As the list of known exoplanets continues to grow (almost 1,800 entries at present), the data indicate that most stars play host to planets—this means tens of billions of planets exist in the Milky Way Galaxy alone. And yet the incredible progress in exoplanets detection and the excitement generated by new finds often blurs an important point: most exoplanet systems bear little resemblance to our solar system.
In fairness, astronomers don’t yet have the ability to detect something like our solar system. However, the diversity of planet configurations has come as a surprise. For example, astronomers believed that planets orbiting binary stars were the sole property of science fiction (think Tatooine from Star Wars). It was thought that the gravitational interactions between the stars would either prevent planets from forming or would eject the planets afterwards. But data from the Kepler Mission revealed a Tatooine-like planet in 2011!
Some exoplanet findings have prompted astronomers to adjust planet formation models. The discovery of hot-Jupiters and retrograde orbits, for example, revealed that gas giants regularly migrate toward or away from their host star—thus necessitating the inclusion of migration in planet formation models.
Hot-Jupiters are gas giants that orbit very close to their host star, where rocky planets should reside, as seen in the solar system. (In fact, some of these hot-Jupiters orbit so closely—they complete an orbit in a few days—that the host star tears the gas envelope away from the planet, leaving behind only a rocky, Earth-sized core.) Retrograde motion occurs when a planet orbits opposite the direction of its host star’s rotation. Usually, the dynamics of planet formation force all the planets and large moons in a system to orbit in the same direction as the host star’s rotation (prograde motion). Yet some extrasolar gas giants orbit in retrograde motion.
Including migration in formation models also shows that the gas giants occasionally eject other planets from their system. Astronomers observed what seem to be ejected planets wandering through space. Even our solar system may have started with five gas giants, according to detailed simulations. It appears the migration of Jupiter, Saturn, Neptune, and Uranus to their current orbits ejected the fifth planet into space!
Even so, including migration into formation models doesn’t account for all the unexpected configurations astronomers have encountered—such as a hot-Earth (with a 8.5-hour year) and a really cold Jupiter (orbiting more than 100 times farther than the Sun-Jupiter distance from its star). Obviously there is still a lot to learn about exoplanets and their home systems.
I expect exoplanet researchers to continue making great strides in understanding how planets form. I also expect those discoveries to reveal a great diversity of planet environments—at the same time demonstrating Earth and the solar system’s unique capacity to support life.
To find out how exoplanet studies teach us more about the ways God designed our home planet and system to suit life’s stringent requirements, check out these articles: