Vivien Parmentier (Oxford University, @V_Parmentier) has produced an explanatory Twitter thread on his latest paper with Jonathan Fortney (@jjfplanet). Since this concerns WASP exoplanets, let’s reproduce it here:
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.
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.
Circumbinary planets are planets that orbit a binary star, rather than a single star, in contrast to most planets known. Possibly they formed out of the same disk of material that formed the two stars. So long as the planet is sufficiently far from the binary, its orbit can be stable.
Several circumbinary planets were discovered by the Kepler mission, but the first such planet found using TESS has recently been announced by Veselin Kostov et al.
Here is the TESS lightcurve of “target of interest” TOI-1338:
Six-percent-deep eclipses recur every 14.6 days, when a small, M-dwarf star eclipses its Sun-like companion. But, when one looks more closely at portions of the lightcurve, one also sees:
Near Day 1483 is a deep eclipse (truncated in this plot). Near Day 1577 is a shallower secondary eclipse, when the Sun-like star eclipses the M-dwarf star. But, in addition, and highlighted by green bands, are two transits where a Saturn-like planet in a wide, 95-day orbit, transits the Sun-like star.
The interest, from the WASP point of view, is that the eclipsing binary had already been found by the WASP survey. The primary eclipses were too deep to be caused by a Jupiter-size planet, and were instead found to be caused by the M-dwarf, but such systems are interesting in their own right, and thus formed the “EBLM” programme offshoot from WASP, led by Amaury Triaud.
Since the eclipsing binary was already known to be an interesting system, this meant it could be observed with 2-min cadence in the TESS survey (instead of the default 30-min cadence data), and that made the detection of the circumbinary-planet transits much easier.
Above is an illustration of how the planet’s orbit precesses around the inner binary over time. The inner binary is illustrated in black and grey. The coloured dots show how the orbit of the planet changes over 2000 days. When the planet passes within the grey ellipses, transits can be seen.
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.