Category Archives: WASP planets

MPIA press release: An exotic water cycle and metal clouds on the hot Jupiter WASP-121 b

Here’s another catch-up on a recent press release from MPIA, reporting on Hubble Space Telescope observations of WASP-121b.

“A group of astronomers, led by Thomas Mikal-Evans from the Max Planck Institute for Astronomy, have made the first detailed measurement of atmospheric nightside conditions of a tidally locked hot Jupiter. By including measurements from the dayside hemisphere, they determined how water changes physical states when moving between the hemispheres of the exoplanet WASP-121 b. While airborne metals and minerals evaporate on the hot dayside, the cooler night side features metal clouds and rain made of liquid gems. This study, published in Nature Astronomy, is a big step in deciphering the global cycles of matter and energy in the atmospheres of exoplanets.”

“To probe the entire surface of WASP-121 b, we took spectra with Hubble during two complete planet revolutions,” co-author David Sing from the Johns Hopkins University in Baltimore, USA, explains. With this technique and supported by modelling the data, the group probed the upper atmosphere of WASP-121 b across the entire planet and, in doing so, observed the complete water cycle of an exoplanet for the first time.

“On the side of the planet facing the central star, the upper atmosphere becomes as hot as about 3000 degrees Celsius. At such temperatures, the water begins to glow, and many of the molecules even break down into their atomic components. The Hubble data also reveal that the temperature drops by approximately 1500 degrees Celsius on the nightside hemisphere. This extreme temperature difference between the two hemispheres gives rise to strong winds that sweep around the entire planet from west to east, dragging the disrupted water molecules along. Eventually, they reach the nightside. The lower temperatures allow the hydrogen and oxygen atoms to recombine, forming water vapour again before being blown back around to the dayside and the cycle repeats. Temperatures never drop low enough for water clouds to form throughout the cycle, let alone rain.”

“Instead of water, clouds on WASP-121 b mainly consist of metals such as iron, magnesium, chromium and vanadium. Previous observations have revealed the spectral signals of these metals as gases on the hot dayside. The new Hubble data indicate that temperatures drop low enough for the metals to condense into clouds on the nightside. The same eastward flowing winds that carry the water vapour across the nightside would also blow these metal clouds back around to the dayside, where they again evaporate.

“Strangely, aluminium and titanium were not among the gases detected in the atmosphere of WASP-121 b. A likely explanation for this is that these metals have condensed and rained down into deeper layers of the atmosphere, not accessible to observations. This rain would be unlike any known in the Solar System. For instance, aluminium condenses with oxygen to form the compound corundum. With impurities of chromium, iron, titanium or vanadium, we know it as ruby or sapphire. Liquid gems could therefore be raining on the nightside hemisphere of WASP-121 b.”

The press release has been taken up by numerous media and press websites.

WASP-132c: A rocky companion to hot-Jupiter WASP-132b

Most hot Jupiters are lonely planets. If they have companions, they are far out in distant orbits. Thus it was a surprise when a K2 lightcurve of WASP-47 detected transits of two small, rocky planets, one orbiting inside the hot Jupiter WASP-47b, and the other orbiting just outside (Becker etal 2015).

TESS light-curves showing transits of hot-Jupiter WASP-132b (bottom) and the newly discovered inner planet, WASP-132c (top).

Now a new paper led by Ben Hord reports a small companion planet to hot-Jupiter WASP-132b. Discovered in TESS light-curves and dubbed WASP-132c, this planet has an orbital period of 1 day and a size just below two Earth radii.

This is only the fourth hot-Jupiter system found to have small companion planets.

Schematics of the four known systems where a hot Jupiter has a rocky planet as a close companion. (From Hord et al, note that the diagram is not to scale.)

As explained by Hord et al: “The existence of a planetary companion near the hot Jupiter WASP-132 b makes the giant planet’s formation and evolution via high-eccentricity migration highly unlikely. Being one of just a handful of nearby planetary companions to hot Jupiters, WASP-132 c carries with it significant implications for the formation of the system and hot Jupiters as a population.”

CHEOPS observes WASP-189b

ESA’s CHEOPS satellite was launched to produce high-quality light-curves of exoplanet systems. A new paper led by Adrien Deline of the University of Geneva now reports CHEOPS observations around the orbit of the ultra-hot-Jupiter WASP-189b. The figure shows the transit (planet passing in front of the star), the eclipse (the planet passing behind the star) and a slower variation caused by the varying visibility of the heated face of the planet.

One notable feature of the transit of WASP-189b is that it is distinctly asymmetrical. This is caused by gravity darkening, which occurs when a star is rapidly rotating. The centrifugal forces cause the equatorial regions to be pushed outwards, producing an equatorial bulge. Since the bulge is then further from the star’s centre, the surface gravity will be lower, and that means that the surface will be cooler and thus dimmer.

The illustrations below show the asymmetry, where the dashed line in the lowest panel shows the difference between a transit model both with and without gravity darkening. The right-hand panel illustrates the polar orbit of the planet.

Tidal deformation of WASP-103b

With ultra-hot Jupiters being so near to their star their shape is predicted to be distorted away from spherical by the tidal effects of the host-star’s gravity. The resulting “rugby-ball” shape (more technically called a “Roche lobe”) will then produce a transit profile that is slightly different from that produced by a spherical planet.

