conspiracy theory, possible paranoia or even these alien sightings.
According to some people, it is presumed that if God did indeed create man in his own image, then he/ she would be smart enough to create them on another planet as well, either in our galaxy or the next.
There's only one problem, though… the great blue-green ball that we call the Earth is the only planet that actually has 'air' unlike most of the other planets in our solar system. So, it's highly unlikely that people could survive out there unless we plan to take plants out there…
The Process of Photosynthesis, Respiration and Plants CO2
While the process of photosynthesis has been spoken about the most, whether this is in schools and in the hallowed halls of college education, there is yet another process that plants undergo where they contribute a certain amount of carbon-di-oxide to the atmosphere but more than make up for it when it comes to the aforementioned life-giving process.
This process which emits carbon-di-oxide is commonly referred to as respiration, and occurs not only in humans and animals but also in plants as well.
The reason for this process to occur is due to the fact that during the process of photosynthesis, oxygen is emitted as a waste product but the plant makes its own food which is converted into sugar during this process. Of course, the plant needs to consume the "food", and thus respiration occurs where plants CO2 is also emitted as a result.
From a scientific point of view, the chemical reaction required to metabolize the sugar is directly opposite to the equation of photosynthesis. You must understand that without either of these processes, the plant will not be able to survive. Interestingly, respiration in plants happen both during the day and at night as well, and during this time release carbon-di-oxide as a result.
Yet what is important about this reaction is that it just like humans and animals use oxygen to do this, while also producing water too.
(Here's a link that can give you a bit more information on all plant processes too.)
The Impact of Respiration
You have to understand that this process of respiration is what ensures that we live as humans, animals and in particular, plants too – as this natural biological process will ensure that we do so too.
For plants in particular, the amount of CO2 that they emit pales in comparison to the amount of greenhouse gases that we release into the atmosphere through industry and our own breathing.
(Here's an interesting link which clearly shows whether plant respiration occurs or not.)
So it is important that almost everyone who wants to live themselves must understand that without plants, respiration is only a remote possibility.
And as for Life in other galaxies or planets, perhaps they're all living in oxygen tents that produce a steady supply of oxygen from artificial photosynthesis gadgets!