On-going radial-velocity monitoring of WASP hot Jupiters has shown that some of them have companions, additional Jupiter-mass planets in much wider orbits.
This might be part of the answer as to why there are hot Jupiters at all. Standard planet-formation theory suggests that they must form much further out, where it is colder and where ice can form, enabling bits of pre-planetary debris to clump together. Thus one solution is that gravitational perturbations by third bodies (wide-orbit massive planets or companion stars) push the inner planets into highly eccentric orbits, where tidal capture then circularises them into hot-Jupiter orbits.
But, if this “Kozai effect” is to work, the outer planets need to be in orbits tilted with respect to the orbits of the hot Jupiters. This requires i < 65 degrees, rather than the co-planar i = 90 degrees.
A new paper by Juliette Becker et al reports an analysis of six hot-Jupiter systems orbiting cool stars that have an outer planetary companion. These are WASP-22, WASP-41, WASP-47, WASP-53, HAT-P-4 and HAT-P-13. Though a statistical analysis they show that the outer planets are most likely co-planar, with orbits tilted by no more than 20 degrees. They thus argue that Kozai-driven high-eccentricity migration is not the dominant way of forming hot Jupiters.
NASA have put out a press release entitled: “Hubble’s Tale of Two Exoplanets: Nature vs. Nurture”.
The article compares WASP-67b and HAT-P-38b, noting how similar they are in size and temperature, both orbiting similar stars at a similar orbital distance. But then Hubble’s Wide Field Camera 3 found that WASP-67b has a very cloudy atmosphere whereas HAT-P-38b has much clearer skies.
From the press release: Perhaps one planet formed differently than the other, under a different set of circumstances. “You can say it’s nature versus nurture,” explains co-investigator Kevin Stevenson. “Right now, they appear to have the same physical properties. So, if their measured composition is defined by their current state, then it should be the same for both planets. But that’s not the case. Instead, it looks like their formation histories could be playing an important role.”
“Astronomers measured how light from each parent star is filtered through each planet’s atmosphere. HAT-P-38 b did have a water signature indicated by the absorption-feature peak in the spectrum. This is interpreted as indicating the upper atmosphere is free of clouds or hazes. WASP-67 b, has a flat spectrum that lacks any water-absorption feature, suggesting most of the planet’s atmosphere is masked by high-altitude clouds.”
The NASA press release has been picked up and reported on several dozen science-related websites.
Credits: Artwork: NASA, ESA, and Z. Levy (STScI); Science: NASA, ESA, and G. Bruno (STScI)
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.