Category Archives: Hot Jupiters

WASP-4 is accelerating toward the Earth

Here is a plot of the timings of the transits of WASP-4b, taken from a new paper led by Luke Bouma:

The curve in the plot shows that the transits are occurring progressively earlier as time passes. One possible explanation is that the planet’s orbit is decaying under the influence of the tidal interaction between the star and planet. This is expected to occur in most hot Jupiters, though how quickly is debated.

However, Bouma have also obtained radial-velocity observations of the system, which show that the star is accelerating towards us. This can result from it being in a wide orbit with another object (the authors suggest a wide-orbiting companion of 10-to-300 Jupiter masses at a distance of 10-to-100 AU). Since the system is accelerating towards us, the light-travel time is decreasing, and this (not orbital decay) means that the transits occur earlier.

Wide companions are expected in hot-Jupiter systems, since, in most theories for the occurrence of hot Jupiters, the gravitational perturbation of a distant companion is needed to shrink the hot-Jupiter orbit down to the current values of only a few days.

Bouma et al recommend continued radial-velocity monitoring of hot Jupiters in order to distinguish orbital decay from accelerations caused by orbiting companions.

The morning and evening terminators are different

Hot Jupiter exoplanets are “phase locked” by tidal forces, meaning that the same face of the planet always faces the star. Being blasted by radiation it is far hotter than the night side. This means that strong winds must be racing around the planet, redistributing the heat.

And that means that the “evening” terminator (where winds flow from the hot day-side face to the cooler night side) will be much hotter than the “morning” terminator (where winds flow from the night side to the day side). Here’s an illustration from a new paper by Ryan MacDonald, Jayesh Goyal and Nikole Lewis:

Of course the terminators are exactly the regions of the planet’s atmosphere that are being sampled by atmospheric-characterisation studies, since that’s the regions that are seen projected against the host star.

As Ryan MacDonald et al point out, most atmospheric-characterisation studies assume that the two limbs are the same, since that’s the easiest thing to do. However, the authors argue, while doing that might produce an acceptable fit to the data, the resulting parameter values could be very wrong.

Thus, the fitted temperature profile could be “hundreds of degrees cooler” than reality. As a result, the fitted abundances of molecular species could also be wrong. MacDonald et al conclude that: “these biases provide an explanation for the cold retrieved temperatures reported for WASP-17b and WASP-12b” and say that: “to overcome biases associated with 1D atmospheric models, there is an urgent need to develop multidimensional retrieval techniques”.

More TESS phase curves of WASP exoplanets

Ian Wong et al have produced a new analysis of the TESS data on previously known WASP exoplanets. Their main interest is the “phase curve”, the variation of the light around the planet’s orbit.

Two examples are the systems WASP-72 and WASP-100:

In addition to the main transit (planet passing in front of the star) the phase curves show secondary eclipses (planet passing behind the star, at phase 0.5) and a sinusoidal variation due to the heated face of the planet. By modelling the phase-curves of these and other similar planets, Wong et al make the tentative suggestion that the hotter the planet (which can be measured from the depth of the secondary eclipse) the more reflective the atmosphere of the planet is.

Here’s a similar plot for WASP-30. Note, though, that the phase-curve variation peaks at phases 0.25 and 0.75, unlike those for WASP-72 and WASP-100. That’s because WASP-30b is not a planet but a brown dwarf, with a mass of 63 Jupiters. That is massive enough for its gravity to distort the host star into an ellipsoidal shape, and so in this system the variation of the light is caused by the varying projection of the distorted star around the orbit.

Detection of iron in the ultra-hot-Jupiter WASP-121b

Three papers this week arrived on arXiv about the ultra-hot-Jupiter WASP-121b. All three report similar findings (and the near-simultaneous arrival on arXiv presumably reflects the teams being aware of the competition). Cabot et al analyse spectra of WASP-121b from the ESO 3.6-m/HARPS spectrograph, Bourrier et al also analyse HARPS data, while Gibson et al analyse data from UVES on ESO’s 8-m VLT.

All three teams then apply velocity shifts to correct for the orbital motion of the star, in order to try to detect features from the planet. The result is a plot looking like (this is the one from Bourrier et al):

As in the plot for WASP-107b, just below, this shows the spectra as a function of time, through transit. The extra absorption during transit (delineated by dashed lines) is from the atmosphere of the planet absorbing starlight while it is projected against the star’s face. The faint diagonal feature (marked by the green line) is the signal from the planet’s atmosphere, moving with the planet’s orbital velocity.

A schematic of WASP-121b as it transits its hot star in a near-polar orbit (from Bourrier et al).

The three papers report the detection of lines from neutral iron in the planet’s atmosphere, and discuss the possible role of iron absorption in producing an atmospheric temperature inversion. The papers also report a blue-shift of the iron absorption, of order 5 km/s, which could be produced by strong winds running round the planet. That is expected in phase-locked planets, where heat from the irradiated face must be transported round to the night side.

Helium reveals the extended atmosphere of WASP-107b

Here’s a plot from a new paper on WASP-107b by James Kirk et al. It shows data taken with a near-infra-red spectrograph on the 10-m Keck II telescope on Mauna Kea, and is focused on the Helium line at 10833 Å. The plot shows the spectra as a function of time (y-axis), though a transit. When the planet passes in front of its host star (white horizontal lines are times of ingress and egress) the helium line shows excess absorption. This helium is in the atmosphere of the planet and is absorbing some of the starlight. There is a slight change in the wavelength of the absorption owing to the orbital motion of the planet (denoted by the dashed white lines).

