Here’s an interesting plot created by Zafar Rustamkulov (@exoZafar), a PhD student at Johns Hopkins University. He has added up all the exoplanets for which we have either transmission spectra (blue), emission spectra (red) or both (pink), and plotted the planet’s size and orbital period.
Most atmospheric characterisation has been done on the hot Jupiters (top left of the plot), since these are the easiest to study. Their large size and often bloated, fluffy outer layers produce the largest spectral signals. Smaller planets are harder to study, unless their host stars are very bright or very small (such that the fraction blocked by the planet during transit is relatively large).
For the planets for which we have over 50 spectra Zafar has added the planet’s name (though the lettering is rather small!). This shows that roughly half of the most-studied exoplanets come from the WASP survey. WASP-12b, WASP-33b and WASP-39b are in the Northern Hemisphere and came from the SuperWASP-North survey. WASP-17b, WASP-19b, WASP-31b, WASP-43b, WASP-80b, WASP-107b, WASP-121b and WASP-127b are in the South and so are from the WASP-South survey.
ESA’s Cheops satellite (the Characterising Exoplanet Satellite) started observing this year, and ESA has just put out a press release announcing its first science results. Cheops looked at transits and occultations of WASP-189b, an ultra-hot Jupiter in a polar orbit transiting a bright star.
“Only a handful of planets are known to exist around stars this hot, and this system is by far the brightest,” says Monika Lendl of the University of Geneva, Switzerland, lead author of the new study. “WASP-189b is also the brightest hot Jupiter that we can observe as it passes in front of or behind its star, making the whole system really intriguing.”
At a visual magnitude of V = 6.6, WASP-189 is the brightest host star of all the WASP planets. The discovery of the transiting hot Jupiter was announced in 2018 in a paper led by David Anderson. The exceptional nature of WASP-189 thus made it a prime target for Cheops.
The Cheops study shows that: “the star itself is interesting – it’s not perfectly round, but larger and cooler at its equator than at the poles, making the poles of the star appear brighter,” says Dr Lendl. “It’s spinning around so fast that it’s being pulled outwards at its equator!”
“This first result from Cheops is hugely exciting: it is early definitive evidence that the mission is living up to its promise in terms of precision and performance,” says Kate Isaak, Cheops project scientist at ESA.
Press coverage has included articles in CNN, The Mirror, The Daily Mail, and The Express.
A new paper by Sarah Millholland et al reconsiders highly bloated, low-mass planets such as WASP-166b. One explanation for the low mass of such planets is that they have small cores and are mostly gaseous envelope. However, having a relatively small core is at odds with core-accretion theory for the formation of such planets, which says that they can only gravitationally attract and then accrete large envelopes if the core is sufficiently massive.
Instead, Millholland et al suggest that the envelope is a smaller fraction of the planet’s mass than it seems, and that instead it has expanded to its current bloated state by tidal heating. A small eccentricity of the orbit is sufficient to produce tidal dissipation that heats the envelope and thus causes it to expand.
In the figure, the authors plot the fraction of the planet that is envelope, assuming no tidal heating, and also the smaller fraction when accounting for the effects of tidal heating. The reduction makes the proportions compatible with core-accretion theory. Millholland et al suggest that: “many sub-Saturns may be understood as sub-Neptunes that have undergone significant radius inflation, rather than a separate class of objects”.
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.
The press release has led to national media coverage in the US and the UK, including by The Sun and Fox News.
Even though WASP has found nearly 200 planets we are still announcing systems that are unlike any previous ones. WASP-148 is an example, as described in the discovery paper by Guillaume Hébrard et al.
WASP first detected transits of the hot Jupiter WASP-148b in an 8.8-day orbit. Spectroscopic observations with OHP/SOPHIE, aimed at measuring the planet’s mass, then found that there was also a second massive planet in a longer, 35-day orbit:
The orbits of both planets are eccentric, likely because they are perturbing each other by their gravitational attraction. Further, the gravitational perturbations mean that the transits of the inner planet vary in time by 15 mins.
We don’t yet know whether the outer planet, WASP-148c, also transits (since its longer period means that there are gaps in WASP’s coverage of its orbit), but this patch of sky is currently being observed by the TESS satellite. The space-based photometry from TESS will be good enough to detect any transits of WASP-148c, to map out transit-timing variations, and to look for additional planets in the system that are too low mass to have been detected in the radial-velocity data. WASP-148 is thus an important system for studying an unusual planetary-system architecture, with two massive planets in relatively close orbits in resonance with each other.
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.
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”.
Ian Wong et al have produced a new analysis of the TESS data on previously known WASP exoplanets. Their main interest is the “phase curve”, the variation of the light around the planet’s orbit.
Two examples are the systems WASP-72 and WASP-100:
In addition to the main transit (planet passing in front of the star) the phase curves show secondary eclipses (planet passing behind the star, at phase 0.5) and a sinusoidal variation due to the heated face of the planet. By modelling the phase-curves of these and other similar planets, Wong et al make the tentative suggestion that the hotter the planet (which can be measured from the depth of the secondary eclipse) the more reflective the atmosphere of the planet is.
Here’s a similar plot for WASP-30. Note, though, that the phase-curve variation peaks at phases 0.25 and 0.75, unlike those for WASP-72 and WASP-100. That’s because WASP-30b is not a planet but a brown dwarf, with a mass of 63 Jupiters. That is massive enough for its gravity to distort the host star into an ellipsoidal shape, and so in this system the variation of the light is caused by the varying projection of the distorted star around the orbit.
Congratulations to Dr Amaury Triaud, now at the University of Birmingham, recipient of the 2020 Fowler Award from the Royal Astronomical Society. The Fowler Award is for scientists making a “particularly noteworthy contribution to Astronomy & Geophysics at an early stage of their research career”.
The citation reads: “Between 2007 and 2017, Dr Triaud led the radial-velocity follow-up of planet candidates south of declination −10 degrees from the Wide-Angle Search for Planets (WASP). His programme led to the discovery of over 130 planets from some 1000 candidates, making WASP the most successful of all ground-based transit searches.”
Amaury started looking for WASP exoplanets as a graduate student at the Geneva Observatory, under the direction of Didier Queloz (himself recipient of the 2019 Nobel Prize for Physics for his discoveries of exoplanets). Didier’s group at Geneva operated the CORALIE spectrograph on the 1.2-m Euler telescope at La Silla in Chile. Euler/CORALIE was the ideal follow-up instrument to vet the transiting exoplanet candidates coming from WASP-South, able to show which ones were genuinely the transits of planetary-mass bodies (only 1-in-10 of all candidates), and which were merely transit mimics. Amaury organised and ran the campaign, observing of order 1500 candidates and leading to the discovery of around 150 planets.
The Euler 1.2-m telescope
While the citation mentions the campaign for Southern candidates south of declination −10 degrees, the Geneva group were also responsible for much of the follow-up in the equatorial strip from −10 to +10 degrees, where the candidates came jointly from data from WASP-South and from SuperWASP-North on La Palma.
Amaury’s work extended into studying the orbits of the WASP exoplanets, showing that many of the orbits were misaligned. He also developed programs identifying and studying the low-mass binary stars that also came from the WASP survey, and is now looking for circumbinary planets orbiting these low-mass binaries.