Category Archives: Hubble Space Telescope

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.

NASA press release: Hubble probes extreme weather on ultra-hot Jupiters

Here’s a quick catch-up on a recent NASA/ESA press release. Based on Hubble Space Telescope observations of WASP-178b and KELT-20b, NASA declare:

“In studying a unique class of ultra-hot exoplanets, NASA Hubble Space Telescope astronomers may be in the mood for dancing to the Calypso party song “Hot, Hot, Hot.” That’s because these bloated Jupiter-sized worlds are so precariously close to their parent star they are being roasted at seething temperatures above 3,000 degrees Fahrenheit. That’s hot enough to vaporize most metals, including titanium. They have the hottest planetary atmospheres ever seen.”

Illustration of an ultra-hot Jupiter (Credit: NASA, ESA, Leah Hustak (STScI) )

“In a paper in the April 6 journal Nature, astronomers describe Hubble observations of WASP-178b, located about 1,300 light-years away. On the daytime side the atmosphere is cloudless, and is enriched in silicon monoxide gas. Because one side of the planet permanently faces its star, the torrid atmosphere whips around to the nighttime side at super-hurricane speeds exceeding 2,000 miles per hour. On the dark side, the silicon monoxide may cool enough to condense into rock that rains out of clouds, but even at dawn and dusk, the planet is hot enough to vaporize rock. “We knew we had seen something really interesting with this silicon monoxide feature,” said Josh Lothringer of the Utah Valley University in Orem, Utah.”

The press release has been taken up by over 50 press and media websites.

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”.

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”.

No clouds on the dayside of WASP-43b

As you’ll likely know from flying in an aircraft above the weather, clouds are bright, they reflect a lot of sunlight. This means that if a hot-Jupiter exoplanet has a cloudy atmosphere, then it should also be relatively bright, and so we should be able to detect a discernible drop in light when it it eclipsed behind its host star.

A new paper by Jonathan Fraine et al analyses data obtained with the Hubble Space Telescope WFCS/UVIS instrument to look for the eclipse of WASP-43b. Here is the result (with the data compared to model eclipse profiles):

The authors find no significant eclipse, deriving only an upper limit to any drop in light of 67 parts-per-million, which means that the dayside face of the planet is reflecting less than 6% of the illuminating starlight. And that means “that we can rule out a high altitude, bright, uniform cloud layer”.

Fraine et al remark that “Because of its observational and atmospheric viability for spectroscopic detections, WASP-43b has become a benchmark planet for current and future hot Jupiter observations. Upcoming … JWST observations [will] map the thermal structure and chemical composition of this exoplanet with exquisite detail … We expect that no other exoplanet has or will be observed with this much precision and wavelength coverage for many years to come.”

The importance of cloud-free skies is that one can then see atomic and molecular spectral features much more readily, and so learn much more about the atmosphere’s composition.

WASP-62b, in James Webb’s continuous-viewing zone, has a clear atmosphere

James Webb’s “Continuous Viewing Zone” is the patch of sky where the satellite can point continuously at a target and so observe it most efficiently. Exoplanets within the CVZ that are suitable for atmospheric characterisation are thus of high importance, and so far WASP-62b is the only gas giant known within the CVZ.

Munazza Alam et al have now pointed the Hubble and Spitzer space telescopes at WASP-62b to see what its atmosphere looks like. Importantly, they find that WASP-62b has clear skies. This matters since cloudy or haze-filled atmospheres tend to produce flat spectra lacking any spectral features, and so don’t tell us much.

Here, Alam et al plot the spectrum near the sodium (Na) line, showing that it has a broad base, akin to that in the clear-skied planet WASP-96b. The broad base of the line means that it is being widened by “pressure broadening”, and that can only happen deep in the planet’s atmosphere where the pressure is high. And we can only see deep into the atmosphere if it is clear rather than cloudy.

Clear skies mean that spectral features produced by the molecules in the atmosphere should be readily detectable with JWST. Here Alam et al simulate what we expect to see with JWST, showing that Na, H2O, NH3, FeH, SiH, CO, CO2, and CH4 can all be detected.

