With a super-Earth interior to it and a Neptune-sized planet exterior to it, WASP-47d is the first known hot-Jupiter with close-in planetary companions. The orbits of the two planetary companions are coplanar with WASP-47b, and all three planets have almost perfectly circular orbits. WASP-47b and its two close-in companion planets form an extremely compact planetary system. The orbits of all three planets fit well within Mercury’s orbit around the Sun. Nevertheless, calculations indicate that the planetary system is dynamically stable in the long run.
Phase-folded K2 transit light curves of WASP-47b, c and d overlaid with the best-fit transit models (red curves), and binned points (purple circles). Becker et al. (2015)
Hot-Jupiters are basically Jupiter-sized planets that formed further out and subsequently migrated closer in to their host stars. There appears to be a lack of hot-Jupiters with close-in planetary companions. This supports the idea that hot-Jupiters form via the high eccentricity migration (HEM) process. Such a process would destabilise the orbits of other planets in the system, resulting in the lack of close-in planetary companions to hot-Jupiters. However, studies have also shown that not all hot-Jupiters form via the HEM process and these hot-Jupiters can have close-in planetary companions. WASP-47b and its close-in planetary companions demonstrate that Jupiter-sized planets can migrate toward their host stars in a dynamically stable manner.
Becker et al. (2015), “WASP-47: A Hot Jupiter System with Two Additional Planets Discovered by K2”, arXiv:1508.02411 [astro-ph.EP]