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GHOST spies ultra-hot Jupiter with ultra-fast winds

In the hunt for exoplanets, many seek out habitable worlds. There's comfort in discovering planets that remind us of home—ones at a perfect distance from their host star, with oceans of water covering their surfaces and breathable atmospheres.

Some astrophysicists, however, are curious about an entirely different type of exoplanet: the treacherous hot Jupiter. One such scientist is Emily Deibert, a science fellow at Gemini South in Chile, one half of the International Gemini Observatory, operated by NSF NOIRLab.

An ultra-hot Jupiter named was the focus of a recent study conducted at Gemini South, led by Adam Langeveld, assistant research scientist at Johns Hopkins University, with a team of astronomers including Deibert.

The results of this study are presented in a appearing in The Astrophysical Journal Letters.

The team's investigation utilized a new instrument on the Gemini South telescope called the Gemini High-resolution Optical SpecTrograph (). GHOST is a powerful instrument that has the ability to observe a wide range of wavelengths simultaneously.

It can also complete observations with high efficiency while achieving world-class resolution. These capabilities allowed the team to peer deep into HAT-P-70 b's atmosphere where they discovered winds blowing at incredible speeds.

Hot Jupiters are gas giants that are similar to our Jupiter in size, but that differ greatly in temperament. They sit much closer to their host stars than our planetary neighbor does, giving them notably different physical properties.

To illustrate their distances, it takes our Jupiter almost 12 years to orbit the sun, while hot Jupiters take 10 days or less. Some have even been observed whipping around their suns in less than a day.

Orbiting at such a close distance gives these planets incredibly high surface temperatures, hence their name. They are oftentimes tidally locked, meaning they have one side constantly facing their star experiencing an extremely hot "day" and one side constantly facing away experiencing a colder "night."

HAT-P-70 b is a very "puffy" planet with a radius almost double that of Jupiter. It sits so close to its host star that its orbit is 2.7 Earth days and it has a temperature of about 2,300° Celsius (around 4,200° Fahrenheit), making it one of the hottest planets known to date. The extreme temperatures give this ultra-hot Jupiter an exotic atmosphere with a diverse array of gaseous metallic atoms and ions.

"These ultra-hot atmospheres are ideal laboratories to study exoplanets on a wider scale due to the fantastic opportunity to detect and study multiple chemical species," Langeveld explains. "By measuring the amounts of different elements—especially comparing 'rocky' elements like calcium and iron to 'icy' elements like water and carbon—we can learn about how they formed and evolved."

To study HAT-P-70 b's atmosphere, the team observed the planet transit, or pass in front of, its host star. As the star's light passes through the planet's atmosphere, the chemicals within the atmosphere act like a filter that absorbs specific wavelengths of light.

Using spectroscopy—a method of observation where an object's light is spread out into a spectrum—the team can determine which chemicals exist in the atmosphere by identifying the fingerprint-like patterns of absorption lines that appear in the spectrum.

In HAT-P-70 b's atmosphere, the team detected signatures of ionized calcium—a gaseous and highly energetic form of calcium that can only exist in environments of incredibly intense heat.

They found that the calcium signal extends tens of thousands of kilometers into the upper atmosphere. But more importantly, GHOST's incredible sensitivity allowed them to "time-resolve" the calcium signal. This means they could track how calcium absorption changes from the planet's day to night side.

Deibert shares her experience probing HAT-P-70 b's atmosphere: "We were surprised by the exceptional sensitivity of GHOST, which allowed us to measure minute variations in the individual absorption lines from the ionized calcium, thereby providing information about different regions of the atmosphere. This level of detail has traditionally been difficult to achieve in exoplanet studies, especially for individual absorption lines in transmission spectra."

From these observations, the team determined that HAT-P-70 b hosts powerful winds that rush from the scorching dayside to the cooler nightside at speeds of up to 18,000 kilometers per hour (11,000 miles per hour). They also used the detected signals to infer the planet's mass, revealing that it is likely much lighter than previously thought—a crucial parameter for future comparisons of ultra-hot Jupiter atmospheres.

"This level of detail is only possible with the most advanced spectrographs," says Langeveld, "making GHOST one of the few instruments in the world capable of such measurements."

Deibert adds, "This study showcases that GHOST has the potential to make major contributions towards advancing our understanding of the 3D nature of exoplanet atmospheres, for which there are still many big unanswered questions."

The time-resolving power of GHOST will continue to push the boundaries of exoplanet studies. In fact, Gemini is changing the game for astronomers like Deibert and Langeveld who are looking to gather spectroscopic data, as well as optical and infrared information through its Large and Long Programs.

Langeveld and Deibert's work is part of an approved Large program that they are co-leading, meaning they and their collaborators have secured observing time with Gemini for multi-year research projects in planetary studies.

Gemini's Large programs will promote collaborations and provide significant impact in all areas of astrophysics, paving the way for the future of the field.