Astronomers may have discovered the presence of water in the atmosphere of an extremely hot planet that is also one of the most “metallic” worlds ever observed. The formation of the planet remains a mystery, which may be solved by discovery.
The extra-solar planet, or “exoplanet” in question is HD 149026 b, which is also known as “Smertrios”, meaning “Purveyor” or “Provider”, worshiped as the god of war in the Gallic tradition. 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, completing an orbit in less than three Earth days. At about three-quarters the diameter of Jupiter, Smertrios is classified as a “hot Saturn,” a class of planets named after the solar system’s smallest gas giant.
Smertrio’s proximity to its star means it is tidally locked with one side always facing its host star, where temperatures rise to 2,600 degrees Fahrenheit (1,420 degrees Celsius). It is relatively cooler on the night side, always facing space. But the density and composition of Smertrio, discovered in 2005 as it passed or “transited” the face of its star, are what make 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 stars at extremely close distances.” 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 metal-rich and gas-dense giants we know so far.”
When astronomers like Ali Rafi refer to “metals,” they are elements that are 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 the 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 bigger a gas giant gets, the less metals it should have. Smertrios opposes this trend. It was also discovered that the planet had an abnormally large solid core, which is why its density is so high.
“The composition of the planet appears to be inconsistent with the current formation scenarios we have for hot gas giants and is still a mystery to this day,” said Ali Rafi. “This implies the importance of observing the planet’s atmosphere as it can help reveal the planet’s formation history through its atmospheric properties, such as metallicity and the ratio of carbon to oxygen.”
Which way do the winds blow in Smertrios?
To investigate Smertrio’s strange atmosphere and hopefully discover why the planet is so “metallic,” Ali Rafi and 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 passes in front of its star, the elements in its atmosphere leave “fingerprints” in the filtered starlight. CARMENES can read these fingerprints and tell astronomers what that atmosphere is made of.
Smertrio’s investigation with CARMENES revealed water vapor fingerprints. This could help to better constrain the abundance of other elements in the planet’s atmosphere.
Ali Rafi explained that assuming elemental oxygen is more abundant than elemental carbon in the atmospheres of hot gas giants like Smertrios means that water and carbon monoxide are two of the most abundant “trace” species that could explain the nature of that atmosphere.
“Therefore, if we can constrain the abundance of both molecules, we are able to constrain the ratio of carbon to oxygen in the atmosphere, which is very important because it can act as a kind of characteristic tracer for the formation and evolutionary history of planets gas giants such as HD 149026 b,” added Ali Rafi. “Finding water in the atmosphere is the first step towards such a characterization.”
However, that was not all the team discovered about Smertrios. The hot planet Saturn brought quite a surprise to the researchers.
Typically, in hot gas giants that are tidally trapped, the massive temperature difference between hot day and coldest night creates extremely fast atmospheric winds that can blow up to 5,000 mph (8,046 km/h) .
These usually blow from day to night on those planets, because winds on most other planets move from hot to cold regions. However, Smertrio’s winds seem to be blowing in the opposite direction, from the planet’s night to day coast.
“Because we’re seeing the planet as it transits, winds blowing on the night side mean they’re blowing towards the observer, and that would cause the planetary absorption spectrum to be blue-shifted,” Ali Rafi explained. “However, what we found is 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 could be winds blowing from night to day. “
One possibility is that these winds are possible in the deep atmospheric layers, causing a red-shifted spectrum, Ali Rafi explained. The researcher added another possibility that Smertrio’s orbit is not circular, but is flattened or “eccentric,” and this could lead to a shift in the spectrum that the team did not account for.
“We need additional observations to confirm any of these scenarios or even if there are any other alternatives,” Ali Rafi added.
Astronomers will now continue to investigate this strangely hot Saturn in order to confirm the discovery of water vapor.
“More observational transit 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 estimate the metal and carbon to oxygen report more precisely”, concluded Ali Rafi. “Hopefully, this can help us clarify the history of the formation and evolution of Smertrio!”
A pre-revised version of the team’s research has been published on the paper’s arXiv repository.