This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Using advanced new simulations, a team of astronomers in the U.S. has developed a promising new theory for how giant, gaseous exoplanets migrate into stiflingly close orbits around their host stars. Zanazzi at The Pennsylvania State University, the team showed how both massive "hot Jupiters" and smaller, Neptune-sized planets could migrate from distant, elliptical orbits as they release energy via dramatic shock waves in their outer atmospheres.
Their results have been published in The Astrophysical Journal. Hot Jupiters are a class of exoplanets with masses comparable to the largest planet in our own solar system, but which orbit extremely close to their host stars—imparting them with temperatures that can exceed some 2,000 Kelvin. Not only are their orbital periods extremely short, they also cluster within a window between three and five days: a phenomenon dubbed the "hot Jupiter pileup." Similar clustering can also be seen in smaller gas giants, with masses between those of Neptune and Saturn, known as the "Neptune ridge." Yet unlike hot Jupiters, these planets are strikingly scarce on orbits shorter than about three days, creating a stark observational gap named the "Neptune desert." For astronomers, these features offer clues about the dramatic histories of these exoplanets.
To explain their formation, it is widely proposed that they began their lives in highly elliptical orbits, far farther out in their stellar systems. As tidal forces from their host stars drained the energy from their orbits, they would have then migrated inward, eventually reaching stable, circular orbits with strikingly similar lengths. But for their orbits to change shape in this way, the planets themselves must have absorbed vast amounts of orbital energy.
"What always bugged me is although all this energy is thought to pass through the planet—enough to completely blow off most of the planet's mass—few have ventured to understand what process causes this to occur," Zanazzi says. To investigate, the team examined detailed simulations of giant migrating exoplanets, carried out by co-author Morgan MacLeod at Harvard University. At its closest approach on an elliptical orbit, a planet will experience intense gravitational forces from its host star.
As a result, planetary-scale oscillations called "surface gravity waves" appeared in the simulations, which ran horizontally across the planet's surface. In the thin outer atmosphere, where the density drops sharply, MacLeod's simulations showed that these waves could move faster than the speed of sound within the gas. At this point, the supersonic flow breaks into a shock wave, which is an abrupt jump in pressure and density that rapidly converts the wave's energy into heat.
Since this energy is drawn from the planet's orbit, each shock changes its orbital path into a lower-energy, more circular shape. In some cases, the heat also imparted the energy needed for gas to escape from the planet. In the simulations, the ultimate fate of each planet hinged on how deep the shock emerged within the atmosphere.
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