Category Archives: exoplanet atmospheres

Night-side clouds on hot Jupiters

Thomas Beatty et al have an interesting new paper on arXiv today, primarily about the transiting brown dwarf KELT-1b. They’ve used the Spitzer Space Telescope to record the infra-red light as it varies around the 1.3-day orbit.

They end up with the following plots (KELT-1b is on the right, with the plot for the planet WASP-43b on the left):

The x-axis is “colour”, the difference in flux between two infra-red passbands at 3.6 and 4.5 microns. The y-axis is brightness (in the 3.6 micron band). The underlying orange and red squares show where typical M-dwarf stars and L and T brown dwarfs fall on the plot.

The solid-line “loops” are then the change in position of the atmospheres of KELT-1b and WASP-43b around their orbits. At some phases we see their “day” side, heated by the flux of their star, and at others we see their cooler “night” side.

The blue line is the track where something would lie if there were no clouds in its atmosphere. The fact that KELT-1b’s loop doesn’t follow the blue track, but moves significantly right (to cooler colours) implies that the night side of the brown dwarf must be cloudy. The night side of WASP-43b, however, appears to be less cloudy, according to its track.

Here are the same plots for two more planets:

The plot for WASP-19b shows a loop with a marked excursion to the right, suggesting a cloudy night side to the planet. For WASP-18b, however, the loop follows a trajectory nearer the blue “no cloud” track, suggesting a clearer atmosphere.

Advertisements

Water Is Destroyed, Then Reborn in Ultrahot Jupiters

NASA JPL have put out a press release about ultra-hot Jupiters including WASP-18b, WASP-103b and WASP-121b.

The work, led by Vivien Parmentier, used the Spitzer and Hubble space telescopes to study how the planets’ atmospheres change from the irradiated day side to the cooler night side.

“Due to strong irradiation on the planet’s daysides, temperatures there get so intense that water molecules are completely torn apart. […] fierce winds may blow the sundered water molecules into the planets’ nightside hemispheres. On the cooler, dark side of the planet, the atoms can recombine into molecules and condense into clouds, all before drifting back into the dayside to be splintered again.”

Simulated views of the ultrahot Jupiter WASP-121b show what the planet might look like to the human eye from five different vantage points, illuminated to different degrees by its parent star. (Credit: NASA/JPL-Caltech/Vivien Parmentier/Aix-Marseille University)

“With these studies, we are bringing some of the century-old knowledge gained from studying the astrophysics of stars, to the new field of investigating exoplanetary atmospheres,” said Parmentier.

Harvard’s CfA have also produced a press release on the work, focusing on the analysis of WASP-103b led by Laura Kreidberg.

“A crucial observational advance by Kreidberg and her team was that they observed the planet for an entire orbit, enabling them to map the climate at every longitude and derive detailed information about the temperatures on the planet’s dayside and nightside. This is only the second time that such a complete exoplanet observation has been performed with HST.”

A bright spot on the host star of WASP-19b

Star spots are cooler regions of a star’s surface, caused by magnetic activity, and emit less light. If a planet transits across a spot it blocks less light, and so we see a slight rise, a bump, in the transit profile.

On the left (in blue) is a transit from a new paper by Espinoza et al, who have observed transits of WASP-19b with the Magellan telescope. A clear bump is seen, indicating that the planet passed over a cooler spot.

On the right (in red), however, is another transit showing a clear dip compared to the expected transit lightcurve. This implies that during this transit the planet passed over a brighter region on the star. This is the first time such an event has been seen.

The authors deduce that the bright spot must have a size of about a quarter of the stellar radius and must be 100 K hotter than the rest of the star. Such regions are not seen on our own Sun.

The main point of the observations, however, was not studying spots but studying the planet’s atmosphere by recording how the transit depth changes with wavelength. Here is the state-of-play for the spectrum of WASP-19b, covering optical to infra-red wavelengths:

The red data-points are from the Hubble Space Telescope, showing a spectral feature, but the new data by Espinoza et al (white points) are consistent with a flat spectrum within the limits of the data.

NASA’s Webb Space Telescope to Inspect Atmospheres of Gas Giant Exoplanets

NASA have written a publicity page on JWST’s plans to study the atmospheres of gas-giant exoplanets, including an animation on how this is done. Since the prime targets for the “Early Release Science” program are three WASP-discovered planets, WASP-18b, WASP-43b and WASP-79b, we “re-blog” the piece here:

“In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS). Its main goal is to locate Earth-sized planets and larger “super-Earths” orbiting nearby stars for further study. One of the most powerful tools that will examine the atmospheres of some planets that TESS discovers will be NASA’s James Webb Space Telescope. Since observing small exoplanets with thin atmospheres like Earth will be challenging for Webb, astronomers will target easier, gas giant exoplanets first.”

Read the full piece here.

Metals and water in the atmosphere of WASP-127b

Puffed-up, low-mass planet WASP-127b is one of the best targets for atmospheric characterisation. Recently Chen etal have announced the results of observations taken with the La Palma telescopes the Gran Telescopio Canarias and the Nordic Optical Telescope.

The result is one of the most detailed and featured spectra of an exoplanet yet.

The spectrum clearly shows the metals sodium (Na), lithium and potassium (K), along with features due to water and haze. A press release from the University of Cambridge has led to coverage of the paper on several dozen websites.

The Instituto de Astrofisica de Canarias have also produced this artist’s depiction of WASP-127b and its host star:

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.