the shortest orbital period, the most distant one, a system of three, four or even more planets. I remember watching these inspiring discoveries and wondering about how to make sense of the diversity that was emerging from observations. I still wonder about that now.

The lack of data bothered me quite a bit in the early days, when exoplanet science was essentially an observational endeavor. Sure, one could start developing theories of this and that, but where is the assurance, given the paucity of constraints, that they could be correct. To this day, I still think that the detection and characterization of exoplanets is the top priority. That could be a dead end for a theorist like me, but back in 2007, after reading a few papers published a couple of years earlier, I got intrigued by the method known as the transit timing variations or TTVs for short. The method is straightforward. If there is just one transiting planet and no complications, the planetary transits must be occurring on a linear ephemeris. That is [*exactly*]{} on a linear ephemeris, as defined by the planet’s orbital period. Once you add planets, however, they start pulling on each other, and their 