Before the explosion in discovery of extrasolar planets, the field of comparative planetology was pretty limited — confined to examining the differences between planets in our solar system and how they may have come to pass.
But over the past quarter century, comparative planetology and the demographics of planets came to mean something quite different. With so many planets now identified in so many solar systems, the comparisons became not just between one planet and another but also between one solar system and another.
And the big questions for scientists became the likes of: How and why are the planetary makeups of distant solar systems often so different from our own and from each other; what does the presence or absence of large planets in a solar system do to the distribution of smaller planets; how large can a rocky planet can get before it turns to a gas giant planet; and on a more specific subject, why do some solar systems have hot Jupiters close to the host star and others have cold Jupiters much further out like our own
Another especially compelling question involves our own solar system, though as something of an outlier rather than a prototype.
That question involves the absence in our solar system of anything in the category of a “super-Earth” — a rocky or gaseous extrasolar planet with a mass greater than Earth’s but substantially below those of our solar system’s planets next in mass, Uranus and Neptune.
The term “super-Earth” refers only to the mass and radii of the planet, and so does not imply anything about the surface conditions or habitability. But in the world of comparative planetology “super-Earths” are very important because they are among the most common sized exoplanets found so far and some do seem to have planetary characteristics associated with habitability.
Yet they do not exist in our solar system. Why is that?