Astronomers may have discovered the presence of water in the atmosphere of an extremely hot planet, which is also one of the most “metallic” worlds ever observed. The formation of the planet remains a mystery, which could be solved by this discovery.
The extrasolar planet, or “exoplanet,” in question is HD 149026 b, also known as “Smertrios,” meaning “the Provider” or “the Supplier,” and is revered as the god of war in Gaelic tradition. Smertrios orbits a yellow subgiant star called HD 149026, located about 247 light-years from Earth.
The planet lies about 6.5 million kilometers from its parent star and orbits it in less than three Earth days. With a width about three-quarters that of Jupiter, Smertrios is classified as a “hot Saturn,” a class of planets named after the smallest gas giant in the solar system.
Smertrios’ proximity to its star means it is tidally locked, with a permanent dayside that always faces its host star, where temperatures can reach 1,420 degrees Celsius. Its relatively cooler nightside always faces space. But the density and composition of Smertrios, discovered in 2005 as it crossed, or “transited,” the face of its star, are what makes it truly strange.
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“A hot Saturn is a type of exoplanet called a ‘hot gas giant.’ Hot gas giants are exoplanets that are similar in size to Jupiter or Saturn, but orbit their host stars at extremely close distances. They typically have an orbital period of less than ten days, meaning a year on these planets could last less than a week!” Sayyed Ali Rafi, a member of the team behind the discovery and an astronomy researcher at the University of Tokyo, told Space.com. “This planet is particularly interesting because it is one of the most metal-rich and dense gas giants we know of so far.”
When astronomers like Ali Rafi talk about “metals,” they’re talking about elements heavier than hydrogen and helium. When the James Webb Space Telescope (JWST) observed Smertrios in 2023, the powerful space telescope found that the planet’s metallicity—its fraction of metals to hydrogen—is much higher than that of most hot Saturns and larger hot Jupiters. The ratio is also much higher for Smertrios than for the solar system giants Jupiter and Saturn.
It is generally considered that metals are inversely proportional to the mass of gas giants. This means that the larger a gas giant is, the less metal it should contain. Smertrios defies this trend. The planet has also been found to have an abnormally large solid core, which explains its high density.
“The composition of the planet does not seem to be compatible with the current formation scenarios we have for hot gas giants, and it remains a mystery to this day,” said Ali Rafi. “This shows the importance of observing the planet’s atmosphere because it could help reveal the planet’s formation history through its atmospheric properties such as its metallicity and carbon-to-oxygen ratio.”
Which direction do the winds blow on Smertrios?
To study Smertrios’ strange atmosphere and hope to discover why the planet is so “metallic,” 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 characteristic wavelengths. When a planet like Smertrios transits in front of its star, elements in its atmosphere leave “fingerprints” in the filtered light from the star. CARMENES can read these fingerprints and tell astronomers what that atmosphere is made of.
Smertrios’ study with CARMENES revealed the fingerprints of 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 atmospheres of hot gas giants such as Smertrios means that water and carbon monoxide are two of the most abundant “tracer” species that can explain the nature of that atmosphere.
“Therefore, if we can constrain the abundance of these two molecules, we can also constrain the atmospheric carbon-to-oxygen ratio, which is very important because it could serve as a characteristic tracer of the formation and evolution of gas giant planets like HD 149026 b,” Rafi added. “The discovery of water in the atmosphere is the first step in such a characterization, hence its importance.”
But that’s not all the team discovered about Smertrios. Saturn’s hot planet had a big surprise in store for the researchers.
Typically, on hot gas giants that are tidally locked, the massive temperature difference between the warm day side and the cooler night side creates extremely fast atmospheric winds that can reach 5,000 mph (8,046 km/h).
These winds generally blow from the dayside to the nightside of these planets, as winds on most other planets move from warm to cool regions. However, Smertrios’ winds appear to blow in the opposite direction, from the nightside to the dayside of the planet.
“Since we observe the planet as it transits, winds that blow toward the night mean that they blow in the direction of the observer, which would cause a blueshift in the planetary absorption spectrum,” Rafi explained. “However, we found that it is redshifted, which was a big surprise to us. We can think of several scenarios that could explain the fact that there could actually be winds blowing from the night side to the day side.”
According to Ali Rafi, it is possible that these winds occur in the deep atmospheric layers, causing a redshifted spectrum. The researcher added that another possibility is that Smertrios’ orbit is not circular but flattened or “eccentric,” which could lead to a shift in the spectrum that the team did not account for.
“We need further observations to confirm any of these scenarios or even if there are other alternatives,” Ali Rafi added.
Astronomers will now continue to study this strange hot Saturn to confirm the detection of water vapor.
“More transit observational data on this planet are needed to follow up on our results. We are currently working to confirm the evidence for water and to search for other atmospheric species, as well as to constrain their abundance to estimate metallicity and carbon-to-oxygen ratio more accurately,” concluded Ali Rafi. “We hope this could help us elucidate the formation and evolutionary history of Smertrios!”
A pre-peer-reviewed version of the team’s research is published on the arXiv repository.