M. Gillon et al. (2014) report the discovery of WASP-103 b,
an ultra-short-period planet at the edge of tidal disruption. WASP-103 b orbits
an F-type star at a distance of just ~2 stellar radii from the star's surface,
taking a mere 22.2 hours to complete an orbit. The WASP transit survey is
sensitive to detecting ultra-short-period giant planets when these planets
happen to cross in front of their host stars. WASP-103 b has 1.49 times the
mass and 1.53 times the diameter of Jupiter. This newfound planet joins a small
group of gas giants that are known to be at the verge of being tidally
disrupted by their host stars. The group include planets such as WASP-12 b and
WASP-19 b.
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Artist’s impression of a gas giant. Credit: Daniel Mallia.
WASP-103 b is significantly inflated and has a bulk density
that is only 55 percent the density of water. The low density of WASP-103 b is
not just because of the intense irradiation it receives due to its extreme
closeness to its host star. Tidal heating is also expected to contribute
significantly to the planet's "bloatedness" since the planet's orbit
is only 15 to 20 percent away the Roche Limit. Any closer, the planet is
expected to be tidally destructed by the gravity of its host star.
Ultra-short-period gas giants that are right at the edge of
being tidally disrupted might experience mass loss and significant tidal
distortion. One such planet, WASP-12 b, is known to be surrounded by planetary
material that has escaped it. In the case of WASP-103 b, the extreme irradiation
it receives, the planet's inflated size and the brightness of its host star
makes it favourable for atmospheric characterisation with existing ground-based
and space-based telescopes. Observing signs of mass loss and tidal distortion
for such extreme planets can shed light on the final stages in the lives of
hot-Jupiters.
Reference:
M. Gillon et al. (2014), "WASP-103b: a new planet at
the edge of tidal disruption", arXiv:1401.2784 [astro-ph.EP]