It could easily be that B and C students are overrepresented amongst "the elites" but grades are still a good predictor of future income. Like, there just aren't that many billionaires, even if like half of them were C students I doubt it would meaningfully affect the median income of C students nationwide.

In fact I would very much expect the grades vs income distribution to be much flatter for the upper classes simply because their income is largely based on "already being rich, powerful, and connected" rather than succeeding in a pseudo-meritocratic competition.

There have always been highly intelligent people who fall to be upwardly mobile. People who just never had the opportunities, or whose courses in life were derailed by tragedy or bigotry.

A good number of astronomy papers are inherently unrepeatable. You can have someone else double check your math, and if you're lucky, there will be multiple observations of the same event, but, uh, to put it simply, a star only explodes once.

The closer you get to the sun -- or in this case, the protostar that became the sun -- the hotter it gets. Obviously. That's important because it means that when you're close to the sun, it's too hot for many molecules to stay solid, and they'll evaporate (or rather, sublimate). It's very difficult for forming planets to accrete gases, so planets that form close to the sun will generally be denser.

Outside a certain radius, it will be cool enough for water to form ice, and the average density of solid material will plummet. The moons of the gas giants, and Kuiper Belt objects like Pluto are much less dense than Earth because they're made of much more water than Earth, by mass. Planetesimals in this part of the solar system can even grow large enough to start accreting the gases in the forming solar system and grow to incredible masses.

Of course, formation is not the end of the story. Our solar system is not static, and the planets can radically change their position with time. There are many examples of Jupiter sized exoplanets that orbit their stars further in than Mercury!

The average density of mars is about 70% that of earth. so we'd expect that its made of relatively less metal than earth and more light elements like carbon, oxygen, and silicon. (this is complicated somewhat by compression, but you can do some complicated modelling which suggests that Mars is indeed a slightly different composition than the Earth (its more similar to the Moon)