Two K2 planets transiting bright stars

With the launch of the James Webb Space Telescope only a year away the exoplanet community is gearing up to exploit its capability for characterising exoplanet atmospheres. A new paper by Yu et al contains a plot of the best targets, giving the expected “signal to noise” for each planet as a function of the planet’s mass. The higher the S/N the better, enabling more atmospheric features to be discerned.

It is notable that most of the best targets do not come from Kepler (which had a relatively small field of view, and so looked at mainly fainter stars), but instead from the ground-based transit surveys (which focus mainly on brighter stars, which are thus better targets for follow-up). WASP features strongly, supplying half of the best targets.

The focus of the Yu et al paper, however, is the discovery of two very good targets from the K2 phase of Kepler‘s mission. K2 is observing more fields for less time than the original Kepler, and so covers more bright stars.

HD 89345b (labelled in red above) is only 10% of Jupiter’s mass but is bloated to 0.6 Jupiter radii. Transiting a bright star of V = 9.4 makes it a prime target.

The transit depth of only 0.15% means that it is too shallow to have been detected by WASP (which can do 0.2–0.3% at best), especially given the 11.8-day orbit, which means that it produces fewer transits than shorter-period planets.

The other new discovery, HD 286123b (which had also been independently found by Brahm et al), is a larger and more massive planet producing a 0.8% dip. This one should have been within the reach of the WASP survey, but happens to lie in a region of the Northern sky where SuperWASP-North has only limited data.

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Comprehensive Spectrum of WASP-39b

NASA, ESA and JPL have put out press releases on the atmospheric spectrum of WASP-39b. The paper by Hannah Wakeford et al combined Hubble and Spitzer data to produce a comprehensive spectrum with broad spectral coverage.

“Using Hubble and Spitzer, the team has captured the most complete spectrum of an exoplanet’s atmosphere possible with present-day technology. “This spectrum is thus far the most beautiful example we have of what a clear exoplanet atmosphere looks like,” said Wakeford.”

“WASP-39b shows exoplanets can have much different compositions than those of our solar system,” said co-author David Sing of the University of Exeter. “Hopefully, this diversity we see in exoplanets will give us clues in figuring out all the different ways a planet can form and evolve.”

The strongest features in the spectrum are caused by water:

“Although the researchers predicted they’d see water, they were surprised by how much water they found in this “hot Saturn.” Because WASP-39b has so much more water than our famously ringed neighbor, it must have formed differently. The amount of water suggests that the planet actually developed far away from the star, where it was bombarded by a lot of icy material. WASP-39b likely had an interesting evolutionary history as it migrated in, taking an epic journey across its planetary system and perhaps obliterating planetary objects in its path.”

Coverage of the press release includes that by Newsweek, the International Business Times, the Daily Mail and about 30 other websites.

How fast do the orbits of hot Jupiters decay?

Tidal interactions between close-in, gas-giant exoplanets and their host star should cause the orbits of the planets to decay. The crucial number in determining how fast that happens is the “quality factor”, Q, which tells us the fraction of the tidal energy that is dissipated in each cycle. A high value of Q, say 107, means that only 1 part in 107 of the energy is dissipated, giving a low rate of orbital decay. A smaller value gives a faster decay.

A new study by Kaloyan Penev et al suggests that Q varies a lot depending on the tidal “forcing period” (that is, the period at which a planet would appear to orbit, if viewed when rotating with the spinning star, with an extra factor of a half since there are two tides per orbital cycle).

Penev estimate the value of Q by comparing the observed spin period of the host star to the most likely spin period expected for that sort of star, if it had no planet, and so modelling how much the star has been spun up by the tidal interaction with the planet.

They find that the Q of the star is high, about 107, when the tidal forcing period is low (< 1 d) but much smaller, about 105.5, when the forcing period is longer.

This work might resolve several puzzles. The Q value expected from studying binary stars is near 105.5, but if that were true for all hot Jupiters then they’d be destroyed too readily, and the current observed population could not be explained. This puzzle is resolved if their orbits decay much more slowly when the forcing period is short.

The different Q values also allow the planets to re-align their orbits with the spin of the star (so that the orbital plane is perpendicular to the star’s spin axis) on a timescale shorter than the orbital period decay, thus explaining why there are many “aligned” hot Jupiters.

Heat redistribution in hot-Jupiter atmospheres by hydrogen ionisation

Since hot Jupiter exoplanets are “phase locked” by tidal interactions (that is, the same side always faces the host star, just as the same side of our moon always faces us), there will be a large flow of heat from the highly irradiated “day side” to the cooler “night side”. This is thought to result in very strong winds rushing around the planet’s atmosphere.

Taylor Bell and Nicolas Cowan have pointed out that hydrogen will tend to be ionised on the day-side face. After flowing to the cooler face in a wind, it will then tend to recombine into neutral atoms, and thus will enhance the transport of heat.

The result is that either heat redistribution will be more effective than previously thought, helping to explain some observations of hot Jupiters, or the winds need be less strong than thought.

Bell and Cowan calculate the difference for WASP-12b. The plot shows models of the difference in temperature (x axis) against the offset of the “hot spot” caused by heat flow (y axis). The different colour coding shows the wind speed. The plot then shows the difference between models including hydrogen recombination, versus previous models by Schwartz. For a given wind speed, including hydrogen recombination results in a larger offset angle, and thus more redistribution of heat.

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:

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.