Jason H. Steffen from the Fermilab Center for Particle Astrophysics et al submitted to The Astrophysical Journal an article entitled “Kepler constraints on planets near hot Jupiters,” a sample study of 63 Hot Jupiter systems.
They found that in planetary systems with Jupiter-size planet candidates in three day orbits, there is no evidence of small, companion candidates. The finding suggests that small candidates were ejected from the system, leaving large planets to later circularize into tight orbits.
Excerpt from the abstract: We present the results of a search for planetary companions orbiting near hot Jupiter planet candidates (Jupiter-size candidates with orbital periods near 3 days) identified in the Kepler data through its sixth quarter of science operations.
There were no companions with sizes ranging from roughly 2/3 to 5 times the size of the Earth.
In contrast, comparison studies of warm Jupiters (with slightly larger orbits) and hot Neptune-size candidates do exhibit signatures of additional companions with these same tests. These differences between hot Jupiters and other planetary systems denote a distinctly different formation or dynamical history.
Source: NASA Kepler