The curiously large sizes of many (though not all) hot Jupiters has become one of the most tantalizing problems in the study of exoplanets. While they account for only a minority of planet systems, they are easiest to detect because they are large and have narrow orbits. Much effort has gone into ingenious ways of keeping hot Jupiters inflated over the long apparent age of their host stars. The most promising processes are some way or other of channelling the radiation heat of the host star into the planet's interior. The mechanisms proposed to achieve this are still hypothetical.

The simple alternative is that the inflated planets are actually as young as they look, while their host stars are actually not quite as ordinary as they appear to be. Low-mass binary stars, with total masses of the order of a solar mass are rather abundant. They are magnetically 'active': that is, they have magnetic fields which drive a strong version of the Sun's 'solar wind'. This causes the orbit of the binary to shrink, until the stars begin to touch. The subsequent sequence of events (Fig. 2) is very fast. The binary merges into a single star resembling the Sun, while the excess angular momentum that was stored in the two stars' orbit around each other causes the merged star to loose mass, forming a disk surrounding the new star. The mass in this disk is enough to form several giant planets. The orbits of these planets are disturbed by the gravitational force they exert on each other, producing inclined (sometimes even retrograde) and eccentric orbits. Some of the planets can even be ejected from the system. These theoretically predicted properties for merged stars agree well with the observed properties of planetary systems with hot Jupiters.

Eduardo Martin, Henk Spruit