Tag Archives: WASP-18b

Night-side clouds on hot Jupiters

Thomas Beatty et al have an interesting new paper on arXiv today, primarily about the transiting brown dwarf KELT-1b. They’ve used the Spitzer Space Telescope to record the infra-red light as it varies around the 1.3-day orbit.

They end up with the following plots (KELT-1b is on the right, with the plot for the planet WASP-43b on the left):

The x-axis is “colour”, the difference in flux between two infra-red passbands at 3.6 and 4.5 microns. The y-axis is brightness (in the 3.6 micron band). The underlying orange and red squares show where typical M-dwarf stars and L and T brown dwarfs fall on the plot.

The solid-line “loops” are then the change in position of the atmospheres of KELT-1b and WASP-43b around their orbits. At some phases we see their “day” side, heated by the flux of their star, and at others we see their cooler “night” side.

The blue line is the track where something would lie if there were no clouds in its atmosphere. The fact that KELT-1b’s loop doesn’t follow the blue track, but moves significantly right (to cooler colours) implies that the night side of the brown dwarf must be cloudy. The night side of WASP-43b, however, appears to be less cloudy, according to its track.

Here are the same plots for two more planets:

The plot for WASP-19b shows a loop with a marked excursion to the right, suggesting a cloudy night side to the planet. For WASP-18b, however, the loop follows a trajectory nearer the blue “no cloud” track, suggesting a clearer atmosphere.

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Water Is Destroyed, Then Reborn in Ultrahot Jupiters

NASA JPL have put out a press release about ultra-hot Jupiters including WASP-18b, WASP-103b and WASP-121b.

The work, led by Vivien Parmentier, used the Spitzer and Hubble space telescopes to study how the planets’ atmospheres change from the irradiated day side to the cooler night side.

“Due to strong irradiation on the planet’s daysides, temperatures there get so intense that water molecules are completely torn apart. […] fierce winds may blow the sundered water molecules into the planets’ nightside hemispheres. On the cooler, dark side of the planet, the atoms can recombine into molecules and condense into clouds, all before drifting back into the dayside to be splintered again.”

Simulated views of the ultrahot Jupiter WASP-121b show what the planet might look like to the human eye from five different vantage points, illuminated to different degrees by its parent star. (Credit: NASA/JPL-Caltech/Vivien Parmentier/Aix-Marseille University)

“With these studies, we are bringing some of the century-old knowledge gained from studying the astrophysics of stars, to the new field of investigating exoplanetary atmospheres,” said Parmentier.

Harvard’s CfA have also produced a press release on the work, focusing on the analysis of WASP-103b led by Laura Kreidberg.

“A crucial observational advance by Kreidberg and her team was that they observed the planet for an entire orbit, enabling them to map the climate at every longitude and derive detailed information about the temperatures on the planet’s dayside and nightside. This is only the second time that such a complete exoplanet observation has been performed with HST.”

NASA’s Webb Space Telescope to Inspect Atmospheres of Gas Giant Exoplanets

NASA have written a publicity page on JWST’s plans to study the atmospheres of gas-giant exoplanets, including an animation on how this is done. Since the prime targets for the “Early Release Science” program are three WASP-discovered planets, WASP-18b, WASP-43b and WASP-79b, we “re-blog” the piece here:

“In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS). Its main goal is to locate Earth-sized planets and larger “super-Earths” orbiting nearby stars for further study. One of the most powerful tools that will examine the atmospheres of some planets that TESS discovers will be NASA’s James Webb Space Telescope. Since observing small exoplanets with thin atmospheres like Earth will be challenging for Webb, astronomers will target easier, gas giant exoplanets first.”

Read the full piece here.

WASP-18b has a smothering stratosphere without water

NASA Goddard Space Flight Center and NASA Jet Propulsion Laboratory have put out press releases about observations of WASP-18b with the Hubble Space Telescope and the Spitzer Space Telescope.

The main finding is that WASP-18b, a highly irradiated hot Jupiter in a tight orbit around a hot F-type star, is “wrapped in a smothering stratosphere loaded with carbon monoxide and devoid of water”.

The team determined this by detecting two types of carbon monoxide signatures, an absorption signature at a wavelength of about 1.6 micrometers and an emission signature at about 4.5 micrometers.”

The findings have been reported in many media outlets including: Newsweek, The Independent, The Sun, the Daily Mail, the International Business Times, phys.org, and more than 20 other websites including Forbes magazine, who have produced the following infographic:

The atmospheres of WASP planets with JWST

The James Webb Space Telescope is expected to revolutionise the study of exoplanet atmospheres following its launch in 2018, and WASP planets will be among the prime targets. Paul Mollière et al have been simulating the data expected, and have produced this illustration of the atmospheric emission spectrum of WASP-18b.

Spectrum of exoplanet WASP-18b as observed with JWST

The different coloured curves result from different assumptions about WASP-18b’s atmosphere. The lines along the bottom illustrate the spectral coverage of the different JWST instruments. In contrast to existing data (Spitzer results are shown as black squares), the JWST data will have both the spectral resolution and signal-to-noise to differentiate clearly between different models.

Mollière et al have also simulated spectra for cooler planets, such as WASP-10b and WASP-32b.

WASP-10b and WASP-32b simulated atmospheres observed with James Webb Space Telescope.

The different models are for different abundances of carbon relative to oxygen (C/O), showing that JWST should be able to settle the issue of which exoplanets have enhanced abundances of carbon relative to the Solar System.

Such simulations show that the results from JWST should be spectacular, opening up whole new areas of enquiry.

Magnetospheres of hot Jupiters

If a hot Jupiter has a magnetic field of a few Gauss it would be surrounded by a magnetosphere that would carve out a hole in the stellar wind of the host star. Since the planet orbits rapidly, this would lead to a “bow shock” where the magnetosphere ploughs through the stellar wind.

In a new paper, Richard Alexander, of the University of Leicester, and co-authors, report computer simulations of this effect for several hot Jupiters, including WASP-12b and WASP-18b.

Hot Jupiter magnetospheres

In the colour-coded figure (see scale on the right) the blue and red show the density of the stellar wind. A low-density (black) magnetosphere surrounds each planet (white dots).

Since these planets orbit edge on to us, the bow shock would absorb ultra-violet light from the star, and so produce a characteristic light-curve with a broad dip preceding the transit.

Hot Jupiter magnetospheric light curves

This magnetospheric bow-shock is a possible alternative explanation for the UV absorption observed in WASP-12, which has previously been attributed to material being lost from the planet owing to Roche-lobe overflow. Alexander et al suggest that WASP-18 is a critical test of these models, since the much higher gravity of the massive planet WASP-18b means that there should not be any Roche-lobe overflow.