The Instituto de Astrofisica de Canarias have put out a press release on a new paper by von Essen et al, reporting a study of WASP-33b using the 10-meter Gran Telescopio Canarias.
WASP-33 is a hard system to analyse since the host star is a delta-Scuti star, which means that it pulsates. That produces transit lightcurves like these, where the usual transit profile has pulsations superimposed on it. The figure shows the transit in different wavebands across the optical, from blue to red, as obtained with the OSIRIS spectrograph. That meant that the authors first had to model and subtract the effect of the pulsations.
After doing that they analysed how the transit depth depended on wavelength, which reveals how the planet’s atmosphere absorbs light. “We find that the feature observed between 450 and 550 nm can best be explained by aluminium oxide in its atmosphere” says lead author, Carolina von Essen.
“The current models of exoplanetary atmospheres predict that the Ultra Hot Jupiters should be free of clouds, and present a range of oxides in the visible spectrum, such as vanadium oxide, titanium oxide, and aluminium oxide”. This work on WASP-33b is the first observational indication of the presence of aluminium oxide.
The latest Hubble Space Telescope spectrum of a WASP exoplanet has just been published by Thomas Evans et al. The spectrum of WASP-121b extends from near-UV wavelengths through the optical to the infra-red, combining data from three different gratings (shown in different colours in the figure):
Of particular interest is the rapid rise in the data in the near-UV (the extreme left of the plot), which is clearly out of line with the fitted model (purple lines). The rise is too rapid to be attributed to Rayleigh scattering in a clear atmosphere.
Instead, the authors suggest that it is due to sulfanyl, a molecule consisting of one sulfur and one hydrogen. Evans et al conclude that the near-UV absorber “likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere”.
The work points to the ongoing importance of the Hubble Space Telescope, even after the James Webb Space Telescope is launched, since the JWST is designed for infrared astronomy, and can’t see the near-UV wavelengths that can be observed with Hubble.
Update: One of the authors, Jo Barstow, has tweeted the following thread on the @astrotweeps account:
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.
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.”
Star spots are cooler regions of a star’s surface, caused by magnetic activity, and emit less light. If a planet transits across a spot it blocks less light, and so we see a slight rise, a bump, in the transit profile.
On the left (in blue) is a transit from a new paper by Espinoza et al, who have observed transits of WASP-19b with the Magellan telescope. A clear bump is seen, indicating that the planet passed over a cooler spot.
On the right (in red), however, is another transit showing a clear dip compared to the expected transit lightcurve. This implies that during this transit the planet passed over a brighter region on the star. This is the first time such an event has been seen.
The authors deduce that the bright spot must have a size of about a quarter of the stellar radius and must be 100 K hotter than the rest of the star. Such regions are not seen on our own Sun.
The main point of the observations, however, was not studying spots but studying the planet’s atmosphere by recording how the transit depth changes with wavelength. Here is the state-of-play for the spectrum of WASP-19b, covering optical to infra-red wavelengths:
The red data-points are from the Hubble Space Telescope, showing a spectral feature, but the new data by Espinoza et al (white points) are consistent with a flat spectrum within the limits of the data.
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.”
Puffed-up, low-mass planet WASP-127b is one of the best targets for atmospheric characterisation. Recently Chen etal have announced the results of observations taken with the La Palma telescopes the Gran Telescopio Canarias and the Nordic Optical Telescope.
The result is one of the most detailed and featured spectra of an exoplanet yet.
The spectrum clearly shows the metals sodium (Na), lithium and potassium (K), along with features due to water and haze. A press release from the University of Cambridge has led to coverage of the paper on several dozen websites.
The Instituto de Astrofisica de Canarias have also produced this artist’s depiction of WASP-127b and its host star: