Tag Archives: Sodium

WASP-62b, in James Webb’s continuous-viewing zone, has a clear atmosphere

James Webb’s “Continuous Viewing Zone” is the patch of sky where the satellite can point continuously at a target and so observe it most efficiently. Exoplanets within the CVZ that are suitable for atmospheric characterisation are thus of high importance, and so far WASP-62b is the only gas giant known within the CVZ.

Munazza Alam et al have now pointed the Hubble and Spitzer space telescopes at WASP-62b to see what its atmosphere looks like. Importantly, they find that WASP-62b has clear skies. This matters since cloudy or haze-filled atmospheres tend to produce flat spectra lacking any spectral features, and so don’t tell us much.

Here, Alam et al plot the spectrum near the sodium (Na) line, showing that it has a broad base, akin to that in the clear-skied planet WASP-96b. The broad base of the line means that it is being widened by “pressure broadening”, and that can only happen deep in the planet’s atmosphere where the pressure is high. And we can only see deep into the atmosphere if it is clear rather than cloudy.

Clear skies mean that spectral features produced by the molecules in the atmosphere should be readily detectable with JWST. Here Alam et al simulate what we expect to see with JWST, showing that Na, H2O, NH3, FeH, SiH, CO, CO2, and CH4 can all be detected.

They conclude by saying that: “As the only transiting giant planet currently known in the JWST Continuous Viewing Zone, WASP-62b could prove a benchmark giant exoplanet for detailed atmospheric characterization in the James Webb era.

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.

WASP-96b: an exoplanet free of clouds

Press Release: Scientists have detected an exoplanet atmosphere that is free of clouds, marking a pivotal breakthrough in the quest for greater understanding of the planets beyond our solar system. (Link to Nature paper)

Figure 1 | Exoplanets in orbits close to the line of sight for us on Earth periodically pass in front (transit) and behind (secondary eclipse) of their host stars. Transits and eclipses are a powerful indirect way to study the composition of exoplanet atmospheres. Image credit: N. Nikolov

An international team of astronomers, led by Dr Nikolay Nikolov from the University of Exeter, have found that the atmosphere of the ‘hot Saturn’ WASP-96b is cloud-free. Using Europe’s 8.2m Very Large Telescope in Chile, the team studied the atmosphere of WASP-96b when the planet passed in front of (“transited”) its host-star (Figure 1). This enabled the team to see the starlight shining through the planet’s atmosphere, and so determine its composition.

Just as an individual’s fingerprints are unique, atoms and molecules have a unique spectral characteristic that can be used to detect their presence in celestial objects. The spectrum of WASP-96b shows the complete fingerprint of sodium, which can only be observed for an atmosphere free of clouds (Figure 2). The result appears today in the prestigious research journal Nature.

Figure 2 | Sodium fingerprint in an exoplanet spectrum. Shown is the absorption due to sodium at each wavelength. More absorption means that we are looking higher up in the atmosphere, and the vertical axis therefore a measure of altitude in the atmosphere of the planet. An atmosphere free of clouds produces an intact sodium fingerprint (left panel). A cloud deck blocks part of the sodium in the atmosphere, partially removing its spectral signature (right panel). Image credit: N. Nikolov/E. de Mooij

“We’ve been looking at over twenty exoplanet transit spectra. WASP-96b is the only exoplanet that appears to be entirely cloud-free and shows such a clear sodium signature, making the planet a benchmark for characterization”, explains lead investigator Nikolay Nikolov from the University of Exeter in the United Kingdom.

WASP-96b was discovered recently by a Keele University team led by Professor Coel Hellier. It is the 96th planet announced by the Wide Angle Search for Planets. WASP-96b is a gas giant similar to Saturn in mass and exceeding the size of Jupiter by 20%. The planet periodically transits a sun-like star 980 light years away in the southern constellation Phoenix.

It has long been predicted that sodium exists in the atmospheres of hot gas-giant exoplanets, and in a cloud-free atmosphere it would produce spectra that are similar in shape to the profile of a camping tent.

“Until now, sodium was revealed either as a very narrow peak or found to be completely missing”, continues Nikolay Nikolov. “This is because the characteristic ‘tent-shaped’ profile can only be produced deep in the atmosphere of the planet and for most planets clouds appear to get in the way”.

“It is difficult to predict which of these hot atmospheres will have thick clouds. By seeing the full range of possible atmospheres, from very cloudy to nearly cloud-free like WASP-96b, we’ll gain a better understanding of what these clouds are made of”, explains Prof. Jonathan J. Fortney, study co-author, based at the Other Worlds Laboratory (OWL) at the University of California, Santa Cruz (UCSC).

The sodium signature seen in WASP-96b suggests an atmosphere free of clouds (Figure 3). The observation allowed the team to measure how abundant sodium is in the atmosphere of the planet, finding levels similar to those found in our own Solar System.

Figure 3 | An artist rendition of ‘hot Saturn’ WASP-96b. A distant observer would see WASP-96b blueish in colour, because sodium would absorb the yellow-orange light from the planet’s full spectrum. Image credit: Engine House

“WASP-96b will also provide us with a unique opportunity to determine the abundances of other molecules, such as water, carbon monoxide and carbon dioxide with future observations “, adds co-author Ernst de Mooij from Dublin City University.

Sodium is the seventh most common element in the Universe. On Earth, sodium compounds such as salt give sea water its salty taste and give the white colour of salt pans in deserts. In animal life, sodium is known to regulate heart activity and metabolism. Sodium is also used in technology, e.g. in the sodium-vapour street lights, where it produces yellow-orange light.

The team aims to look at the signature of other atmospheric species, such as water, carbon monoxide and carbon dioxide with the Hubble and James Webb Space Telescopes as well as telescopes on the ground.

Update: The story has been covered on over 50 websites, including Newsweek, Astronomy Magazine, the International Business Times, the Irish Times and others.

The clear atmosphere of WASP-39b, seen from the ground

Most of the best detections of features in the atmospheres of transiting exoplanets have come from the Hubble Space Telescope, but time on hugely expensive satellites is in high demand and limited. Thus a recent paper led by Nikolay Nikolov from Exeter University is a welcome development. Nikolov and his team observed WASP-39b and detected a strong Sodium line from the planet, which indicates a clear atmosphere. The result came from the newly upgraded FORS2 spectrograph on ESO’s Very Large Telescope.

Sodium in the atmosphere of exoplanet WASP-39b

The important feature of the plot is that the VLT data (black) are every bit as good as those from a previous detection of the same line using the Hubble. While Hubble has the advantage of being in space, the VLT has a much larger mirror and can observe whole transits without the gaps seen in Hubble data owing to its low-Earth orbit.

The similar result from a very different facility also gives confidence in the correctness of such detections of features in exoplanet atmospheres, which are, after all, pushing current technology to its limits.