Here is a plot of the timings of the transits of WASP-4b, taken from a new paper led by Luke Bouma:
The curve in the plot shows that the transits are occurring progressively earlier as time passes. One possible explanation is that the planet’s orbit is decaying under the influence of the tidal interaction between the star and planet. This is expected to occur in most hot Jupiters, though how quickly is debated.
However, Bouma have also obtained radial-velocity observations of the system, which show that the star is accelerating towards us. This can result from it being in a wide orbit with another object (the authors suggest a wide-orbiting companion of 10-to-300 Jupiter masses at a distance of 10-to-100 AU). Since the system is accelerating towards us, the light-travel time is decreasing, and this (not orbital decay) means that the transits occur earlier.
Wide companions are expected in hot-Jupiter systems, since, in most theories for the occurrence of hot Jupiters, the gravitational perturbation of a distant companion is needed to shrink the hot-Jupiter orbit down to the current values of only a few days.
Bouma et al recommend continued radial-velocity monitoring of hot Jupiters in order to distinguish orbital decay from accelerations caused by orbiting companions.
On-going radial-velocity monitoring of WASP hot Jupiters has shown that some of them have companions, additional Jupiter-mass planets in much wider orbits.
This might be part of the answer as to why there are hot Jupiters at all. Standard planet-formation theory suggests that they must form much further out, where it is colder and where ice can form, enabling bits of pre-planetary debris to clump together. Thus one solution is that gravitational perturbations by third bodies (wide-orbit massive planets or companion stars) push the inner planets into highly eccentric orbits, where tidal capture then circularises them into hot-Jupiter orbits.
But, if this “Kozai effect” is to work, the outer planets need to be in orbits tilted with respect to the orbits of the hot Jupiters. This requires i < 65 degrees, rather than the co-planar i = 90 degrees.
A new paper by Juliette Becker et al reports an analysis of six hot-Jupiter systems orbiting cool stars that have an outer planetary companion. These are WASP-22, WASP-41, WASP-47, WASP-53, HAT-P-4 and HAT-P-13. Though a statistical analysis they show that the outer planets are most likely co-planar, with orbits tilted by no more than 20 degrees. They thus argue that Kozai-driven high-eccentricity migration is not the dominant way of forming hot Jupiters.