Category Archives: exoplanet atmospheres

Night-side temperatures of hot Jupiters

A team from McGill University have put out a press release about the nightsides of hot Jupiter exoplanets, which, given that hot Jupiters are phase-locked, always point away from their star. Dylan Keating et al collected observations with the Spitzer Space Telescope for a sample of 12 hot Jupiters, including 7 WASP exoplanets.

They find that, while the heated daysides show a range of temperatures, the nightsides always have a similar temperature:

“The uniformity of the nightside temperatures suggests that clouds on this side of the planets are likely very similar to one another in composition. Our data suggest that these clouds are likely made of minerals such as manganese sulfide or silicates, or rocks”, Keating explained.

Caption: Schematic of clouds on the night side of a hot Jupiter exoplanet. The underlying atmosphere is over 800 C, hot enough to vaporize rocks. Atmospheric motion from the deep atmosphere or from the hotter dayside bring the rock vapour to cooler regions, where it condenses into clouds, and possibly rains down into the atmosphere below. These clouds of condensed rock block outgoing thermal radiation, making the planet’s nightside appear relatively cool from space. Credit: McGill University

The work has led to press coverage by Fox News, Sci News, UPI, and other websites.

Hints of volcanic exo-moons?

A new paper by Apurva Oza et al has proposed the interesting idea that spectral features of sodium, previously attributed to the atmospheres of hot-Jupiter exoplanets, could actually be caused by volcanos on exo-moons orbiting the planets. The volcanos would produce a cloud of material surrounding the planet:

They suggest that WASP-49b might be the prime candidate for such a system. The idea has been discussed in a press release by the University of Bern. In our Solar System, Jupiter’s moon Io has continuous volcanic activity because of tidal stresses owing to the moon being close to Jupiter’s strong gravity. The authors produce an artist’s impression of how a volcanic exomoon might look:

All this is, of course, currently speculative, but the press release has led to widespread coverage of the idea, including by the International Business Times, Fox News, ZME Science, Sputnik News and other media outlets.

Metals streaming from the atmosphere of WASP-121b

The Hubble Space Telescope Science Institute have put out a press release about Hubble observations of WASP-121b, as reported in a new paper led by David Sing of Johns Hopkins University.

WASP-121b is one of the hottest WASP planets, since it is fiercely irradiated by being in a very tight orbit of only 1.27 days around a hot F star. The Hubble spectra show clear absorption features caused by metals including Magnesium and Iron:

“Heavy metals have been seen in other hot Jupiters before, but only in the lower atmosphere,” explains David Sing, “So you don’t know if they are escaping or not. With WASP-121b, we see magnesium and iron gas so far away from the planet that they’re not gravitationally bound.”

“The heavy metals are escaping partly because the planet is so big and puffy that its gravity is relatively weak. This is a planet being actively stripped of its atmosphere.”

The Hubble press release continues: “This exoplanet is also a perfect target for NASA’s upcoming James Webb Space Telescope to search in infrared light for water and carbon dioxide, which can be detected at longer, redder wavelengths. The combination of Hubble and Webb observations would give astronomers a more complete inventory of the chemical elements that make up the planet’s atmosphere.”

STSci have produced an artist’s impression of WASP-121b, showing how the planet’s shape is tidally distorted by the gravity of the star that it orbits:

Artwork: NASA, ESA, and J. Olmsted (STScI)

The press release has led to coverage on over 50 news and science websites, including Newsweek, CNN, Fox News, Metro, The Daily Mail, The Express, and countries including Switzerland, Germany, India, and Malaysia.

Spectral contamination from starspots on WASP-4

Here’s a topic we’ll be hearing much more about: how the observed spectrum of a transiting exoplanet is affected by transiting across star-spots. In “transmission spectroscopy” the starlight shines through the planet’s atmosphere during transit, and the easiest thing to do is assume that the star itself is a uniform light source.

But as discussed by papers led by Ben Rackham, if the planet passes over a dark region (star spot) or bright region (faculae), this would change the observed spectrum.

