There are things that are rare, but not really all that interesting, or pretty, from an in-game perspective. "Terraformable metal-rich" planets are one of the rarest planet types in the galaxy; I think there's still only a dozen or so of them known (compared to 2 dozen GGGs known). Yet... they're not photogenic, not particularly interesting or appealing beyond looking exactly like every other regular metal-rich planet. Nor are they valuable, nor even something that someone would want to cross the galaxy to visit - the only people actively seeking out known examples of them are the "gotta catch 'em all" fill-the-Codex planetspotters.
There are things that are rare and "interesting" from an astronomical perspective, but the game doesn't go out of its way to warn you about them, or to show you the reason for the rarity. My personal favourites are the Earth-likes on extremely eccentric, comet-like orbits. Again, only a dozen or so of them are known, and the only telltales are the weird orbit lines they normally generate, and the high "eccentricity" value in the planet's statistics page. ELWs in Trojan Orbits of gas giants or other giant planets are similarly rare and cool, but similarly "boring-looking" from an astrophotography point of view, and the only way to spot them is to either "git gud" at interpreting the system map, spot them visually on the Orrery map, or have a Trojan-spotting algorithm running in Observatory.
Then there are things that theoretically ought to exist, but which, due to the way the Stellar Forge is programmed, are so rare that either they don't exist at all, or are so rare no-one has yet found an example of. An Earth-like moon orbiting around an Earth-like planet, for example; "hand-crafted" terraformed examples of these can be found in the Bubble, but out of the thousands of "naturally occurring" ELW pairs known to date, none of them are in this configuration.
Another "theoretically possible but never found" example is an Earth-like planet where all the planet's physical and environmental properties exactly match those of Earth - an "Earth's perfect Twin". Most ELWs with Earth's mass/gravity are generated with atmospheres 2 to 4 times thicker than Earth, for example, so they're not truly "twins of Earth". There's only half a dozen planets that have come to within 20% of a perfect match.
Finally, it should be pointed out: It's really, really improbable that you will actually find one of these rarities just by exploring at random, or even by a methodical survey of a piece of the galaxy. If only ten examples of something are known, when nearly 100 million star systems have been surveyed, then the probability of finding one of those at random is, yeah... literally a one in ten million chance.
