For the first time, scientists have taken a closer look at the “dark side” of a hot Jupiter. And that leads to some interesting findings.

Hot Jupiters are large planets that are close to their star and therefore have a synchronous rotation. This means that their orbital time is equal to their rotational time, which results in the same side of the planet always being pointed towards the parent star. That side – also called the day side – is very bright and has already been studied for several hot Jupiters. However, it is much more difficult to get a grip on the night side: the side facing away from the parent star. But for the first time, researchers have managed to get a pretty good picture of that dark side of a hot Jupiter. And with that, for the first time, we also have a fairly detailed picture of the entire atmosphere of this exoplanet.

WASP-121b

For their study, the scientists focused on WASP-121b. The gas giant is nearly twice the size of Jupiter and is about 850 light-years away from Earth. There, the planet orbits the parent star in just 30 hours. And so the same side of the planet is always aimed at that parent star. That day side – which is about 10 times brighter than the night side – astronomers had previously examined. During that investigation, water vapor had already been discovered on the day side. Researchers had also determined how the temperature of the atmosphere on the day side of WASP-121b changes as you measure higher in that atmosphere.

night side

So researchers had already obtained a picture of the atmosphere of WASP-121b, but because the night side was not taken into account, that picture was incomplete. The new research changes that; In the new study, scientists have not only been able to measure the huge temperature differences between the atmospheres on the day and night sides, but also determine how water is transported from that day to the night side and due to the different conditions transforms into that atmosphere.

Water cycle

Here on Earth, the water cycle works like this: water evaporates, condenses and falls back down in the form of rain, only to evaporate again, etc. On WASP-121b, the water cycle works a bit differently, according to the new observations. On the day side, water molecules are ripped apart under the influence of the extremely high temperatures. The released hydrogen and oxygen atoms are then transported to the night side by powerful winds at speeds of up to 5 kilometers per second, or 18,000 kilometers per hour. There the temperatures are much lower, allowing the atoms to reunite and form water molecules. These then blow through to the day side, where the cycle starts again.

exotic rain

But it may not just be water molecules that circulate in this way. The observations also indicate that the night side of WASP-121b is cold enough to harbor clouds of iron and corundum (a mineral that can form sapphires and rubies). If those clouds are actually there, they will be transported – just like water molecules – to the day side by powerful winds. Due to the high temperatures there, the metals would evaporate and a little further on might come down again in the form of rain. In that case, it would be raining liquid gems on WASP-121b!

Method

The researchers base their conclusions – which have been published in the journal Nature Astronomy on observations using the Hubble Space Telescope. On board this telescope is the Cosmic Origins Spectrograph† This is an instrument that can provide more insight into the temperature and composition of the planet’s atmosphere using the light spectrum of WASP-121b. The day side of hot Jupiters has been studied in this way before. Studying the night side is a lot trickier, though, as astronomers have to look for small changes in the full spectrum of light from the planet orbiting the parent star.

To get a better handle on that night side, the astronomers studied the light spectrum of WASP-121b during two complete orbits. And that provided a much more complete picture of WASP-121b’s atmosphere. For example, based on the Hubble observations, the researchers can conclude that the atmospheric temperature on the daytime ranges from 2500 Kelvin (in the deepest observable layer) to 3500 Kelvin (in the top layer). On the night side, the temperature varies from 1800 Kelvin in the deepest layer to 1500 Kelvin in the top layer. Models were then used to find out which chemicals could be present in those different atmospheric layers with different temperatures. And this shows again that iron and corundum clouds on the night side are a real option.

Heat

Mapping temperatures on WASP-121b also reveals that the hottest spot on the planet isn’t exactly where you’d expect it to be. Instead, it is located slightly east of the expected location. According to the researchers, this is because the gases heated by the parent star are flung eastward by strong winds before they can release their heat into space. And that shift from where WASP-121b releases most of the heat provides further insight into the speed of the winds that sweep through the atmosphere. This leads researchers to the conclusion that these have a speed of about 5 kilometers per second. “These winds travel much faster than our jet stream and could potentially transport the clouds around the entire planet in about 20 hours,” said study researcher Tansu Daylan.

webb

While researchers have learned a lot more about WASP-121b thanks to Hubble, they are far from satisfied. There are still many loose ends. That’s why they’re eager to take a closer look at the planet, this time with the help of James Webb’s space telescope. They hope to use this telescope to detect carbon monoxide in the atmosphere of WASP-121b. “It would be the first time that we can measure a molecule containing carbon in the atmosphere of this planet,” said Mikal-Evans. And that would be a valuable addition. “The amount of carbon and oxygen in the atmosphere can tell us more about where these types of planets form.”

The formation of hot Jupiters is still shrouded in mystery. It’s hard to imagine that they originated in their current locations; the radiation from young stars is so intense that the gases that hot Jupiters must amass to become mighty gas giants evaporate at lightning speed. Many researchers therefore suspect that hot Jupiters appear at a much greater distance from the parent star and then move towards the parent star. But even this theory that seems plausible at first sight is surrounded by question marks, because why are the gas giants taking off? And at what stage of their existence? There’s only one way to find out: study as many hot Jupiters as possible. Thousands of these exotic worlds have already been discovered, but now it’s time to take a closer look at them. The James Webb telescope launched in December can help with that; this super-powerful space telescope is currently being readied for its first observations, due sometime in June. Some time ago, scientists were able to get a hold of the first 6,000 hours of observation time. It led to numerous proposals, of which eventually 266 have been approved, including a proposal by Mikal-Evans and colleagues to take a closer look at WASP-121b.