Each glory is unique, depending on the composition of the planet’s atmosphere and the colours of the light from the star that illuminates it. WASP-76 (the «Sun» of WASP-76b) is a yellow and white main sequence star like our Sun, but different stars create glories with different colours and patterns.
© ESA, work performed by ATG under contract for ESA. CC BY-SA 3.0 IGO.
Scientists working at the CHEOPS space telescope control centre in the University of Geneva (UNIGE), Switzerland, working in collaboration with the European Space Agency (ESA) and the University of Bern (UNIBE), have discovered something which should ring alarm bells in the minds of Bible literalists.
It is the discovery of a rainbow in the atmosphere of an exoplanet known to science as WASP-76b.
WASP-76b is an exoplanet discovered in 2013 and confirmed in 2016 by a team of astronomers led by Neale P. Gibson using data from the Wide Angle Search for Planets (WASP) project. It belongs to the class of exoplanets known as "hot Jupiters," which are gas giants similar in size to Jupiter but with much higher temperatures due to their close proximity to their parent stars. Here are some key characteristics of WASP-76b:The reason this should set alarm bells ringing in the minds of Bible literalists, is because they believe that 'the' rainbow was sent by the god of the Bible to show believers that he wouldn't ever inflict another genocide on the planet because, although being omniscient and perfect, he regretted the time he did it in a fit of temper and flooded the planet to a depth that covered the highest mountains.Overall, WASP-76b represents a fascinating example of the diverse range of exoplanets that exist in our galaxy and provides valuable insights into the atmospheric characteristics and physical processes occurring on these distant worlds.
- Discovery: WASP-76b was discovered using the transit method, which involves observing the slight dimming of a star's light as an orbiting planet passes in front of it. This dimming effect is periodic and can be used to infer the presence and characteristics of the planet.
- Physical Characteristics: WASP-76b is approximately 1.8 times the size of Jupiter but significantly more massive. It has a high surface temperature, estimated to be around 2,400 degrees Celsius (4,350 degrees Fahrenheit). This extreme heat is due to the planet's close orbit around its host star, WASP-76, which is located about 640 light-years away from Earth in the constellation Pisces.
- Atmospheric Composition: One of the most intriguing features of WASP-76b is its atmospheric composition. Observations using the European Southern Observatory's Very Large Telescope (VLT) in Chile revealed the presence of iron and titanium vapor in the planet's atmosphere. This finding is significant because it provides insights into the atmospheric chemistry and physical processes occurring on hot Jupiter exoplanets.
- Extreme Conditions: The extreme temperature and atmospheric conditions on WASP-76b make it an inhospitable world, unlikely to support life as we know it. Its atmosphere is thought to be dominated by high-speed winds and extreme atmospheric dynamics, which may lead to unusual weather patterns and atmospheric phenomena.
- Importance for Exoplanet Research: WASP-76b is one of many exoplanets discovered in recent years that are helping astronomers better understand the diversity of planetary systems beyond our solar system. Its unique atmospheric composition and extreme conditions make it a valuable target for further study, particularly in the field of exoplanet atmospheres and planetary formation theories.
Or so the tale goes.
Before about 4,000 years ago, obviously, sunlight wouldn't have been diffracted into its component colours by passing through raindrops, or so those who didn't understand how rainbows are formed wrote.
And God spake unto Noah, and to his sons with him, saying, And I, behold, I establish my covenant with you, and with your seed after you; And with every living creature that is with you, of the fowl, of the cattle, and of every beast of the earth with you; from all that go out of the ark, to every beast of the earth.
And I will establish my covenant with you, neither shall all flesh be cut off any more by the waters of a flood; neither shall there any more be a flood to destroy the earth. And God said, This is the token of the covenant which I make between me and you and every living creature that is with you, for perpetual generations: I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth.
And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud: And I will remember my covenant, which is between me and you and every living creature of all flesh; and the waters shall no more become a flood to destroy all flesh. And the bow shall be in the cloud; and I will look upon it, that I may remember the everlasting covenant between God and every living creature of all flesh that is upon the earth.
Genesis 9: 8-17
Apparently, God is so forgetful that he needs the rainbow to remind him not to lose self-control and commit genocide again.
But the question for creationists is, did God commit genocide on another planet too and put a rainbow there to remind him not to do it again? If not, how do rainbows form on another planet if they only appear on Earth because God puts them there?