They conclude by saying that: “As the only transiting giant planet currently known in the JWST Continuous Viewing Zone, WASP-62b could prove a benchmark giant exoplanet for detailed atmospheric characterization in the James Webb era.

Dayside spectrum of the ultrahot-Jupiter WASP-121b

Thomas Mikal-Evans et al have released a new paper analysing the heated, dayside face of WASP-121b. Teams studying the atmospheres of exoplanets either look at the transit, when the planet’s atmosphere is projected against the host star, such that molecules produce absorption features in the spectrum, or they study the eclipse, when the heated face of the planet disappear and then reappears. In the latter, atmospheric molecules produce emission features in the spectrum.

Here is the spectrum of the heated face of WASP-121b, based on recording five eclipses using the WFC3 spectrograph on the Hubble Space Telescope. The orange line and yellow banding show the spectrum expected for a pure black body of the same temperature as the planet. The red lines then show model fits, which reveal emission features caused by H ions and water (H2O) molecules.

No Rayleigh scattering gives yellow skies to exoplanet WASP-79b

Here’s a catch-up on a press release recently put out by NASA, Hubble and Johns Hopkins University, who led an analysis of WASP-79b. Lead author of the paper, Kristin Sotzen, combined spectroscopy from the ground-based Magellan II telescope in Chile with data from the HST and Spitzer satellites.

As explained in the press release: “The surprise in recently published results, is that the planet’s sky doesn’t have any evidence for an atmospheric phenomenon called Rayleigh scattering, where certain colors of light are dispersed by very fine dust particles in the upper atmosphere. Rayleigh scattering is what makes Earth’s skies blue by scattering the shorter (bluer) wavelengths of sunlight. Because WASP-79b doesn’t seem to have this phenomenon, the daytime sky would likely be yellowish, researchers say.”

“This is a strong indication of an unknown atmospheric process that we’re just not accounting for in our physical models.” said Sotzen.

WASP-79b also was observed as part of the Hubble Space Telescope’s Panchromatic Comparative Exoplanet Treasury (PanCET) program, and those observations showed that there is water vapor in WASP-79b’s atmosphere. Based on this finding, the giant planet was selected as an Early Release Science target for NASA’s upcoming James Webb Space Telescope.

The press release has led to national media coverage in the US and the UK, including by The Sun and Fox News.

Aluminium oxide in the atmosphere of hot-Jupiter WASP-43b?

WASP-43b is one of the favourite planets for atmospheric characterisation, being in such a tight, short-period orbit that it is heated up by its host star, such that the molecules in its atmosphere should be easy to discern.

A new paper by Katy Chubb et al re-analyses observations made with the Hubble Space Telescope and concludes that the observations show signs of aluminium oxide.

It is important to realise that this work is not easy, being right at the limit of what can be done, even with Hubble. Neither the spectral resolution nor the signal-to-noise ratio of the data are sufficient to unambiguously discern features of particular molecules. Instead, the art is to guess the molecules that might be present, simulate the resulting spectrum if the guess were right, and then compare that to the observed spectrum. This leads to figures such as this, from Katy Chubb’s paper:

The grey lines are the data (shown as error bars). The coloured lines are the calculated model (with the coloured bands then allowing for uncertainties), and the grey diamonds are where error-free data would be if the model were perfect. The x-axis is wavelength, and the y-axis is the effective radius of the planet’s atmosphere at that wavelength, which tells us how good it is at absorbing light of that wavelength.

The bottom panel (orange) fits the data with water vapour only, while the upper panel (blue) includes both water and aluminium oxide. The later gives a significantly better fit. The authors write that, in addition to water, “AlO is the molecule that fits the data to the highest level of confidence”, while “We find no evidence of the presence of CO, CO2, or CH4“.

However this could be a puzzle, since: “AlO is not expected from the equilibrium chemistry at the temperatures and pressures of the atmospheric layer that is being probed by the observed data. Its presence therefore implies direct evidence of some disequilibrium processes with links to atmospheric dynamics.”

As with all current characterisation of exoplanet atmospheres, we await the James Webb Space Telescope (which has been designed to do this work; Hubble was designed before exoplanets were even known), to tell us how reliable the current results are.

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.