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Curious, these inflated hot Jupiters ...

By now, astronomers have found more than five hundred 'exoplanets', i.e. planets orbiting other stars. A group of these are large planets with orbits very close to their host stars, the so-called 'hot Jupiters'. Their mass is similar to our Jupiter but they are often much bigger, indicating that their interior is much hotter. Left to themselves, they should cool down and deflate fairly rapidly to a size similar to the Jupiter in our solar system. This however, poses a problem: the associated stars do not look like they formed very recently. So why do young planets orbit old stars? The solution: the old-looking host star could be the result of a recent process. When two small, old stars merge, planets can be formed as a by-product. In other words, these Jupiters are inflated because they are actually still young...

Fig. 1: Copyright: MPA

Fig. 2: Schematic of how planets are formed in the merging of two stars. A close binary system of two small stars (a) is slowly getting smaller by the loss of energy due to magnetic winds from the stars until they touch (b). The stars merge, the excess of angular momentum in their orbits causes a dense 'excretion' disk to spread outwards, in which planets form (c). As the disk dissipates further outward the planets disturb each other's orbits by their gravitational pull, resulting in eccentric, inclined orbits out of the disk plane. In this sketch the smallest planet is kicked out; in many cases only one (large) planet remains orbiting close to the star.
Copyright: MPA

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

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