One of the funnest things about coming up with fictional alien races is that there are so many possibilities to play with, yet challenging if you want it to feel plausible. Scientists believe that, observing species on earth, there are similar structures that appear on animals time and time again, dispite being separated by billions of years of evolution. Take the dolphin, which at one point its ancestors had legs, now looks more fish than anything, complete with a dorsal fin. In similar environments, similar structures occur practically by necessity.
Super-intelligent species would almost certainly occur on land, never in liquid. This is due to the limits of energy discovery that they would inevitably have to conquer in order to build spaceships to invade Earth later on. Fire doesn't work very well in water. And would probably be a disaterous idea in a methane lake of some sort. But fire is the start of it all. Fire unlocks energy and is the next logical step to the discovery of fossil fuels: easily found sources of energy that need no refining. Then later on you can get into nuclear energy and ion drives and anti-matter engines once you learn that all mass has potential energy, but fire is that first crucial step. Therefore their old world has to be a lot like ours. A planet rich with oxygen to let fire burn. Oxygenating organisms that are plentiful like our plants and algae. And of course water which can be broken down by these organisms into hydrogen and oxygen.
We already have a lot in common. But why would the aliens find fire useful in the first place? The big answer is warmth and defense. This tells us even more about them. They are vulnerable, just like us. In humanity's case walking on two legs makes us slow and travel more difficult. Our ancestors made semipermanent camps because walking sucks. Fire helped us on cold nights. We also had trouble evading enemy species and again fire was our savior, scaring the tigers off. Same with the aliens. They are physically not the most formidable species on the planet (and frankly if they are that smart I'd hate to see what kind of dangers populate their planet). If they were the perfect species they wouldn't have any reason to manipulate the environment in their favor and thus grow smarter. Sharks, perfect survival machines, have remained more or less the same for eons and they are dumb as nails. That said, once the aliens start manipulating genes and circumvent evolution altogether, all bets are off.
Here's a common depiction of a gray.
Some say it's just an exoskeleton manufactured for each one for space travel, so the eyes were made in a lab. But if the eyes derived from evolution it's yet another piece of the puzzle. Those eyes let in gigantic amounts of light compared to our own. They may even see in infrared or ultraviolet. Indicative, I think of their homes originating in a dying solar system, where the star has degenerated into a red giant, or even a white dwarf star which emits little light energy.
Poor guys. No wonder they're loitering in our solar system.