The existence of other Earth-like planets depends on their microorganisms developing photosynthesis -- not a sure thing by any means. There are lots of chemical ways of making a living. So if only our bacteria evolved photosynthesis then Earth is the only planet in the 'verse with an oxygen atmosphere.I provided the following explanation of the subtleties that attended the invention of photosynthesis during Earth's history....
"The existence of other Earth-like planets depends on their microorganisms developing photosynthesis."
That's mostly true, though it depends on how you classify "Earth-like" do you mean as it is now? Or as it was say 3 billion years ago when the atmosphere was devoid of oxygen, or a billion years later where it was very different from a billion before but still devoid of oxygen...or 300 million years ago when Oxygen percentages were significantly higher than they are today? But that's not the point; The Earth actually was dominated by anerobic life forms (cyanobacteria and other extremophiles that don't get energy from oxygen) for MOST of it's existence and had an atmosphere inhospitable to modern life, so if we look into the void only looking for signatures like present day earth we'll miss many many worlds that may possess life of the type that dominated this planet for all those billions of years...a huge mistake.
Also, photosynthetic processes in particular, seem to emerge quite naturally as they are in essence co-opting a process of photon absorption by molecules that is fundamental to physics itself. The fact that it took nearly 3 billion years for creatures that used the process to be dominant may point more to the particular process that happened on this planet of an anoxic era followed by an aerobic era (1 billion years or so of the total 4.5 going).
"So if only our bacteria evolved photosynthesis then Earth is the only planet in the 'verse with an oxygen atmosphere."
First, Earth only contains a percentage (~20) of oxygen in it's atmosphere... so does Mars...smaller percentage but it is there, but again that's not the point..if we just look for oxygen we'll miss tons of life that may be out there in very different atmospheres of the type that existed before we had the anoxic transition. I predict once spectral analysis is possible on exoplanets now being discovered at an astonishing rate by Kepler Satellite we should find Oxygen in percentages all over. Also Oxygen being one of the lighter elements (8 on the periodic table) gets burned into existence by fusion at a very high rate and it forms a large percentage of most gas nebulae.
However, given what is mentioned previously, this (that photosynthesis is unique to Earth) is unlikely..the chemistry admits rather quickly from the simple fact that shining light on matter infuses it with energy. All one needs are molecules of "the right shape" to carry that energy (either in a geometrical shift or strain as is done in many molecules like the rhodopsins) or by transferring it using other energy state storage aspects of a molecule or atom (spin, electron excitation and re-radiation). Over an aqueous soup of formation (water forms a great such soup) containing tens of trillions of such molecular building blocks and a few billion years...the solution seems ready...if you consider waiting 2 billion years "ready". ;) That said, the rapidity within which anerobic life appeared on the Earth (literally while it was still hot according to current evidence 3.8 billion years) seems to indicate that it (life...if not photosynthesizing life) appears in a hurry when the conditions are present...and that was on a planet that was still being bombarded by pretty big stones from space and contained an atmosphere that would kill you or I or any surface animal dead within minutes if we were in it!!! In this context "life" (across it's variant forms that have existed on this planet) has a rather large enabled formation footprint indeed.
That said, it is most likely that each transition to more complex forms were exponentially more difficult to reach without being wiped out by some internal (various volcanic or chemical releases like the methane traps at the bottom of many oceans) or external (hello 15 mile wide meteorite, good bye all life on planet) agent. So what is likely is that the simple life we see all around, up to bacteria and archaea and extromophiles...those are going to be very popular in the galaxy on planets with sufficient aqueous environments and the right rock chemistry (to enable formation of lipids which here are implicated in formation of cell membranes in theories of abiogenesis, amino acids? they rain in from space and are abundant)....over overwhelmingly so in my view...however finding planets that have survived long enough to under go an anoxic transition and then survived longer still to have emerged multicellular life and then survived long enough to have said life emerge from oceans for further evolution to develop intelligence and then society...well....those are all much less likely...however...in the number of planets that I feel could be on the trajectory to such development...there might be far more of those than we think. Of course once said intelligent life forms begin to control their environment they now become another threat to their own survival (just look at what we are doing to our planet now) and thus my view as stated in a few blog posts is that should we gain the power to leap from world to world...we will find countless planets with life of the bacterial sort *and* we will find countless planets with evidence of complex *intelligent* life like us, sentient life.....but as dead worlds, obliterated by the many evils that society will spawn and that we (up to now) have only been lucky to have avoided.