If he did commit genocide on another planet, this means there must have been life there too, yet WASP-76b is far too hot to sustain life as we know it, being a 'hot' Jupiter-like gas giant which orbits its sun (WASP-76) closer than mercury orbits ours.
How this discovery was made is the subject of a news release by the University of Geneva:
The CHEOPS space telescope, whose scientific operations centre is based at the University of Geneva (UNIGE), is providing new information on the mysterious exoplanet WASP-76b. This ultra-hot giant is characterised by an asymmetry between the amount of light observed on its eastern terminator - the fictitious line that separates its night side from its day side - and that observed on its western terminator. This peculiarity is thought to be due to a "glory", a luminous phenomenon similar to a rainbow, which occurs if the light from the star - the "sun" around which the exoplanet orbits - is reflected by clouds made up of a perfectly uniform substance. If this hypothesis is confirmed, this would be the first detection of this phenomenon outside our solar system. This work, carried out in collaboration with the European Space Agency (ESA) and the University of Bern (UNIBE), is published in Astronomy & Astrophysics.The team have also published their findings, open access in the journal, Astronomy Astrophysics:
WASP-76b is an ultra-hot giant planet. Orbiting its host star twelve times closer than Mercury orbits our Sun, it receives more than 4,000 times the Sun’s radiation on Earth. ‘‘The exoplanet is ‘inflated’ by the intense radiation from its star. So, although it is 10% less massive than our cousin Jupiter, it is almost twice as big,’’ explains Monika Lendl, assistant professor in the Department of Astronomy of the UNIGE Faculty of Science, and co-author of the study.ELEMENTS THAT WOULD FORM ROCKS ON EARTHSince its discovery in 2013, WASP-76b has been the subject of intense scrutiny by astronomers. A strangely hellish picture has emerged. One side of the planet is always facing its star, reaching temperatures of 2,400 degrees Celsius. Elements that would form rocks on Earth melt and evaporate here, before condensing on the slightly cooler night side, creating clouds of iron that drip molten iron rain.
MELT AND EVAPORATE, CREATING CLOUDS OF IRON
THAT DRIP MOLTEN IRON RAIN.
The crucial contribution of CHEOPS
One of the most disturbing observations for astronomers is the asymmetry between the planet’s two terminators. The terminator is the imaginary line that separates the day and night sides of a planet. In the case of WASP-76b, the observations show an increase in the amount of light from the terminator to the east of the planet compared with the one to the west.
To solve this mystery, astronomers used no fewer than twenty-three observations with the CHEOPS space telescope, spread over three years. The ESA satellite, which is piloted by Switzerland and has its scientific operations centre at the UNIGE Department of Astronomy, observed numerous secondary eclipses of the planet (when it passes behind its star) and several phase curves (continuous observation during a complete revolution of the planet).
Combining these new data with those from other telescopes (TESS, Hubble and Spitzer), the astronomers were able to put forward a surprising hypothesis to explain the excess luminous flux on the eastern side of the planet:
Artistic representation of the CHEOPS space telescope.© ESA / ATG medialab
A first outside our solar systemThis unexpected glow could be caused by a strong, localised and anisotropic reflection - i.e. one that depends on direction - what we call a glory.
Olivier Demangeon, lead author
Instituto de Astrofísica e Ciências do Espaço, Portugal.
Glories are common phenomena on Earth. They have also been observed on Venus. The effect, similar to a rainbow, occurs when light is reflected by clouds made up of perfectly uniform droplets. In the case of Earth, the droplets are made out of water, but the nature of these droplets on WASP-76b remains mysterious. It could be iron, as this has already been detected in the planet’s extremely hot atmosphere. The detection of this phenomenon on WASP-76b is the first of its kind outside our solar system.
DETECTING SUCH TINY PHENOMENA
AT SUCH A GREAT DISTANCE WILL ENABLE SCIENTISTS
TO IDENTIFY OTHERS THAT ARE JUST AS CRUCIAL.Results to be confirmedThe reason why no such glory has ever been observed outside our solar system is that this phenomenon requires very specific conditions. First of all, the atmospheric particles must be almost perfectly spherical, completely uniform and sufficiently stable to be observed throughout a long time. These droplets have to be directly illuminated by the planet’s host star, and the observer - in this case CHEOPS - must be in the right position.
Olivier Demangeon.
