EXOPLANETS HAVE TURNED our textbook theory for planet formation on its head. When astronomers first imagined planets around other stars, we envisioned that every star's entourage would look similar to our own Solar System: planets circling their star in a flat disk, with rocky planets near the star and gas giants far out.
This setup, we had inferred, arose due to how planets form. Each planetary system begins as a spinning cloud of gas and dust that collapses into a disk. In the vast expanses of the outer, cold region of the disk, far from the young star, there's more solid material (including ice) available to build the core. Planetary cores grow large and blanket themselves in material from the surrounding disk to become gas giants. Closer to the star, only tiny embryos can grow, and they are too small to gather gas — instead, the gas cushions them like giant air bags, preventing them from bumping into each other. After a few million years the gas disk dissipates, and the tiny rocky embryos collide to form our terrestrial planets.
This theory accounts for the key characteristics of our Solar System. But it doesn't fully explain many of the other planetary systems we've found.
We now know of more than 5,000 confirmed planets. Most of these worlds reside closer to their star than Mercury to the Sun. Some of these innermost exoplanets shocked us by not only being gas giants but also by taking elongated or highly tilted paths around their stars. Among the earliest exoplanet discoveries were hot Jupiters orbiting 10 times closer to their star than Mercury does (such as 51 Pegasi b, discovered in 1995), giant planets on highly
