Congratulations to the HATSouth project for the discovery of HATS-18b, a hot Jupiter with the very short orbital period of only 0.84 days. The other known hot Jupiters with periods below 1 day are all WASP-South discoveries (WASP-19b at 0.79 d, WASP-43b at 0.81 d, WASP-103b at 0.93 d and WASP-18b at 0.94 d).
Since such short-period systems are the easiest to find in transit surveys (owing to lots of transits!) they must be very rare, presumably because tidal forces are causing the orbits to decay, so that the planets spiral into their stars on relatively short timescales of tens of millions of years.
The HATSouth team note that the rotational periods of the host stars of HATS-18b and WASP-19b are much shorter than expected given the ages of the stars, and suggest that the stars have been spun up by the same tidal interaction that caused the planet’s orbit to decay. By modelling the in-spiral process Penev et al arrive at constraints on the “quality factor” Q‘* of the star. This is a measure of how efficient the star is at dissipating the tidal energy resulting from the planet’s gravitational tug on the star, and this sets the timescale for the tidal decay. Penev et al argue that the log of Q‘* is between 6.5 and 7, one of the tightest constraints yet estimated.
Estimates of the tidal quality factor, from modelling the HATS-18b and WASP-19b systems. The different models use different assumptions and are explained in the text. Figure by Penev et al.
New HATSouth planets gives us at WASP a check on our methods, since we can look for them in our own data (and if we don’t see them we can ask why not). At V = 14.1, HATS-18 is fainter than any of the WASP host stars, and fainter than we would adopt as a candidate (HATSouth is optimised to get better photometry on a slightly fainter magnitude range, whereas WASP-South is optimised for a wider field). Nevertheless, 26 000 data points from WASP-South do detect the transit of HATS-18b, giving a detected signal at the 0.837-day period and its first harmonic (1.67-d) in the period search:
There is then a clear detection of the transit when the data are folded on the transit period:
This is thus the faintest detection of a planet yet by WASP-South and so is reassuring about WASP data quality.
It is fairly amazing what one can deduce about planets orbiting distant stars. A new paper by Peter Buhler et al reports constraints on the rigidity of the hot-Jupiter exoplanet HAT-P-13b.
The essential data comes from an observation of the occultation of the planet (when it passes behind the host star), as observed in infra-red light by the Spitzer Space Telescope.
If the planet’s orbit were exactly circular the occultation would occur exactly half a cycle after the transit. But this occultation is 20 minutes early. That means that the orbit is slightly elliptical, amounting to an eccentricity of 0.007 +/– 0.001, a small but non-zero value.
Most hot Jupiters are expected to have orbits which have been completely circularised by tidal forces. Thus an eccentric orbit implies either that the planet has only relatively recently moved into that orbit, or that the eccentricity is being maintained by the gravitational effects of a third body.
In this case another planet, HAT-P-13c, a 14-Jupiter-mass planet in a longer 446-day orbit, is thought to be perturbing the close-in hot Jupiter HAT-P-13b.
The extent of the perturbation then tells us about the rigidity of the hot Jupiter. Tidal forces result from the fact that gravity differs across an extended body such as a planet, and how a planet reacts to the tidal stress depends on its rigidity.
The rigidity is parametrised by the “Love number”, and the authors find that the eccentricity of HAT-P-13b’s orbit implies a Love number of 0.3. This in turn implies that the planet likely has a rocky core of about 11 Earth masses, with the rest being an extended gaseous envelope.
An interesting new paper by von Paris et al has explored the effect of the cloudiness of a planet on transit lightcurves. If a planet were cloudy on one limb, but clear on the other limb, then that could make the transit slightly asymmetric. The authors show that, in principle, this effect could be detectable with good-enough quality lightcurves.
An apparent shift in the transit:
Would then lead to residuals, relative to a “perfect” transit, looking like:
The authors then claim a possible detection of such an effect in the hot Jupiter HAT-P-7b.
This might open up a new way of exploring the atmospheres of exoplanets. Whether this can ever be done reliably, however, is debatable. A big assumption in the authors’ simulations is that the star being transited is uniform. However, we know that stars are usually magnetically active and so are patchy. Star spots and bright patches on the star are likely to have a greater effect on the transit profile than the cloudiness of the planet’s atmosphere. Still, the effect is worth exploring, particularly for planets transiting magnetically quiet stars.