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.
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.
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.
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.
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.”
“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.”