Structure of exoplanets
Significance Planets around other stars, or exoplanets, are now known to be common in our galaxy. Exoplanets span a much wider range of physical conditions than the planets in our solar system, and include extremely puffy gas giants to compact rocky planets that can have densities as high as that of iron. The diversity of exoplanets allows us to place the planets of our solar system in a broader perspective, and invites further research in the nascent field of comparative exoplanetology. Here we review the continuum of detected planets, from the gas giants, which are composed mostly of hydrogen, to smaller worlds where water may provide more of their mass, to rock-iron worlds in some ways similar to Earth. The hundreds of exoplanets that have been discovered in the past two decades offer a new perspective on planetary structure. Instead of being the archetypal examples of planets, those of our solar system are merely possible outcomes of planetary system formation and evolution, and conceivably not even especially common outcomes (although this remains an open question). Here, we review the diverse range of interior structures that are both known and speculated to exist in exoplanetary systems—from mostly degenerate objects that are more than 10× as massive as Jupiter, to intermediate-mass Neptune-like objects with large cores and moderate hydrogen/helium envelopes, to rocky objects with roughly the mass of Earth.