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.
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.
A new paper by Shreyas Vissapragada and colleagues reports a new technique for detecting material boiling off hot-Jupiter exoplanets. The idea is that helium atoms in escaping material should be strong absorbers of light at the wavelength of 1083.3 nm, one of the transitions of neutral helium. Thus, if one records a transit in an ultra-narrow-band filter around that wavelength, the planet should look bigger and so the transit should be deeper.
Vissapragada et al pointed the 200-inch Hale Telescope at a transit of WASP-69b. Here’s the result:
The blue line is the usual transit depth expected in continuum light. The data and fitted red line are the transit observed in the 1083.3-nm helium line. The authors compute that the extra depth of the transit implies that 30 million kilos of material is evaporating off the planet each second, as a result of stellar irradiation. This sounds a lot, but adds up to only a few percent of the planet’s mass over the host star’s lifetime.
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.
Hot Jupiter exoplanets are “phase locked” by tidal forces, meaning that the same face of the planet always faces the star. Being blasted by radiation it is far hotter than the night side. This means that strong winds must be racing around the planet, redistributing the heat.
And that means that the “evening” terminator (where winds flow from the hot day-side face to the cooler night side) will be much hotter than the “morning” terminator (where winds flow from the night side to the day side). Here’s an illustration from a new paper by Ryan MacDonald, Jayesh Goyal and Nikole Lewis:
Of course the terminators are exactly the regions of the planet’s atmosphere that are being sampled by atmospheric-characterisation studies, since that’s the regions that are seen projected against the host star.
As Ryan MacDonald et al point out, most atmospheric-characterisation studies assume that the two limbs are the same, since that’s the easiest thing to do. However, the authors argue, while doing that might produce an acceptable fit to the data, the resulting parameter values could be very wrong.
Thus, the fitted temperature profile could be “hundreds of degrees cooler” than reality. As a result, the fitted abundances of molecular species could also be wrong. MacDonald et al conclude that: “these biases provide an explanation for the cold retrieved temperatures reported for WASP-17b and WASP-12b” and say that: “to overcome biases associated with 1D atmospheric models, there is an urgent need to develop multidimensional retrieval techniques”.
ESO have produced a press release about a Nature paper on WASP-76b. The study was led by David Ehrenreich of the Geneva Observatory, and used observations with the new ESPRESSO spectrograph on the ESO VLT.
“The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets.”
“One could say that this planet gets rainy in the evening, except it rains iron,” says Ehrenreich. “The observations show that iron vapour is abundant in the atmosphere of the hot day side of WASP-76b,” adds María Osorio, chair of the ESPRESSO science team. “A fraction of this iron is injected into the night side owing to the planet’s rotation and atmospheric winds. There, the iron encounters much cooler environments, condenses and rains down.”
ESO have produced an artist’s impression of iron rain as dusk on WASP-76b:
ESO have also produced videos of WASP-76b and its host star.
Media coverage from the press release includes The BBC, CNN, The Guardian, The Times, The Independent, the NY Times, Newsweek, NBC News, the Canadian Broadcasting Company, the Canberra Times, and others amounting to over 50 English-language articles plus coverage in German, French, Chinese, Polish, and other languages.
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.
Here’s a plot from a new paper on WASP-107b by James Kirk et al. It shows data taken with a near-infra-red spectrograph on the 10-m Keck II telescope on Mauna Kea, and is focused on the Helium line at 10833 Å. The plot shows the spectra as a function of time (y-axis), though a transit. When the planet passes in front of its host star (white horizontal lines are times of ingress and egress) the helium line shows excess absorption. This helium is in the atmosphere of the planet and is absorbing some of the starlight. There is a slight change in the wavelength of the absorption owing to the orbital motion of the planet (denoted by the dashed white lines).
The paper shows, firstly, that ground-based telescopes such as Keck can do a fine job of discerning the compositions of exoplanet atmospheres. Secondly, the fact that the absorption extends beyond transit-egress indicates that the atmosphere is boiling off the surface of WASP-107b, under the fierce irradiation of the star, and is forming a comet-like tail.
The plot shows the measured water abundance versus the planet’s mass. Welbanks et al state that: “We find a mass–metallicity trend of increasing H2O abundances with decreasing mass”, and also that: “The H2O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms”.
The press release explains that: “The researchers found that while water vapour is common in the atmospheres of many exoplanets, the amounts were surprisingly lower than expected, while the amounts of other elements found in some planets were consistent with expectations”.