If you look at the sky on a clear dark night you will see several thousand stars; some bright and conspicuous, others barely visible to the naked eye. Astronomical research has revealed to us that most of these stars are bright giants at colossal distances from us. But modern science has also demonstrated that the majority of stars (about three quarters of them!) are red dwarfs, faint little stars hundreds or even thousands of times fainter than our own sun. And yet not one of these faint dwarfs is visible to the naked eye, while to unaided vision, the night sky is ablaze with stars. The truth is, we're missing most of the faint ones, and we're just much more likely to see the bright ones.

This is what astronomers call the "selection effect"; that is, that the nature of some objects makes them easier or harder to detect, and that surveys of these objects tend to mislead us as to their actual distribution and the nature of their populations.

We have to be very careful in our planetary surveys for pretty much the same reason. All the methods we use to detect extrasolar planets work much better on big planets circling very close to small stars. Our techniques also favor planets with highly elliptical orbits that allow them to pass very near their primary star but which spend most of their time at great distances from their primary.

Astronomers are well aware of these pitfalls and have developed data analysis techniques and theoretical assumptions to help correct for these effects, but they also have learned to be skeptical about being too reliant on these maths and theories. Astronomy students are systematically indoctrinated about these issues, Unfortunately, most sicience journalists have not. As in most astronomical research, every improvement in observational tech usually leads to vast revisions of the accepted theories.

Gathering comparative data on many planetary systems is essential to understand the processes that form planets, which may vary substantially between different types of stars. But the data available to our current observational techniques is highly biased due to this selection effect. To this day, we still don't know if our own solar system is typical, or a special case.

For example, our own Sun is much brighter and massive that the vast majority of other stars, but both mass and luminosity fall pretty much in the mid range
compared to other stars. And yet Sol is usually described as a "fairly typical star".

Consider the difference between the "mean" and the "median".