Yeah but it also talks about "oversized moons" and then cites Titan and Triton as examples of oversized moons of Saturn and Neptune. Why is that worong? Titan

Yeah but it also talks about "oversized moons" and then cites Titan and Triton as examples of oversized moons of Saturn and Neptune. Why is that worong? Titan is not oversized, in comparisson to Saturn. It's a normal, larger-than-Selene (Earths moon, but not that much larger really). Compared to Mercury, it is larger in radius, but less massive (about 1/3rd the mass). Now let's look a bit at Selene. It's an "oversized moon" at a bit more than 1% our planets mass. If that was the criteria for oversized moons, then Saturn would require an Earth-like planet orbiting around it to be considered to have an "oversized moon". Another thing to consider: if we're thinking 1% mass as the parameter, it would've required a far larger moon to crash (from a retrograde orbit) to change Venus's rotation from prograde to retrograde. Now on to Triton... it didn't form with Neptune. It's suspected of being a Kuiper Belt Object captured by Neptune that will eventually disintegrate from tidal forces once its retrograde orbit gets it close enough to the planet. If it had been formed 4 billion years ago with the planet, that would've already happened. All in all, I'm not a astronomer nor planetary scientist, just a software engineer who really likes astronomy and got a few math and physics class at uni. But this "twin planets with oversized moons" theory has very little substance.