WASP-18b has a smothering stratosphere without water

NASA Goddard Space Flight Center and NASA Jet Propulsion Laboratory have put out press releases about observations of WASP-18b with the Hubble Space Telescope and the Spitzer Space Telescope.

The main finding is that WASP-18b, a highly irradiated hot Jupiter in a tight orbit around a hot F-type star, is “wrapped in a smothering stratosphere loaded with carbon monoxide and devoid of water”.

The team determined this by detecting two types of carbon monoxide signatures, an absorption signature at a wavelength of about 1.6 micrometers and an emission signature at about 4.5 micrometers.”

The findings have been reported in many media outlets including: Newsweek, The Independent, The Sun, the Daily Mail, the International Business Times, phys.org, and more than 20 other websites including Forbes magazine, who have produced the following infographic:

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Wide-coverage spectrum of exoplanet WASP-39b

WASP-39b is turning out to be one of the more important WASP discoveries, being observed with the Hubble Space Telescope, the Spitzer Space Telescope and large ground-based telescopes such as the VLT. This is because, as a Saturn-mass planet with a bloated radius, it has a low surface gravity and so is ideal for atmospheric characterisation. Further, it has relatively clear skies showing spectral features.

Now a team led by Hannah Wakeford from Exeter University have put the different data-sets together to produce the widest-coverage spectrum of the planet so far:

The dominant spectral features are due to water vapour, while there are narrower lines due to sodium (Na) and potassium (K) and a Rayleigh-scattering slope at the blue end.

The main finding from fitting the water features is that the atmospheric metallicity must be at least 100 times that of the sun. This high value shows the diversity of exoplanets. The authors conclude that “WASP-39b is an ideal target for follow-up studies with the James Webb Space Telescope”.

A first planet for the Next Generation Transit Survey

The latest transit survey to announce their first planet is the Next Generation Transit Survey. While the planet NGTS-1b has a fairly normal mass and radius for a hot Jupiter, it is unusual in being found transiting an M0-type dwarf, a star of only 0.6 solar radii. Thus the planet is nearly a quarter as big as the star, in terms of radius, the highest planet-to-star ratio yet found.

NGTS is an array of twelve 20-cm telescopes sited at Cerro Paranal in Chile, and has been accumulating survey data since 2016.

Next-Generation Transit Survey

It is important to realise that the newer survey NGTS does not supersede WASP, but instead complements it, being designed to do a different task. WASP, and similar surveys such as HATnet and KELT, use camera lenses (typically 200-mm f/1.8 or 85-mm f/1.2) to survey large swathes of sky. The data is good enough to detect transits of Jupiter- and Saturn-sized planets, but not smaller ones.

NGTS was designed to find smaller planets, down to Neptune and possibly super-Earth size. To do that it uses bigger optics, being telescopes rather than camera lenses, with a much better plate scale (more CCD pixels per chunk of sky). This gives much better photometry, but at the price of a much smaller field of view. A smaller field of view means covering many fewer bright stars.

Indeed, NGTS has a field of view comparable to the Kepler field (1% of the sky), though since it will raster several fields it will add up to sky coverage comparable to that of the Kepler K2 mission phase.

Thus WASP, running with 200-mm lenses surveying much of the sky, finds Jupiters and Saturns transiting stars of typically V = 9 to 13. NGTS can find smaller planets, and is aimed at finding Neptunes, but will likely find them transiting fainter stars of typically V = 13 to 14 (and perhaps, as with K2, an occasional brighter one).

Meanwhile, WASP-South has recently been running with wider, 85-mm lenses, which cover the whole Southern sky and target stars of V = 6.5 to 11.5. Hence the two surveys are entirely complementary: WASP aiming for large, Jupiter-sized planets around very bright stars, while NGTS aims for Neptune-sized planets around much fainter stars.

The main competition for WASP is now KELT and MASCARA, whereas the main competition for NGTS is the ongoing K2 mission. Of course NASA’s forthcoming TESS mission, set for launch in 2018, should out-compete all of the ground-based surveys.

WASP planets selected for James Webb Space Telescope ERS and GTO

Studying the atmospheres of exoplanets is one of the main goals of the James Webb Space Telescope, now scheduled for launch in mid 2019. The mission recently asked for proposals for “Early Release Science”, observations to test out the instruments, show what JWST can so, and supply the community with data to start analysing.

Of 13 ERS proposals accepted, the “The Transiting Exoplanet Community ERS Program”, led by Kepler lead-scientist Natalie Batalha, got all the time it asked for.

WASP planets feature heavily in the ERS program, since many transit relatively bright stars. Large, puffy gaseous planets will also give the strongest and clearest signals of atmospheric features, and so are optimum early targets. While JWST will want to look also at atmospheres of smaller, rocky planets, “Astronomers initially will train their gaze onto gaseous Jupiter-sized worlds like WASP-39b and WASP-43b because they are easier targets on which to [look for the chemical fingerprints of the atmosphere’s gases]”.

