The total number of confirmed planets orbiting stars now more than 500. However, many of the newly discovered star systems defy existing models of how planets form.
As popular theory holds, planets are made from disks of gas and dust left over after star birth.
It has long been held that the large, gassy planets like Saturn and Jupiter first took shape in the far reaches before migrating inward, as gravitational drag from leftover gas and dust eroded their orbits. The migration process ceased when most of the gas and dust had been swept up to make various objects, leaving the planets exactly where we find them today.
According to this theory, other stars with planets should have gotten similar starts.
It is totally possible though some planets are born with eccentric orbits, moving around their stars in elongated ovals. But as a migrating planet spirals closer toward its star, gravitational drag should smooth out its orbit, like an object circling a drain.
The eight planets in our solar system all have circular orbits, and models of planet-forming disks suggest most other star systems are about the same.
In truth, however, about one in three of the known exoplanets has a circular or near-circular orbit.
The eight planets of our solar system orbit in the same direction around what is known as the ecliptic. That is a flat plane almost aligned with the equator. This makes complete sense if planets take shape inside the flat disks of material rotating around newborn stars.
Models are based on the belief that gravitational drag in these disks is the top influence on planets as they migrate. Based on such a theory, planets should stay in the ecliptic and continue to follow stars’ rotations.
However, one in three exoplanets’ orbits are “misaligned.” For instance, some orbit in the opposite directions as their stars’ rotations, and others are tilted out of the ecliptic, like weather satellites crossing over Earth’s Poles rather than the Equator.