Astronomers may have discovered the presence of water in the atmosphere of a blazing-hot planet, which is also one of the most “metallic” worlds ever seen. The formation of the planet remains a mystery that the discovery could solve.
The extrasolar planet or “exoplanet” in question is HD 149026 b, which is also known as “Smertrios”, meaning “Provider” or “Provider”, worshiped in Gaelic tradition as the god of war. Smertrios orbits a yellow subgiant star called HD 149026, located about 247 light-years from Earth.
The planet is about 4 million miles from its parent star and completes orbits in less than three Earth days. At about three-quarters the width of Jupiter, Smertrios is classified as a “hot Saturn,” a class of planets named after the Solar System’s smaller gas giant.
Smertrios’ proximity to its star means it is tidally locked with its permanent dayside always facing its host star, where temperatures soar to 2,600 degrees Fahrenheit (1,420 degrees Celsius). The relatively cooler night side always faces space. But the density and composition of Smertrios, discovered in 2005 as it cruised, or “passed” the face of its star, is what’s really special.
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“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 usually have an orbital period of less than ten days, which means that one year on these planets can 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 particularly interesting because it is one of the richest and densest gas giants known to date.
When astronomers like Ali Rafi talk about “metals,” they are referring to 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 ratio of metals to hydrogen, was much higher than most hot Saturns and larger hot Jupiters. This ratio is also much larger for Smertrios than for the solar system giants Jupiter and Saturn.
Metals are normally considered to be inversely proportional to mass in gas giants. This means that the larger a gas giant is, the fewer metals it should have. Smertrios bucks this trend. The planet was also found to have an anomalously large solid core, which is why its density is so high.
“The composition of the planet does not seem to be compatible with the current formation scenarios we have for hot gas giants and is still a mystery to this day,” said Ali Rafi. “This signifies the importance of observing a 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 does the wind blow on Smertrios?
To probe the strange atmosphere of Smertrios and perhaps find out why the planet is so “metallic”, Ali Rafi and colleagues turned to the CARMENES spectrograph, an instrument at the Calar Alto Observatory in Spain.
Elements and chemical molecules absorb and emit light at characteristic wavelengths. When a planet like Smertrios passes the face of its star, the elements in its atmosphere leave “fingerprints” in the filtered starlight. CARMENES can read these fingerprints and tell astronomers what this atmosphere is made of.
Smertrios investigation with CARMENES revealed water vapor fingerprints. 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 like Smertrios, this means that water and carbon monoxide are two of the most abundant “tracer” species that can explain the nature of that atmosphere .
“So if we can limit the amount of both molecules, we are able to limit the atmospheric ratio of carbon to oxygen, which is very important because it could act as a kind of indicator characteristic of the formation and evolution history of gas giant planets such as HD 149026 b,” added Ali Rafi. “Finding water in the atmosphere is the first step towards such a characterization, hence its importance.”
However, that was not all the team discovered about Smertrios. The hot planet Saturn brought a big surprise to researchers.
On hot gas giants that are tidally locked, the usually huge temperature difference between the hot day side and the cooler night side creates extremely fast atmospheric winds that can rage at speeds of up to 8,046 km/h.
These usually blow from the day side to the night side of these planets, as the winds on most other planets move from high temperature areas to low temperature areas. However, the winds of Smertrios seem to blow in the opposite direction, from the night side to the day side of the planet.
“Because we see the planet as it transits, the wind blowing on the night side means that it is blowing towards the observer, and this would cause a blue shift in the planetary absorption spectrum,” explained Ali Rafi. “However, we found that it is redshifted, and that was a big surprise to us. We can think of several scenarios that could explain the fact that there may actually be nightside to dayside winds.”
One possibility is that these winds are possible in the deep layers of the atmosphere, causing the red-shifted spectrum, Ali Rafi explained. The researcher added the additional possibility that Smertrios’ orbit is not circular, but flattened or “eccentric,” and this could lead to a shift in the spectrum that the team hadn’t anticipated.
“We need more observations to confirm any of these scenarios, or even if there are any other alternatives,” Ali Rafi added.
Astronomers will now continue to study this strange hot Saturn to confirm the detection of water vapor.
“We need more data from transit observations of this planet to follow up on our result. We are now working to confirm the evidence for water and look for other atmospheric species as well as constrain their abundances to estimate the metallicity and carbon-to-oxygen ratio.” more precisely, the ratio,” concluded Ali Rafi. “Hopefully this could help us clarify the history of the origin and development of the Smertrios!”
A preliminary peer-reviewed version of the team’s research is published on the paper repository arXiv.