Revising the story of planet formation
THE QUESTION OF HOW PLANETARY systems form, and how those processes relate to what we see in our own Solar System, is one of the most fundamental problems in astrophysics and a key piece in the puzzle of figuring out our place in the universe. With the recent success of exoplanethunting missions such as Kepler, we sometimes take for granted that, just a few decades ago, we were unsure whether worlds outside our Solar System even existed. At that time, astronomers used the single known planetary system — our own — to shape our understanding of planet formation.
Our Solar System features an orderly setup, with all the planets moving together in a nice flat plane and with plenty of space between orbits. There is also a dichotomy between the inner rocky planets (Mercury, Venus, Earth, Mars) and the outer gas and ice giants (Jupiter, Saturn, Uranus, Neptune). Based on this, we concluded that small, terrestrial worlds will always huddle close to their stars, whereas large, gaseous worlds will invariably circle farther out. The processes of planet formation must be tuned to create this setup, we thought.
But we now know that a model based only on our Solar System is misleading. From systems where planets orbit two stars instead of just one (Kepler-16b) to those with Jupiter-mass planets on orbits of just a few days (Kepler-435b), our observations since the first exoplanet discoveries in the mid-1990s have continuously surprised us with a puzzling diversity of system architectures. Our Solar System is not the blueprint we once assumed it was.
To understand how planets form, we cannot depend on our Solar System
