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.
Hot Jupiter planets are in tight orbits around their host star, and since that star will not be perfectly spherical, small gravitational perturbations should cause the orbit to precess. A team led by Marshall Johnson has now shown that this is indeed happening in WASP-33.
WASP-33 is a very hot, rapidly rotating A-type star. This means the planet is only detected by the “shadow” that it causes in the profiles of the spectral lines of the star during transit.
Since the star is rotating the spectral lines will be broadened by the Doppler effect, with photons from the approaching limb being blue-shifted and photons from the receding limb being red-shifted. As the planet transits the star, it blocks the light from one small region of the star’s surface. This removes the photons that are Doppler shifted with the velocity of that part of the star’s surface.
The trace of the planet across the star’s surface during transit can therefore be seen as a stripe moving in velocity across the profile of the star’s spectral lines. This is seen in these false-colour images of the spectral line of WASP-33, taking during two transits, six years apart:
The white diagonal stripe is the path of the planet, blocking out the photons below it. The stripe is clearly in a different place in the two observations. This means that the path of the orbit has changed. Johnson et al give the following schematic of how they think the orbit of the planet has changed between the two observations.
This observation validates the theory that the orbit should be precessing, and is only the second detection of nodal precession in an exoplanet orbiting a single star, after the example of Kepler-13 Ab.
NASA have put out a press release about Hubble Space Telescope observations of WASP-33b.
WASP-33b is the hottest of the WASP planets, being the only one so far found orbiting a very hot A-type star. A team led by Korey Haynes from NASA’s Goddard Space Flight Center, have used Hubble to show that WASP-33b has a “stratosphere”. The spectrum in the infra-red is best explained by a temperature inversion caused by the presence of Titanium Oxide in the atmosphere.
Titanium Oxide is noted for its ability to absorb light, which is why it is often used in sunscreen lotion. NASA’s graphic shows how an absorbing layer in the atmosphere produces a “temperature inversion” with a hotter layer higher up:
WASP-33b’s stratosphere was detected by measuring the drop in light as the planet passed behind its star (top). Temperatures in the low stratosphere rise because of molecules absorbing radiation from the star (right). Without a stratosphere, temperatures would cool down at higher altitudes (left). [Image: NASA/GSFC]
By comparing models with and without a temperature inversion to the spectrum of WASP-33b, as observed with Hubble’s WFC3 instrument, Haynes et al “make a very convincing case that we have detected a stratosphere on an exoplanet”.
The figure shows the spectrum of WASP-33b (left) and the temperature profile of the atmosphere (right), both for models with a temperature inversion (red) and without an inversion (blue). (From Haynes et al 2015)
The work has been reported widely, in over 100 news and science websites, such as by SciTechDaily, Pioneer News, The Daily Mail, and NY City Today.