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...
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Fig. 1:
Copyright: MPA
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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
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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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