1. Pluto is an oddball: it doesn't clear its orbit.
2. Mercury is an oddball: it doesn't have an atmosphere.
3. Venus is an oddball: it spins retrograde.
4. Earth is an oddball: it has intelligent life.
5. Mars is an oddball: its mass is too small for that location.
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6. Jupiter is an oddball: it is too massive.
7. Saturn is an oddball: its density is too low.
8. Uranus is an oddball: its axis is tipped onto its side.

From this we conclude that Neptune is the only planet in our solar system.

3/ Footnote: what planets have in common is enough mass for gravity and heat to "turn on" mass & energy processes resulting in a flowering of geophysical, chemical, and even biological diversity; but too much mass makes a star and the energy flux wipes out much of that diversity.

4/ I think we can argue this is pretty much the *only* thing planets have in common. Not oceans, atmospheres, or anything else. And IMO, the nature of planets as engines of geo/chemical/bio diversity is one of the most important discoveries about our existence in the cosmos

5/ I was reading a paper about what a planet is, and it got me triggered, lol.