The paper shows, firstly, that ground-based telescopes such as Keck can do a fine job of discerning the compositions of exoplanet atmospheres. Secondly, the fact that the absorption extends beyond transit-egress indicates that the atmosphere is boiling off the surface of WASP-107b, under the fierce irradiation of the star, and is forming a comet-like tail.

The tidal shape of the exoplanet WASP-121b

The moon’s gravity causes a tidal bulge in Earth’s oceans, so that the water facing the moon is raised several metres. Similarly, close-orbiting exoplanets will have a tidally distorted shape, with a tidal bulge facing the host star. The amount of distortion can be quantified by the “Love number” h (named after the mathematician Augustus Love)

Specifically, h2 tells us the relative height of the tidal bulge, and would be zero for a perfectly rigid body that did not distort at all, and would be 2.5 for a perfectly fluid body that adapted fully to the tidal potential. Gas-giant planets have large envelopes of gaseous fluid, so would be expected to have fairly high values of h2. However, they also might have rocky or metallic cores, and so would have values less than 2.5. For example Jupiter has h2 = 1.6 while Saturn has h2 = 1.4.

Transit of WASP-121b observed by HST with a model fit by Hellard et al.

A new paper by Hugo Hellard et al discusses whether h2 for a hot-Jupiter exoplanet can be measured from the shape of the transit lightcurve, given good-enough photometry such as that from the Hubble Space Telescope.

The main problem is that the transit profile is heavily affected by variations in the brightness of the stellar disc, in particular the limb darkening (a star’s limbs appear a bit dimmer, because a tangential line-of-sight into a gas cloud skims only the cooler, upper layers). Thus the Hellard et al paper discusses at length different ways to model the limb darkening.

A star’s disk is dimmer at the edges, so a transiting exoplanet removes less light (here Venus, top right, is transiting the Sun).

The end-result, however, is a claim to have measured h2 for WASP-121b, with a value of h2 = 1.4 ± 0.8. This is not (yet) a strong constraint, but points to the potential in the future, and also flags up the need to understand and properly parametrise limb darkening.

TESS phase curve of WASP-19b

The space-based photometry from the TESS satellite is producing high-quality light curves of many of the WASP exoplanets. Here is the lightcurve of WASP-19b, from a new paper by Ian Wong et al:

In addition to the transit (phase zero), the lightcurve shows a shallower eclipse of the planet (phase 0.5) and a broad variation caused by the changing aspect of the heated face of the planet. Unlike in some planets, the hottest part of the planet directly faces the star, so there is no offset in the phase of the broad modulation.

Wong et al deduce that the dayside face of the planet is heated to 2240 ± 40 K, that there is no flux detected from the colder night side, and that the planet reflects 16 ± 4 percent of the light that falls on it. The last value is relatively high compared to other planets.

The atmosphere of the inflated hot Jupiter WASP-6b

Atmospheric characterisation of hot Jupiters continues apace, using both ground-based telescopes such as ESO’s Very Large Telescope and satellites such as Hubble.

Aarynn Carter et al have just produced a new analysis of WASP-6b:

The spectrum shows absorption due to sodium (Na), potassium (K) and water vapour, while the modelling implies that the atmosphere is partially hazy. Carter et al state that: “despite this presence of haze, WASP-6b remains a favourable object for future atmospheric characterisation with upcoming missions such as the James Webb Space Telescope.

The orbit of WASP-12b is decaying

Here’s the latest update on the changes in the orbital period of WASP-12b, from a new paper by Samuel Yee et al.

The times of transit are getting earlier, which means that the period is decreasing slightly. By also considering the times of occultation (when the planet passes behind the star), and also the radial-velocity measurements of the system, the authors deduce that the changes are not the effect of some other planet, but are a real decay in the orbit of WASP-12b. This is expected to occur as a result of tidal interactions between the planet and its host star.

One notable conclusion is that the rate of period decay in WASP-12b is much faster than that in WASP-19b, which shows no detectable period change yet, despite it being an even shorter-period hot Jupiter, which should increase tidal interactions. Yee et al suggest that the difference could arise if the host star WASP-12 is a sub-giant star, whereas WASP-19 is not.

Update: Following an article on WASP-12b’s orbital decay, supplied by Liz Fuller-Wright of Princeton University, and appearing in phys.org and Science Daily, the work has gained media attention from CNN, Science Times, Universe Today, and the UK’s Metro.

Gravity darkening of planet-host MASCARA-4

MASCARA is one of WASP’s competitor transit-search projects, so let’s celebrate a neat result from TESS data of transits of MASCARA-4b. The host star, MASCARA-4, is a hot, fast-rotating A-type star. As a result of its fast rotation, the equatorial regions are being flung outwards by centrifugal forces, such that the star has a flattened, oblate shape. As a result, the force of gravity will be less at the equator than at the poles of the star, and that means that the equatorial regions will be slightly cooler and so a bit dimmer (in outline, that’s because gravity inward pull is balanced by gas pressure, and so lower gravity means lower pressure, and the temperature of a gas is related to its temperature through the perfect gas law). This effect is called “gravity darkening”.

The star spins around its axis (thick line) while the planet orbits at an oblique angle.

In a new paper, John Ahlers et al have detected the effect of gravity darkening on a transit lightcurve of the hot Jupiter MASCARA-4b. The planet has a misaligned orbit, first coming onto the stellar face near the equator, and then moving towards a pole. That means it moves from slightly cooler regions to slightly hotter regions, and that changes the amount of light occulted by the planet.

If gravity darkening is not taken into account then the model fit is a bit too deep at the start and a bit too shallow at the end of the transit. One of the benefits of detecting this effect of gravity darkening is that it then tells us the true angle between the star’s spin axis and the planet’s orbit (whereas other methods, such as Doppler tomography, only tell us the projection of that angle onto the sky).