Clearest View Yet Of Hot Jupiter's Dark Side Obtained. Does 'Exotic Rain' Occur On The Exoplanet?

Astronomers from Massachusetts Institute of Technology (MIT), Cambridge have obtained the clearest view yet of the perpetual dark side of an exoplanet which is "tidally locked" to its star.

An artists’s impression of WASP-121 b. (Credit: Engine House VFX via Massachusetts Institute of Technology)

New Delhi: Astronomers from Massachusetts Institute of Technology (MIT), Cambridge have obtained the clearest view yet of the perpetual dark side of an exoplanet which is "tidally locked" to its star. Tidal locking is a phenomenon by which an astronomical body has the same rotational period as its orbital speed around a partner. For instance, the Moon is tidally locked to the Earth because it rotates in exactly the same time as it takes to orbit the Earth. 

The researchers' observations, combined with measurements of the planet's permanent day side, provide the first detailed view of an exoplanet's global atmosphere. The study marks the first time the dark side of a "hot Jupiter" is revealed in detail. Hot Jupiters are gas giant planets with an orbital period of less than 10 days.

Also, the researchers explored the possibility of 'exotic rain' on the exoplanet.

The study was recently published in the journal, Nature Astronomy. 

Thomas Mikal-Evans, the lead author of the study, said that researchers are now moving beyond taking isolated snapshots of specific regions of exoplanet atmospheres, to study them as the 3D systems truly are, according to a statement issued by MIT.

The Planet Is An Ultra Hot Jupiter

The planet at the centre of the new study is WASP-121b. This is a massive gas giant nearly twice the size of Jupiter, according to the study. WASP-121b is an ultra hot Jupiter and was discovered in 2015 orbiting a star about 850 light years away from Earth. 

WASP-121b Has One Of The Shortest Orbits

The planet circles its star in just 30 hours, and has one of the shortest orbits detected to date, according to the study. WASP-121b is tidally locked, such that its "day side" is permanently roasting. Meanwhile, the planet's "night side'' is turned forever toward space.

Tansu Daulan, an MIT postdoc working on the Transiting Exoplanet Survey Satellite (TESS) mission, said that hot Jupiters are famous for having very bright day sides, but the night side is a different beast. Daylan said that WASP121b's night side is about 10 times fainter than its day side, according to the statement.

Earlier, astronomers had detected water vapour and studied how the atmospheric temperature changes with altitude on the planet's day side.

Dramatic Temperature Changes On The Exoplanet

The new study captured a much more detailed picture because the researchers were able to map the dramatic temperature changes from the day to the night side, and to see how these temperatures change with altitude. 

The scientists also tracked the presence of water through the atmosphere to show, for the first time, how water circulates between a planet's day and night sides, the study said.

The water cycle on Earth includes evaporation, condensation into clouds, and precipitation. However, on WASP-121b, the water cycle is far more intense. This is because, on the day side, the atoms that make up water are ripped at temperatures greater than 3,000 Kelvin, according to the study.

The atoms are blown around to the night side, where colder temperatures allow hydrogen and oxygen atoms to recombine with water molecules. 

These molecules subsequently blow back to the day side, and the cycle commences again.

High-Speed Winds On WASP-121b

According to the study, the researchers calculated that the planet's water cycle is sustained by winds that whip the atoms around the planet at speeds of up to five kilometres per second, or more than 11,000 miles per hour.

Does Exotic Rain Occur On The Exoplanet?

The study found that water is not alone in circulating around the planet. The night side was observed to be cold enough to host exotic clouds of iron and corundum. 

Corundum is a mineral which makes up rubies and sapphires. According to the study, these clouds, like water vapour, may whip around to the day side. 

High temperatures at the dayside vaporise the metals into gaseous form. While the clouds are on their way to the day side, exotic rain may be produced, such as liquid gems from the corundum clouds, according to the study.

Mikal-Evans said that with this observation, the researchers are really getting a global view of an exoplanet's meteorology.

How Astronomers Observed Day & Night On WASP-121b

Using a spectroscopic camera aboard NASA's Hubble Space Telescope, the researchers observed WASP-121b, the study said.

The spectroscopic camera observed the light from a planet and a star, and broke that light down into its constituent wavelengths. Astronomers obtain clues about the atmosphere's temperature and composition from the intensities.

Scientists have observed atmospheric details on the day sides of many exoplanets through spectroscopic studies. However, it is far trickier to perform the same activity for the night side, the statement said.

This is because it requires watching for tiny changes in the planet's spectrum as it circles its star.

In the new study, the researchers observed WASP-121b throughout two full orbits. 

In 2018 and 2019, the scientists looked through the light data for a specific line, or spectral feature, that indicated the presence of water vapour.

How Temperatures Change With Altitude

Mikal-Evans said that the researchers saw this water feature and mapped how it changed at different parts of the planet's orbit. He explained that this encodes information about what the temperature of the planet's atmosphere is doing as a function of altitude. 

The team mapped the temperature profile for both the day and night side with the help of the changing water feature, according to the study. The researchers found that the day side ranges from 2,500 Kelvin at its deepest observable layer, to 3,500 Kelvin in its topmost layers. 

The study found that the night side ranges from 1,800 Kelvin at its deepest layer, to 1,500 Kelvin in its upper atmosphere. 

The researchers made an interesting observation. They found that the temperature profiles appeared to flip-flop. 

The temperatures increase with altitude on the day side, and decrease with altitude on the night side, according to the study. This phenomenon is called "thermal inversion" in meteorological terms.

Metal Clouds On The Planet?

Then, the researchers passed the temperature maps through various models to identify chemicals that are likely to exist in the planet's atmosphere. The team found that there was a potential for metal clouds, such as iron, corundum, and titanium on the night side. 

The researchers also observed that the planet's hottest region is shifted to the east of the "substellar region" directly below the star, the study said. Extreme winds could have resulted in this shift, the scientists deduced.

The scientists analysed the size of the shift, and estimated that the wind had a speed of around five kilometres per second, the study said.

According to the statement, Daylan said that winds can probably move across the entire planet in about 20 hours. 

JWST To Observe WASP-121b This Year

The astronomers suspect that carbon monoxide also resides in the atmosphere. They have reserved time on the James Webb Space Telescope to observe the exoplanet later this year.

Mikal-Evans said that would be the first time they could measure a carbon-bearing molecule in this planet's atmosphere. He explained that the amount of carbon and oxygen in the atmosphere provides clues on where these kinds of planets form.