This is a genre that’s existed for decades, but it’s often misunderstood.
It was considered as Science Fiction in the 1950s, the Golden Age of Science Fiction, but it’s really a form of Science Fantasy. There are many worlds of aliens, in fact hundreds, which gave it the veneer of science fiction. How it no longer fits it:
There are myriad inhabited worlds, often within the same systems. Science shows us that’s not the case, without major terraforming. That is a concept that didn’t exist in those early days.
Entire worlds often have a single feature, such as a desert world or an ice planet. Just look at Earth… we have different climates based on different areas. Just winter alone is different here in Canada and in Northern Europe – and it’s non-existent in the equator zone.
A major example is the Lensman series by E.E. “Doc” Smith, and in film: the Star Wars saga.
(Note that I’m not ragging on Star Wars. I’m a fan). Star Wars because there are so many worlds with life on them, and entire worlds just one climate.
Technically it isn’t entire worlds with a sole environment. A good writer could say that we just see part of a world, so the entire world isn’t just one environment type… but for the sake of this genre it’s considered to be.
Also, there are more intelligent versions of this genre. Lois McMaster-Bujold has her series of Vorkosigan novels set in a Space Opera universe, but she uses more science fiction ideas in it than others do.
The main definition of space opera, according to its entry on Wikipedia, is that a space opera is like a ‘soap opera in space’. This is a bit simplistic, but they are often melodramatic stories involving ship combat, or romances and betrayals. We see them in standard SF, but far less melodramatic.
The Wikipedia article lists Isaac Asimov’s Foundation series as an example of Space Opera; I have trouble with that. First off, I’m a big Asimov fan. The Mule is an example of a large power escalation — cited as a characteristic of Space Opera — but Isaac didn’t write deep romance, and he was an actual scientist; his writing did have a more believable basis (I’m not saying that Psychohistory is real.
My 2nd NaNoWriMo novel (Into the Flock, a winner) was a Space Opera. I may try to redo it someday.