by Astronomers may have discovered the presence of water in the atmosphere of a blisteringly hot planet that is also one of the most “metallic” worlds ever seen. The formation of the planet remains a mystery, one that could be solved with the discovery.
HD 149026 b is the extrasolar planet, or “exoplanet,” also known as “Smertrios,” meaning the “Purveyor” or “The Provider,” worshiped as a god of war in the Gaelic tradition. Smertrios orbits a yellow subgiant star called HD 149026, located approximately 247 light years from Earth.
The planet is about 4 million miles from its parent star, completing an orbit within three Earth days. With a width of about three-quarters that of Jupiter, Smertrios is classified as a “hot Saturn,” a class of planets that takes its name from the gas giant of the smaller solar system.
Because of Smertrios’ proximity to its star, it is tidally locked to a permanent day that always faces its host star, where temperatures rise to as high as 2,600 degrees Fahrenheit (1,420 degrees Celsius). It is relatively cooler on the night side which always faces space. But it is the density and composition of Smertrios, discovered in 2005 as it crosses or “shifts” the face of its star, that makes it strange.
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“A hot Saturn is a type of exoplanet known as a ‘hot gas giant.’ Hot gas giants are exoplanets that are similar in size to Jupiter or Saturn but usually orbit very close to their host stars, with an orbital period of less than ten days, meaning that one year on the that planet to be less than a week! ” Sayyed Ali Rafi, part of the team behind the discovery and an astronomy researcher at the University of Tokyo, told Space.com. “This planet is of particular interest because it is one of the most dense and metal-rich gas giants known to date.”
When astronomers like Ali Rafi refer to “metals,” they are elements heavier than hydrogen and helium. When the James Webb Space Telescope (JWST) looked at Smertrios in 2023, the powerful space telescope found that the planet’s metallicity, its fraction of metals to hydrogen, is much higher than most hot Saturns and Hot Jupiter more. This ratio is also much higher for Smertrios than it is for the giants of the solar systems Jupiter and Saturn.
Metals are usually considered to be inversely proportional to mass in gas giants. That means the more a gas giant gets, the less metal it should have. Smertrios bucks this trend. The planet was also found to have a large anomalous solid core, which is why its density is so high.
“The composition of the planet seems to be incompatible with our current formation scenarios for hot gas giants, and it remains a mystery to this day,” said Ali Rafi. “This highlights the importance of looking at the planet’s atmosphere as it could help reveal the planet’s formation history through its atmospheric properties such as its metallicity and carbon to oxygen ratio.”
Which way do the winds blow on Smertrios?
To investigate the strange atmosphere of Smertrios and hopefully find out why the planet is so “metallic,” Ali Rafi and his colleagues turned to the CARMENES spectrograph, an instrument at the Calar Alto Observatory in Spain.
Chemical elements and molecules absorb and emit light at specific wavelengths. When a planet like Smertrios moves in front of its star, the elements in its atmosphere leave “fingerprints” in the filtered starlight. CARMENES can read these fingerprints and tell the astronomers what that atmosphere is made of.
Smertrios’ investigation with CARMENES revealed the fingerprints of the water vapor. This can help better constrain the abundance of other elements in the planet’s atmosphere.
Ali Rafi explained that assuming that elemental oxygen is more abundant than elemental carbon in the atmosphere of hot gas giants such as Smertrios, it means that water and carbon monoxide are two of the most abundant “trace” species that can change the nature of the atmosphere to explain that.
“Therefore, if we can constrain the abundance of both molecules, we are able to constrain the atmospheric ratio of carbon to oxygen, which is very important because it could act as a kind of tracer of the characteristics of the formation and evolution of the planets gas giants. HD 149026 b,” said Ali Rafi. “Finding water in the atmosphere is the first step to such a characterization, hence its importance.”
However, this is not all the team he discovered about Smertrios. The researchers were quite surprised by the hot planet Saturn.
Typically, on tidally locked hot gas giants, the huge temperature difference between the warm day side and the cooler night side creates very fast atmospheric winds that can rage as fast as 5,000 mph (8,046 kph).
These tend to blow from day to night on those planets because winds on most other planets shift from high temperature regions to cold temperature regions. However, the winds of the Smertrios seem to be blowing in the opposite direction, from the night side to the planet.
“Because we see the planet as it moves, winds blowing to the night side mean that they blow in the direction of the observer, and the planetary absorption spectrum would be changed,” explained Ali Rafi. “However, what we found is that it is red-shifted, and that was a big surprise to us. We can think of several scenarios that could explain the possibility of winds which blows from night to day.”
One possibility is that these winds are occurring in the deep layers of the atmosphere, causing a red-shifted spectrum, explained Ali Rafi. The researcher also added another possibility that the orbit of the Smertrios is not circular but is flattened or “eccentric,” and this could be the result of a change in the spectrum that the team did not take into account.
“We need additional comments to confirm any of these cases or even if there are any other options,” said Ali Rafi.
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Astronomers will continue to investigate this strange hot Saturn to confirm the detection of water vapor.
“More transit observational data on this planet is needed to follow up our result. We are now working on confirming the evidence of water and searching for other atmospheric species, as well as constraining their abundance to the metallicity and estimate the carbon-to-oxygen ratio more accurately,” concluded Ali Rafi. “Hopefully this could help us clarify the formation and evolution history of the Smertrios!”
A pre-reviewed version of the team’s research is published on the working paper site on Xiv.
