Category Archives: Hot Jupiters

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,, and more than 20 other websites including Forbes magazine, who have produced the following infographic:

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.

First results on the atmosphere of WASP-107b

Being a Neptune-mass planet (0.12 MJ) bloated to a near-Jupiter radius (0.94 RJ) makes WASP-107b’s atmosphere very fluffy, and that, coupled with it transiting a moderately bright K star (V = 11.6) makes it a superb target for atmospheric characterisation.

Laura Kreidberg et al have pointed the Hubble Space Telescope at WASP-107b to make the first atmospheric study. Here’s the WFC3 spectrum:

Hubble Space Telescope spectrum of WASP-107b

The broad features at 1.15 and 1.4 microns are due to water absorption in WASP-107b’s atmosphere. Kreidberg et al model the features, finding that they are compatible with expectations given solar abundances. They are not deep enough, though, to be produced by fully clear skies, and a layer of high-altitude cloud is also required.

WASP-107b is one of the prime exoplanets already chosen for early observations with the imminent James Webb Space Telescope, so it is exciting to know that its atmosphere does show prominent molecular features.