Tag Archives: WASP-121b

The changing atmosphere of WASP-121b

NASA have put out a press release about observations of hot-Jupiter exoplanet WASP-121b. A team led by ESA scientist Quentin Changeat analysed observations with the Hubble Space Telescope made in 2016, 2018 and 2019 to find distinct differences in the atmosphere of the planet, revealing that planets have weather patterns.

The figure below shows the “phase curve”, a smooth variation caused by the varying visibility of the heated face of the planet (there are also sharp dips caused by the eclipse and transit of the star/planet). Differences between the phase curve in different years indicate changes in the atmosphere.

Heat maps of WASP-121b show the changing temperatures of the irradiated face of WASP-121b.

The press release has led to articles on WASP-121b on dozens of websites across the world.

Detection of barium in the atmospheres of WASP-76b and WASP-121b

ESO have put out a press release reporting the discovery of barium in the atmospheres of ultra-hot Jupiters WASP-76b and WASP-121b, based on observations with the ESPRESSO spectrograph on ESO’s Very Large Telescope. Barium is the heaviest element so far detected in an exoplanet.

“The puzzling and counterintuitive part is: why is there such a heavy element in the upper layers of the atmosphere of these planets?” says Tomás Azevedo Silva, lead author of the paper.

This artist’s impression shows an ultra-hot exoplanet, a planet beyond our Solar System, as it is about to transit in front of its host star. When the light from the star passes through the planet’s atmosphere, it is filtered by the chemical elements and molecules in the gaseous layer. With sensitive instruments, the signatures of those elements and molecules can be observed from Earth. Using the ESPRESSO instrument of ESO’s Very Large Telescope, astronomers have found the heaviest element yet in an exoplanet's atmosphere, barium, in the two ultra-hot Jupiters WASP-76 b and WASP-121 b.

The work has been reported by dozens of newspapers and media websites, such as CNN and The Independent.

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.

Weather on ultra-hot-Jupiter WASP-121b?

Here’s a plot of the spectrum of the ultra-hot-Jupiter WASP-121b. It’s from a new paper led by Jamie Wilson of Queen’s University Belfast.

The plot compares results from different instruments at different times. In particular the green points are from the ground-based Gemini/GMOS instrument, and are fitted by the model in red. The light-blue points (and fitted purple model) are from the space-based HST/STIS instrument.

Clearly the two datasets are not consistent. One possible explanation would involve instrumental systematics that are not properly accounted for in the analysis. Such analyses are right at the edge of what can be done, pushing the instruments beyond their designed capabilities, and reducing the datasets to a properly calibrated spectrum is a demanding task.

The other possible explanation is that WASP-121b really was different on the two occasions, and that “weather” on the planet is affecting its atmosphere. Just as Earth’s atmosphere can change from clear to cloudy, we expect that the same could be occurring on exoplanets.

The authors say that: “WASP-121b is expected to have wind speeds of 7 km/s and a pressure–temperature profile which lies near the condensation curves of a number of species”, and thus: “It is therefore perhaps not all that surprising that small temperature fluctuations could result in significant spatial and temporal variations in atmospheric constituents and could lead to measurable variations in transit measurements.”

ESPRESSO looks at ultra-hot-Jupiter WASP-121b

ESPRESSO is ESO’s state-of-the-art spectrograph for the Very Large Telescope, specifically designed to get the best data possible on planetary systems.

Francesco Borsa et al have pointed ESPRESSO at transits of the ultra-hot-Jupiter WASP-121b, and the ESPRESSO team have put out a series of Tweets explaining the paper:

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.

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.

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.

WASP-121b observed by TESS

As is sometimes the way when prime observations are open access, two independent papers (Daylan et al 2019; Bourrier et al 2019) have, on the same day, announced independent analyses of the TESS lightcurve of the ultra-hot Jupiter WASP-121b.

The phase curve shows the transit (time zero), a “phase curve” modulation caused by the varying visibility of the heated face of the planet (illustrated by schematics of the planet), and the eclipse (when the planet passes behind the star, at −15 hr).

Both analyses report similar findings, saying that the heated “hot spot” directly faces the star, rather than being offset in phase, which suggests that any re-circulation of heat by planetary winds is inefficient.

The planet’s atmosphere shows a temperature inversion (it is hotter at higher altitudes), which could result from absorption of heat by molecules of titanium and vanadium oxide, and H-minus ions.