A new paper led by Alex Bixel about WASP-4b is the first to attempt to correct for this effect. The authors’ transit observations show a clear crossing of a starspot (the feature is shown in blue, the spot shows as a upward bump since the planet is then removing less light):

And here is the difference it makes. The blue curve is the observed spectrum, presumed to be of the planet’s atmosphere. The orange curve is then the spectrum corrected for the presence of the star spot.

The details of how to do this are complex, and are discussed at length in the above papers. The central message is that “active FGK host stars can produce such features and care is warranted in interpreting transmission spectra from these systems”.

However, there is good news in that: “stellar contamination in transmission spectra of FGK-hosted exoplanets is generally less problematic than for exoplanets orbiting M dwarfs”, and that such signals “are generally minor at wavelengths of planetary atomic and molecular features”. Overall the authors say that their study “bodes well for high-precision observations of these targets”.

Helium in WASP-69b, HAT-P-11b and HD 189733b

Earlier this year helium was found in the outer atmosphere of WASP-107b, the first detection of helium in an exoplanet. Several teams have now used similar techniques to find helium in WASP-69b, HAT-P-11b and HD 189733b, leading to a slew of papers and accompanying press releases from the Instituto de Astrofísica de Andalucía, the University of Exeter and others (see [1], [2], [3] and [4]).

Artist’s impression of an escaping envelope of helium surrounding WASP-69b. (Credit: Gabriel Perez Diaz, IAC)

Lisa Nortmann, lead author of the WASP-69b paper, explains that the helium is escaping from the atmosphere, forming a comet-like tail: “We observed a stronger and longer-lasting dimming of the starlight in a region of the spectrum where helium gas absorbs light. The longer duration of this absorption allows us to infer the presence of a tail.”

The press releases have led to extensive coverage including by CNN, the Daily Mail and Tech Times.

The IAA press release includes a video illustration of WASP-69b, created by Gabriel Perez Diaz of the IAC:

Aluminium Oxide in the atmosphere of WASP-33b?

The Instituto de Astrofisica de Canarias have put out a press release on a new paper by von Essen et al, reporting a study of WASP-33b using the 10-meter Gran Telescopio Canarias.

WASP-33 is a hard system to analyse since the host star is a delta-Scuti star, which means that it pulsates. That produces transit lightcurves like these, where the usual transit profile has pulsations superimposed on it. The figure shows the transit in different wavebands across the optical, from blue to red, as obtained with the OSIRIS spectrograph. That meant that the authors first had to model and subtract the effect of the pulsations.

After doing that they analysed how the transit depth depended on wavelength, which reveals how the planet’s atmosphere absorbs light. “We find that the feature observed between 450 and 550 nm can best be explained by aluminium oxide in its atmosphere” says lead author, Carolina von Essen.

“The current models of exoplanetary atmospheres predict that the Ultra Hot Jupiters should be free of clouds, and present a range of oxides in the visible spectrum, such as vanadium oxide, titanium oxide, and aluminium oxide”. This work on WASP-33b is the first observational indication of the presence of aluminium oxide.

Sulfanyl in the atmosphere of WASP-121b?

The latest Hubble Space Telescope spectrum of a WASP exoplanet has just been published by Thomas Evans et al. The spectrum of WASP-121b extends from near-UV wavelengths through the optical to the infra-red, combining data from three different gratings (shown in different colours in the figure):

Of particular interest is the rapid rise in the data in the near-UV (the extreme left of the plot), which is clearly out of line with the fitted model (purple lines). The rise is too rapid to be attributed to Rayleigh scattering in a clear atmosphere.

Instead, the authors suggest that it is due to sulfanyl, a molecule consisting of one sulfur and one hydrogen. Evans et al conclude that the near-UV absorber “likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere”.

The work points to the ongoing importance of the Hubble Space Telescope, even after the James Webb Space Telescope is launched, since the JWST is designed for infrared astronomy, and can’t see the near-UV wavelengths that can be observed with Hubble.

Update: One of the authors, Jo Barstow, has tweeted the following thread on the @astrotweeps account: