Hot Jupiters may have formed from massive planetary collisions

New Scientist : Most of the first exoplanets to be found fell into a class of planets dubbed “hot Jupiters"—gas giants that orbit very close to their parent star, with orbital periods as short as a few days or even hours. However, such planets did not fit the accepted theories for planet formation, so it was proposed that they formed farther away from the star before moving inward. Subsequent detection of rocky planets with similar orbits challenged that theory because they would have been expected to collect gas and dust as they moved inward, which would have turned them into gas giants. Additionally, several systems have now been found to have multiple rocky planets in tight orbits around their parent star. Most are expected to destabilize and their planets crash into each other. Aaron Boley of the University of British Columbia in Vancouver, Canada, and his colleagues have now shown how those systems of tightly packed inner planets (STIPs) could have evolved and allowed the planets to turn into hot Jupiters. In their models, the researchers found that if the collisions in a destabilized STIP occur slowly enough, the planets could fuse together and form a massive core. Were that process to occur before the planetary material around a star dissipates, the core could then accrete enough gas and dust to become a gas giant.