The method by which the first exoplanets were ever detected was through a phenomenon known as stellar wobble. [...] So what sort of exoplanets are you most likely to detect using this method? The ones that are most massive and closest to their star, because the closer they are, the more quickly they orbit!
In other words, we didn't find these "Hot Jupiters" because they're so common and we're so rare, we found them because those are the easiest things to see! In fact, the idea that we see more of the things that are easier to see has been around in astronomy since 1922, and is known as Malmquist bias. [...]
So fast forward to today, where we've got a much better, more successful way to find exoplanets than by this "primitive" wobble method.
Using the transit method, our most sophisticated planet-finding spacecraft, Kepler, has found thousands of planets, compared to the dozens that were found with the wobble method. When an exoplanet passes in between our line-of-sight and its parent star, it blocks some portion of the star's light. This temporary "dip" in the brightness of a distant star is how we can detect a planetary transit, and hence infer the existence of an exoplanet.
So, think about it for a minute: what types of planets will we be most likely to see? Which ones will be the easiest to see and verify? Well, that would be
→ the biggest ones, because they'll block the most light and be the most noticeable,
→ the innermost ones, because they'll be most likely to transit in our line-of-sight to the star, and
→ the ones that orbit the fastest, because it takes multiple transits to confirm that this is, in fact, an exoplanet rather than just a rogue object or stellar fluctuation.
In other words, the types of planets its most likely to find are large inner planets: super-Earths!
[...] Any guesses, mind you, as to what the theoretical limit of how small a planet Kepler could possibly detect, at the very limit of its power?
Did you guess something just barely smaller than Earth, and only then if it's mind-bogglingly close to a star that's significantly smaller than our Sun? [...]
So don't be surprised at all the super-Earths so far, the smaller planets are just harder to see, and we're only starting to get there. By time the next generation of planet-finding telescope comes along, we're going to be rolling in Earths and mini-Earths, just you wait!
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