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A team of astronomers has used observations from the Hobby-Eberly Telescope (HET) at the McDonald Observatory at the University of Texas at Austin to discover some of the longest tails of gas ever seen escaping from a planet.
The planet, HAT-P-32b, is nearly twice the size of Jupiter and is losing its atmosphere to dramatic jets of helium that unfurl in front of and behind it as it travels through space. These tails are more than 50 times the length of the planet’s radius. The discovery was published June 7 in the journal The progress of science.
Tails of escaping material around planets are not unheard of. They may be the result of a collision that releases a trail of dust and debris. Or they may be caused by heat from a nearby star energizing and blowing a planet’s atmosphere into space. However, tails as long as those of HAT-P-32b are truly remarkable.
“It’s exciting to see how gigantic the extended tails are compared to the size of the planet and its host star,” said Zhoujian Zhang, a NASA Sagan fellow at the University of California, Santa Cruz. She led the team that made this discovery while part of the HET Exospheres project at the University of Texas at Austin. The HET Exospheres project studies the atmospheres of planets outside our solar system.
Detection of the dramatic queues of HAT-P-32b
To learn about the atmospheres of planets outside our solar system, astronomers can observe their parent star as the planet passes in front of it. This is what is referred to as “transit”. An example would be when Venus passes between the Earth and the sun.
During a transit, the star radiates light through the atmosphere of the passing planet, if there is one. Through a method called “spectroscopy,” astronomers can study this light to identify which elements are present in the atmosphere. With spectroscopy, light is split into a spectrum, just like white light shining through a prism. Different color bands in the spectrum correspond to different elements.
Previous studies had detected the tails of HAT-P-32b. However, because astronomers had only observed the planet as it passed in front of its star, the true sizes of the tails remained unknown.
“We wouldn’t have seen this without the long-term observations we can get with the Hobby-Eberly telescope,” said Caroline Morley, assistant professor at the University of Texas at Austin and principal investigator for the HET Exospheres project. “It allowed us to observe this planet throughout its orbit.”
Zhang’s team observed HAT-P-32b over the course of several nights, capturing the moment the planet passed through the star and the observations in the days before and after. This covered the entire time it took for the planet to orbit its star, ensuring that the full extent of its tails were revealed.
HAT-P-32b’s tails are likely caused by the boiling of its parent star in the planet’s atmosphere. The planet is what astronomers call “hot Jupiter,” meaning it’s large, hot, gaseous, and has a tight orbit around its star. Its orbit is so tight that heat from its parent star is causing gas in HAT-P-32b’s atmosphere to expand. The atmosphere expanded so much that some of it escaped the planet’s gravitational pull and was pulled into orbit around the nearby star.
“Our findings on HAT-P-32b may help us understand how other planets and their stars interact,” Morley said. “We are able to make high-precision measurements on hot Jupiters, like this one, and then apply our findings to a wider range of planets.”
Hobby-Eberly Telescope (HET) and the study of planetary atmospheres
HET is particularly well suited to studying the atmospheres of planets outside our solar system. Its high-resolution instrument, the Habitable-Zone Planet Finder spectrograph, is capable of observing objects at near-infrared wavelengths. This includes the wavelength associated with helium, allowing astronomers to observe the gas venting from HAT-P-32b and other similar planets.
Another benefit of observing with HET is that it captures the same arc of the sky every night. Unlike most other telescopes, which tilt up and down, the HET’s 10-by-11-meter mirror is always tilted 55 degrees above the horizon. This can lead to long-duration, high-precision observations of the same patch of sky every night.
“Because we can observe the system every night for several consecutive days, we can detect physically large structures like this one,” said Zhang. “Other planets may also have extensive runaway atmospheres waiting to be discovered through similar monitoring.”
Zhoujian Zhang et al, Giant helium tidal tails escaping hot Jupiter HAT-P-32 b, The progress of science (2023). DOI: 10.1126/sciadv.adf8736
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The progress of science
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