A team of astronomers have 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 observed escaping from a planet. Called HAT-P-32b, this world is nearly twice the size of Jupiter and is losing its atmosphere through dramatic helium jets fanning out in front of and behind it as it travels through space. These tails are more than 50 times the length of the radius of the planet.
Tails of material escaping around planets are not unheard of. They can be the result of a collision releasing a trail of dust and debris. Or, they may be caused by heat from a nearby star energizing and ejecting 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, who led the team that made this find while part of the HET Exospheres Project at the University of Texas at Austin. . The huge structures of HAT-P-32b To learn about the atmosphere of planets outside our solar system, astronomers can observe their parent star as the planet passes in front of it. This is what is known as a “transit”. An example would be when Venus passes between the Earth and the Sun. During a transit, the star emits light through the passing planet’s atmosphere, if any. Through a method called “spectroscopy,” astronomers can study this light to identify what elements are present in the atmosphere. With spectroscopy, light is divided into a spectrum, 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 actual 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, an assistant professor at the University of Texas at Austin and principal investigator for the HET Exospheres Project. “It allowed us to observe this planet in its full orbit.”
Zhang’s team observed HAT-P-32b over the course of several nights, capturing the moment the planet crossed in front of the star, as well as observations in the days before and after. This covered the entire time it takes for the planet to orbit its star, ensuring that the full extent of their tails was revealed.
HAT-P-32b’s tails are likely caused by the boiling of its parent star in the atmosphere of the planet, which astronomers characterize as a “hot Jupiter”—meaning it’s big, hot, gaseous, and has an orbit close around its star. Its orbit is so tight that the heat from its parent star causes the gas in HAT-P-32b’s atmosphere to expand. The atmosphere has expanded so much that part of it has escaped the gravitational pull of the planet and has been drawn into orbit around the nearby star. “Our findings at HAT-P-32b may help us understand how other planets and their stars interact,” Morley said. “We can take high-precision measurements on hot Jupiters, like this one, and then apply our findings to a broader range of worlds.”
The HET is particularly suitable for studying atmospheres on planets outside of our solar system. Its high-resolution instrument, the Habitable-Zone Planet Finder Spectrograph, can observe objects at near-infrared wavelengths. This includes the wavelength associated with helium, allowing astronomers to observe gas escaping from HAT-P-32b and other similar planets. Another advantage of observing with HET is that it examines the same sweep of sky each night. Unlike most other telescopes, which tilt up and down, the HET’s 10-by-11-meter mirror is always tilted at 55 degrees above the horizon. This can lead to high-precision, long-timeline observations of the same strip of sky each night.
source: https://www.science.org/doi/10.1126/sciadv.adf8736