by Duncan Lunan

The atmospheric effect known as ‘The Glory’ is sometimes encountered on Earth in special conditions.  It requires the droplets in cloud to be perfectly spherical, the distance between them to be just right for diffraction effects, and the angle between the Sun and the observer to be just right. 

It’s sometimes seen from high mountains, but more often from aircraft  (Fig. 1), from where I’ve seen it myself.  Remarkably it was seen in the sulphuric acid clouds of Venus, by Europe’s Venus Express spacecraft, in July 2011  (Figs. 2 & 3).  The sulphuric acid droplets are much smaller than the water ones of Earth’s clouds, generating much broader and more widely separated diffraction rings – so explaining a simulated comparison image which has puzzled me for the last 13 years  (Fig. 4).

And now it has been seen in the clouds of WASP-76b  (Fig. 5), an exoplanet 637 light-years away in the constellation Pisces, by the CHEOPS space telescope launched in 2019  (Fig. 6). 

The planet itself was discovered in 2016 by SuperWASP  (Wide Angle Search for Planets), one of two exoplanet-searching telescopes operated by an international consortium of eight universities, many of them British – SuperWASP is located in South Africa.  CHEOPS, the Characterizing Exoplanet Satellite, has been studying WASP-76b in nearly two dozen observations over three years, and found the effect consistently appearing over the planet’s eastern terminator, where day becomes night  (Fig. 7). 

It’s thought that water droplets form on the nightside of the planet, and must evaporate as the day goes on.  On the dayside temperatures reach 2,400 degrees C, which is hot enough to vaporise iron.  WASP-76b has 1.8 times the diameter of Jupiter but only 92% of its mass, and is tidally locked, keeping the same face towards its sun as it circles it in only 1.8 Earth days.  Observations by the Hubble Space Telescope and the Spitzer infrared space telescope have detected a large variety of metals and metal oxides in its atmosphere, and it’s thought that iron rain falls immediately after sunset – a challenge taken up by artists  (Fig. 8)  including those producing NASA’s ongoing series of retro space tourism posters  (Fig. 9).  Discovering the Glory effect in observations over several years suggests that the clouds and winds on WASP-76b have a remarkably stable pattern, something of a surprise when conditions must be so violent at the surface.

Although WASP-76b is described as a ‘Hot Jupiter’, because of its mass and tight orbit, its composition must be very different from Jupiter itself.  Jupiter is a gas giant planet, often described as a ‘failed star’ although it’s not massive enough to trigger fusion reactions inside.  Back in the 1960s astronomers including Patrick Moore featured the ‘Wildt model’ of Jupiter, with a huge water ocean and an icy crust overlying a very large rocky core.  The discovery by the Pioneer probes of heat emanating from Jupiter, due to continuing contraction, ruled that out, and none of the spacecraft flying past the planet detected a rock and metal core, though they did so for Saturn, Uranus and Neptune.  Though Patrick continued to insist that there ‘must’ be a rock and metal core to the end of his life, the current Juno mission has established that it does not exist – Jupiter is gaseous all the way to the centre, with ever-higher temperatures and pressures all the way.

Back in the 1950s, before the Wildt model, many astronomers believed that Jupiter would still retain heat from its formation at its surface, which would be rocky and strewn with volcanoes.  Chesley Bonestell painted Jupiter like that for The Conquest of Space by Willy Ley  (Sidgwick & Jackson, 1951), and for the George Pal version of The War of the Worlds in 1953.  Captain W.E. Johns had a similar description in Return to Mars, the second of his young adult space novels  (1955).  To have such a variety of metals in its atmosphere, WASP-76b must have a similar composition, more like a giant Earth than a failed star, and it’s looking as if many exoplanets may be that way.  I’ve seen very little discussion of that, let alone any campaign claiming ‘W.E. Johns was right!’, though he does have a fan club, like many favourite authors of my childhood.

The discovery of The Glory on WASP-76b improves the possibility of detecting another predicted effect.  Paul Scott Anderson writes ‘In addition, being able to detect glories will help astronomers see other faint phenomena including glints of sunlight on oceans or lakes on exoplanets’.  (‘Rainbow-like glory on an exoplanet is 1st-ever detected’, Earthsky, online, April 11^th, 2024). 

The same thought had occurred to me:  Sun glint from Earth’s oceans and lakes  (Fig, 10)  is so often seen from space that it’s become something of a cliché in advertising, and it has also been glimpsed through the clouds on the methane and ethane lakes of Titan, Saturn’s largest moon  (Figs. 11 & 12). 

It occurs to me that while bodies of liquid on exoplanets might be more stable than cloud patterns, the rotations of their planets might make them hard to spot – unless they have trapped rotations like WASP-76b, or resonant rotations like Mercury’s, which fooled astronomers into thinking it had a trapped rotation, for so many years.  (See ‘Mercury’, ON, June 6^th 2021).  If the exoplanets were entirely covered in water, which may often be the case – so often that the name ‘Hycean’ has been coined for that hypothetical class of planet  (Fig. 13), and several possible examples are under Study  (Figs. 14 & 15) – then sun glint might well be a permanent condition.

Another possibility which I haven’t seen mentioned elsewhere, is that the same techniques could be used to detect the city lights of extraterrestrial civilisations  (Fig. 16). 

Of course, looking for ‘water glint’ or for ET city lights requires the planets to be situated in their stars’ habitable zones, much further out, smaller, cooler,  and harder to detect than ‘hot Jupiters’.  Looking for city lights also requires them to be as careless with such lighting as we are  (Fig. 17):  as I pointed out at the close of ‘Dark Sky Part 1 & 2’, ON, September 11^th and 18^th, 2022), a nationwide policy of dark sky compatible lighting would be far more effective in energy savings than forcing people to install heat pumps, and could be done for effectively no cost on a replacement basis, as existing municipal lighting comes up for renewal.  Savings of 35% or more could be made on municipal energy bills, road lighting etc..  Although it’s being done on a piecemeal basis in the UK, it’s not being done as a matter of policy, and one wonders if that’s because it’s to save money, not to generate it.  But it’s as true for extraterrestrial civilisations as it is here – any light which you can see from a satellite 2000 miles up, let alone across interstellar space, wastes energy, and all that light going upwards serves no useful purpose.