The CHEOPS team now report that they have detected this distortion in the case of WASP-103b. A press release presents the infographic:

The CHEOPS observations of transits of WASP-103b are shown below (grey points). The blue model is the expected profile for a deformed planet, while the green line (lowest panel) is the expected difference in transit profile between a deformed planet and a spherical planet. The CHEOPS team show statistically that the data prefer the deformed shape, at a confidence level of 3σ.

The authors, Susana Barros et al, explain that the degree of tidal deformation constrains the distribution of mass within the planet, since the gaseous hydrogen envelope is much easier to deform than the rocky core. ESA have produced an artist’s illustration showing the distorted shape of WASP-103b:

Following ESA’s press release, the work has been reported by CNN, Newsweek, the BBC, the Daily Mail, The Sun, The Independent and numerous other websites in multiple languages.

Hot Jupiters often have polar orbits

Once a planet is found to transit its star, astronomers often try to figure out whether the planet’s orbit is aligned with the spin of the star. This is called the “obliquity”, denoted by Ψ, the angle between the orbital and stellar-spin axes.

This angle Ψ can be measured if we have enough information , including the broadening of the stellar lines caused by the star’s rotation, the perturbation of the stellar line profiles as the planet transits the star (called the Rossiter–McLaughlin effect), and the star’s rotation period.

It has long been known that many hot-Jupiter exoplanets are in aligned orbits (where the star’s spin axis is perpendicular to the orbital plane), but that a significant fraction are misaligned. Now a new paper led by Simon Albrecht reports that the misaligned planets tend to be in polar orbits, where the planet passes directly over the star’s poles.

The plot shows values for all the hot Jupiters where Ψ can be measured — of which roughly half are WASP planets — and reveals that obliquity values (y-axis) imply that the planets tend to be either aligned (low values of Ψ) or in polar orbits (Ψ near 90 degrees).

In the illustration below the planets orbit in the equatorial plane (we look along the z axis), and the arrows point along the stellar spin axes. The arrows collected around the y axis are thus the aligned systems. The rest are not evenly distributed, but are preferentially close to the orbital plane.

Although the authors discuss several mechanisms that can be causing misaligned orbits, the reason for the preponderance of planets in polar orbits is not yet understood.

Aerosol particles make WASP-69b’s atmosphere hazy

“Aerosols have a critical role in establishing energy budgets, thermal structure, and dynamics in planetary atmospheres”, declares a new paper by Raissa Estrela et al.

Aerosols make the planet’s atmosphere hazy, an effect which is more pronounced at the blue end of the spectrum. Here is the spectrum of hot-Jupiter exoplanet WASP-69b, combining Hubble Space Telescope data from several observations.

The steeply rising spectrum (the y-axis shows effective planet size, with a larger size indicating more atmospheric absorption) is modelled (blue line) by including haze from aerosol scattering. The aerosols are found to extend from millibar pressures to microbar pressures.

The authors don’t yet know the composition of the aerosols, but suggest possibilities including hydrocarbons or magnesium silicate condensates. Overall they conclude that: “These results are consistent with theoretical expectations based on microphysics of the aerosol particles that have suggested haze can exist at microbar pressures in exoplanet atmospheres”.

CHEOPS observations of WASP transits

CHEOPS, the CHaracterising ExOPlanet Satellite is ESA’s Small-class mission dedicated to recording transits of exoplanets. A new paper led by Luca Borsato presents some early observations of transits of WASP, KELT and HATnet planets.

Here, for example, are the lightcurves of two transits of WASP-8b, both plotted against phase.

The paper focuses on the transit timing, which can be as good as timing a transit to an accuracy of 13 to 16 seconds, depending on the brightness of the host star and the amount of transit covered by the observations.

One aim of such work is to look for variations in the timing of transits, caused by the gravitational perturbations of additional unseen planets in the system.

Iron in the atmosphere of the ultra-hot-Jupiter WASP-33b

Here’s a plot from a new paper by David Cont et al, of the University of Göttingen. The plot shows spectra of the WASP-33 system, obtained with the CARMENES near-infra-red spectrograph on the 3.5-m telescope at the Calar Alto Observatory.

The image shows features caused by iron absorption lines, as a function of time (y-axis). The spectra have all been adjusted so that zero velocity (RV) is centred on the host star, WASP-33. The star’s rotation then causes features over the spread of velocities marked by the dashed yellow lines.

One can clearly see the rippling effect of pulsations as they run around the star. The pulsations are likely being excited by the tidal pull of the planet.

In addition, though, and marked by yellow arrows, is a faint diagonal line. This is caused by the planet, WASP-33b, and is the effect of iron absorption lines in the planet’s atmosphere. It moves diagonally across the image owing to the orbital motion of the planet around the star.

By comparing their analysis of iron lines to a similar analysis for Titanium Oxide, the authors show that there is a temperature inversion (higher temperature at greater height) in the atmosphere of the planet.

The spectrum of hot Jupiter WASP-79b

Bloated hot-Jupiter WASP-79b, one of the largest known exoplanets with a radius near twice that of Jupiter, is among the planets scheduled to be observed with the keenly-awaited JWST. In a new paper, Alexander Rathcke et al report on observations made with Hubble. Here’s the spectrum:

The clearest spectral feature, in the range observed with the WFC3 G141 grating, is attributed to water vapour. The authors also interpret the spectrum as showing opacity due to H ions and the effects of faculae on the host star, that are 500 K hotter than most of the star’s surface. They say that this “underscores the importance” of observing a wide wavelength range in order to “disentangle the influence of unocculted stellar heterogeneities from planetary transmission spectra”.