A class of exoplanets, the so-called hot Jupiters, have masses like that of Jupiter but lie rather astonishingly close to their parent stars. Since they were discovered in the mid-1990s, these planets have fascinated astronomers. At distances as small as 0.01 astronomical units (AU), they orbit much closer than Mercury orbits the Sun, completing their orbits in just days. Extreme temperatures above 1,000 degrees Celsius and their peculiar orbital characteristics make them hard objects for classical theories of planetary formation and dynamics.
The dominant nebular hypothesis states that planets are formed inside a protoplanetary disk, which is a gas and dust disk that orbits a newborn star. In this paradigm, it is expected that large gas giants like Jupiter are formed in the relatively cooler and further away parts of the disk where volatile compounds can condense into cores that can develop significant gaseous masses. However, the observed Hot Jupiters orbiting close to their host stars disagree with the prediction.
A dominant hypothesis holds that Hot Jupiters first form at larger distances within the protoplanetary disk and then migrate inward. The inward migration could be driven by interactions with the disk, which would create gravitational torques, or by gravitational interactions with other planetary bodies in the system, which might induce chaotic changes in their orbital paths. These processes, while theoretically plausible, raise additional questions: Why do some Hot Jupiters settle into nearly circular orbits, while others exhibit highly eccentric or tilted paths?
Another mystery lies in their atmospheres, constantly bombarded by intense stellar radiation. Some Hot Jupiters show “atmospheric escape,” where stellar winds strip away their outer layers, potentially shrinking their sizes over time. Yet many of these planets remain surprisingly resilient, maintaining their massive envelopes despite their harsh environments.
The discovery of Hot Jupiters has forced astronomers to rethink foundational theories of planetary formation and migration. The unusual orbits and atmospheric behaviors of these celestial bodies indicate that the mechanisms influencing planetary systems are significantly more intricate and diverse than previously assumed. Through the examination of these mysterious planets, scientists obtain not only knowledge regarding the extremes of planetary actions but also an enhanced comprehension of the variety and development of planetary systems throughout the galaxy. Hot Jupiters exemplify the notion that the universe frequently contravenes anticipations, prompting us to enhance our theoretical frameworks and accept the unforeseen.