Further data will be needed to confirm with certainty that this intriguing excess of light on the eastern terminator of WASP-76b is a glory. This confirmation would attest to the presence of clouds made up of perfectly spherical droplets that have existed for at least three years, or that are constantly renewing themselves. For such clouds to persist, the temperature of the atmosphere would also have to be stable over time - a fascinating and detailed insight into what could be happening on WASP-76b. Detecting such tiny phenomena at such a great distance will enable scientists and engineers to identify others that are just as crucial. For example, the reflection of starlight off liquid lakes and oceans - a necessary condition for habitability.
Abstract
Context. WASP-76 b has been a recurrent subject of study since the detection of a signature in high-resolution transit spectroscopy data indicating an asymmetry between the two limbs of the planet. The existence of this asymmetric signature has been confirmed by multiple studies, but its physical origin is still under debate. In addition, it contrasts with the absence of asymmetry reported in the infrared (IR) phase curve.
Aims. We provide a more comprehensive dataset of WASP-76 b with the goal of drawing a complete view of the physical processes at work in this atmosphere. In particular, we attempt to reconcile visible high-resolution transit spectroscopy data and IR broadband phase curves.
Methods. We gathered 3 phase curves, 20 occultations, and 6 transits for WASP-76 b in the visible with the CHEOPS space telescope. We also report the analysis of three unpublished sectors observed by the TESS space telescope (also in the visible), which represents 34 phase curves.
Results. WASP-76 b displays an occultation of 260 ± 11 and 152 ± 10 ppm in TESS and CHEOPS bandpasses respectively. Depending on the composition assumed for the atmosphere and the data reduction used for the IR data, we derived geometric albedo estimates that range from 0.05 ± 0.023 to 0.146 ± 0.013 and from <0.13 to 0.189 ± 0.017 in the CHEOPS and TESS bandpasses, respectively. As expected from the IR phase curves, a low-order model of the phase curves does not yield any detectable asymmetry in the visible either. However, an empirical model allowing for sharper phase curve variations offers a hint of a flux excess before the occultation, with an amplitude of ~40 ppm, an orbital offset of ~ −30°, and a width of ~20º. We also constrained the orbital eccentricity of WASP-76 b to a value lower than 0.0067, with a 99.7% confidence level. This result contradicts earlier proposed scenarios aimed at explaining the asymmetry observed in high-resolution transit spectroscopy.
Conclusions. In light of these findings, we hypothesise that WASP-76 b could have night-side clouds that extend predominantly towards its eastern limb. At this limb, the clouds would be associated with spherical droplets or spherically shaped aerosols of an unknown species, which would be responsible for a glory effect in the visible phase curves.
1 Introduction
Whether they are viewed as stepping stones towards the study of the atmosphere of Earth-like planets or as ideal laboratories to unravel the composition and physical processes at work in extreme and fascinating worlds, ultra-hot Jupiters (UHJs) have been the subject of many observations and studies over the past years (e.g. Parmentier et al. 2018; Kreidberg et al. 2018.1; Hoeijmakers et al. 2019). Due to their high equilibrium temperature and the low mean molecular weight of their atmosphere, they possess the largest atmospheric pressure scale heights (e.g. Seager 2010, pp. 185–186). Cooler hot Jupiters (HJs) commonly harbour clouds and hazes which hide the signatures of their atmospheric constituent (Sing et al. 2016). On the contrary, UHJs tend to be cloud-free, at least on their day-sides (e.g. Parmentier et al. 2018; Kitzmann et al. 2018.2). Thanks to the combination of these two elements, UHJs are ideal targets for ground and space-based observatories. Their atmospheres can be probed with most of the available observational techniques, which provide a more extensive view than any other class of exoplanets.
Dozens of atomic and molecular species, some ionised, have already been detected in the atmosphere of UHJs (e.g. Azevedo Silva et al. 2022; Borsa et al. 2021). We have also seen a tremendous effort from the community to more closely model the physical processes driving these atmospheres in an attempt to explain the wealth of observational constraints (e.g. the identification of hydrogen dissociation as a dominant heat transport mechanism for UHJs; Bell & Cowan 2018.3; Tan & Komacek 2019.1; Mansfield et al. 2020). One-dimensional (1D) radiative transfer models embedded in Bayesian inference tools have successfully modelled low-resolution and high-resolution transmission spectroscopic observations. They have allowed us to infer the presence of many species and retrieve abundance ratios (e.g. Kitzmann et al. 2023; Pluriel et al. 2020.1; Gibson et al. 2020.2; Brogi & Line 2019.2). Three-dimensional (3D) global circulation models coupled with radiative transfer models and other types of models capable of accounting for the spatial inho-mogeneity of these atmospheres enabled the interpretation of orbital phase curves (PCs) and minute line-shape deformations (e.g. Kreidberg et al. 2018.1; Tan & Komacek 2019.1; Helling et al. 2021.1; Changeat et al. 2022.1; Jones et al. 2022.2; Seidel et al. 2023.1; Pelletier et al. 2023.2).