The target list for the ERS proposal is currently being finalised in the light of the recent delay in JWST launch from 2018 to 2019, though an earlier draft of the proposal featured 7 WASP planets out of 12 targets.

Further, the four GTO teams have also selected WASP planets for early JWST observations. GTO time (“Guaranteed Time Observations”) is time allocated to the teams who built the JWST instruments as a reward and incentive. All four instrument teams have picked WASP planets, including WASP-17b, WASP-52b, WASP-43b, WASP-69b, WASP-77Ab, WASP-80b, WASP-107b and WASP-121b.

Meanwhile, Kevin Heng, of the University of Bern, has written a popular-level account for American Scientist of how JWST is expected to revolutionise the study of exoplanet atmospheres.

Hot Jupiter irradiation and the efficiency of heat recirculation

Here’s an interesting plot from a new paper by Michael Zhang et al.

The x-axis is the irradiation temperature for a sample of hot-Jupiter exoplanets; that is, how blasted the day-side of their atmosphere is by irradiation from the host star. This depends on the temperature of the star, its size, and the closeness of the orbit.

The heat of the day side of the planet is then transported to the night side by winds (the planets are phase-locked, so the same side always faces the star). The efficiency of this re-circulation of heat then determines whether the hottest regions of the planet are directly facing the star, or whether they are offset by some angle. This angle can by measured by looking at the “phase curve” radiation in the infra-red.

The y-axis then shows the observed offset angle as a function of the irradiation. The plot shows that the offset angle appears to be highest for cooler planets, and then decreases as irradiation increases, but then perhaps increases again for the very hottest planets such as WASP-33b.

There is, however, also a lot of scatter in the plot. The authors speculate that this might result from differing metallicities of the planets, which affects how well they form clouds, which can then determine the albedo of the planet, and thus how much irradiation is simply reflected.

Outer-orbiting companions of hot-Jupiter planets appear to be co-planar

On-going radial-velocity monitoring of WASP hot Jupiters has shown that some of them have companions, additional Jupiter-mass planets in much wider orbits.

This might be part of the answer as to why there are hot Jupiters at all. Standard planet-formation theory suggests that they must form much further out, where it is colder and where ice can form, enabling bits of pre-planetary debris to clump together. Thus one solution is that gravitational perturbations by third bodies (wide-orbit massive planets or companion stars) push the inner planets into highly eccentric orbits, where tidal capture then circularises them into hot-Jupiter orbits.

But, if this “Kozai effect” is to work, the outer planets need to be in orbits tilted with respect to the orbits of the hot Jupiters. This requires i < 65 degrees, rather than the co-planar i = 90 degrees.

A new paper by Juliette Becker et al reports an analysis of six hot-Jupiter systems orbiting cool stars that have an outer planetary companion. These are WASP-22, WASP-41, WASP-47, WASP-53, HAT-P-4 and HAT-P-13. Though a statistical analysis they show that the outer planets are most likely co-planar, with orbits tilted by no more than 20 degrees. They thus argue that Kozai-driven high-eccentricity migration is not the dominant way of forming hot Jupiters.

Precise masses for the WASP-47 exoplanetary system

In the discovery paper the exoplanet WASP-47b was introduced to the world with the description: “With an orbital period of 4.16 d, a mass of 1.14 MJup and a radius of 1.15 RJup, WASP-47b is an entirely typical hot Jupiter”.

And it did appear to be entirely typical until Juliette Becker et al looked at K2 lightcurves and found two more planets, a super-Earth orbiting inside the hot Jupiter (WASP-47e in a 0.79-d orbit) and a Neptune orbiting just outside it (WASP-47d in a 9-d orbit). Around the same time Neveu-VanMalle et al announced long-term monitoring showing another Jupiter-mass planet (WASP-47c), this one in a much wider orbit of 580 days. Thus WASP-47 was shown to host a whole exoplanetary system, one that is so-far unique.

Since then WASP-47 has been observed intensively in order to measure the planet masses and investigate the dynamics of the exoplanetary system. The state of play is now reported by Andrew Vanderburg et al. The planets’ host star is tugged around by the gravitational pull of the orbiting planets, leading to the following cyclical variations in the observed radial velocities:

Combining all the information, Vanderburg et al deduce that the innermost “super-Earth”, WASP-47e, is not dense enough to be made only of rock. Instead it likely has a liquid or gaseous envelope (possibly water or steam) surrounding an Earth-like core. That is unlike other ultra-short-period super-Earths which appear to be fully rocky.

From modelling the dynamical history of the system Vanderburg et al also deduce that the outermost planet, WASP-47c, is likely in an orbit that is in the same plane as those of the inner planets. If this were not the case then the system would not be stable. Thus they conclude that the likelihood that WASP-47c also transits its star, as seen from Earth, is relatively high, which should motivate a campaign to look for those transits.