WASP-76 b is (arguably) considered an archetypal UHJ (West et al. 2016.1). From the ground, multiple studies have reported the detection of close to 20 different species (primarily atoms and ions), along with many more upper limits using high-resolution spectroscopic observation in the visible and the near infrared (NIR) obtained during transit (Tsiaras et al. 2018.4; Seidel et al. 2019.3, 2021.2; Ehrenreich et al. 2020.3; Azevedo Silva et al. 2022; Pelletier et al. 2023.2; Kesseli et al. 2020.4, 2022.3; Sánchez-López et al. 2022.4; Gandhi et al. 2022.5; Kawauchi et al. 2022.6; Kesseli & Snellen 2021.7; Casasayas-Barris et al. 2021.11; Landman et al. 2021.12; Deibert et al. 2021.3; Tabernero et al. 2021.4; Edwards et al. 2020.5). High-resolution observations in the NIR taken close to occultation have shown the presence of CO and hints of H2O emission lines indicative of the presence of a thermal inversion in the planet’s upper atmosphere (Yan et al. 2023.3). Space-based observatories not only provided low-resolution transit and emission spectroscopy, but also full PC in the NIR and mid-IR (May et al. 2021.5; Fu et al. 2021.6; Garhart et al. 2020.6; Tsiaras et al. 2018.4). This plethora of observations provides a unique opportunity to piece together one of the most comprehensive views of an exoplanet’s atmosphere.
The scientific interest in UHJs and, in particular, WASP-76 b has increased drastically after the detection and later confirmation of the asymmetric signature coming from the two limbs of WASP-76 b in transmission detected at ultra-high spectral resolution (Ehrenreich et al. 2020.3; Kesseli & Snellen 2021.7; Pelletier et al. 2023.2). Based on a toy model, Ehrenreich et al. (2020.3) interpreted these observations as an asymmetry in the iron composition: the upper atmosphere of the morning limb (also called leading limb or western limb) of WASP-76 b would be depleted in gaseous iron compared to its evening limb (also called trailing or eastern limb) due to the condensation of iron over the night-side. Naturally, Savel et al. (2022.7), Wardenier et al. (2021.8) and May et al. (2021.5) attempted to confront these observations with global circulation models (GCMs). All three studies have agreed that standard GCM models cannot reproduce the asymmetry detected by Ehrenreich et al. (2020.3). Savel et al. (2022.7) and Wardenier et al. (2021.8) both argued that an asymmetry in the composition of the limbs is not the only way to explain the observations. Savel et al. (2022.7) invoked the presence of clouds and a slight eccentricity of the planet’s orbit, while Wardenier et al. (2021.8) highlighted the importance of temperatures and wind dynamics. Both studies have argued that a weak drag, or long drag timescale (from 105 to 107 s), would be necessary to explain the asymmetry. May et al. (2021.5) took a slightly different approach to the problem, as they also presented the IR phase curves of WASP-76 b. The phase curves show little to no asymmetry which indicates a strong drag, or short drag time-scale (≤l04 s), in contrast with the results of high-resolution transmission spectroscopy.
In this paper, we provide new observational constraints on the puzzling atmosphere of WASP-76 b with phase curve, transit, and eclipse observations in the visible with the CHEOPS and TESS instruments (Sects. 2 and 4). We refine the properties of WASP-76 a and its stellar companion WASP-76 b (Sect. 3). We revisit the emission spectra of this planet (Sect. 5.1) and derive its geometric albedo (Ag). We discuss the available phase curves at visible and IR wavelengths (Sects. 5.2 and 5.2.2). Finally, we argue that the visible phase curves of WASP-76 b hold crucial elements to reach a comprehensive view of its atmosphere (Sect. 6).
O. D. S. Demangeon, P. E. Cubillos, V. Singh, et al,
Asymmetry in the atmosphere of the ultra-hot Jupiter WASP-76 b A&A, 684 (2024) A27 DOI: https://doi.org/10.1051/0004-6361/202348270
Copyright: © 2024 The authors/The European Southern Observatory (ESO).
Published by EDP Sciences. Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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