Giant Helium Tails Escaping Jupiter-like Planet Observed by Astronomers.

A team of astronomers used observations from the Hobby Eberle Telescope (HET) at the University of Texas at Austin’s MacDonald Observatory to discover some of the longest gas tails observed escaping from a planet.

HAT-P-32b is roughly twice the size of Jupiter and loses its atmosphere with dramatic jets of helium darting before and behind it as it travels through space. These tails are more than 50 times the radius of the planet. The discovery was published June 7 in the journal Nature Science advances.

The tails of material spilling around the planets are almost unheard of. It can be the result of a collision and release of a trace of dust and debris. Or they could be caused by the heat of a nearby star energizing and blowing the planet’s atmosphere out into space. However, the HAT-P-32b’s long tails are really cool.

“It’s exciting to see how huge the elongated 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. He led the team that made the discovery while part of the University of Texas at Austin’s HET Exospheres project. The HET Exospheres project studies the atmospheres of planets outside our solar system.

Detection of HAT-P-32b excitable tails

To get a sense of 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 “crossing over”. One such example is when Venus passes between Earth and the Sun.

During the transit, the star shines light through the passing planet’s atmosphere, if any. Through a method called “spectroscopy,” astronomers can study this light to identify elements in the atmosphere. With spectroscopy, the light is broken down into a spectrum, much like white light shining through a prism. Different ranges of colors in the spectrum correspond to different elements.

Previous studies have detected tails of HAT-P-32b. However, since 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 witnessed this without the long time frame observations that we can get with the Hobby Eberle telescope,” said Carolyn Morley, assistant professor at the University of Texas at Austin and principal investigator for the HET Exospheres project. “It has allowed us to observe this planet for its full orbit.”

Zhang’s team observed HAT-P-32b over 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 a planet to orbit its star, ensuring that the full extent of its tails was revealed.

HAT-P-32b’s tails are likely caused by its parent star boiling off the planet’s atmosphere. The planet is what astronomers refer to as a “hot Jupiter,” meaning it’s large, hot, gaseous and has a close 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 gravity and was drawn 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’re able to take high-resolution measurements on hot Jupiters, like this one, and then apply our results on a wider planetary scale.”

Hobby Eberle Telescope (HET) and the study of planetary atmospheres

HET is particularly suitable for studying the atmospheres on planets outside our solar system. The High Resolution Planet Finder in the Habitable Zone instrument is capable of observing objects in near-infrared wavelengths. This includes the wavelength associated with helium, allowing astronomers to monitor gas escaping from HAT-P-32b and other similar planets.

Another advantage of observing with HET is that it scans the same sweep of the sky each night. Unlike most other telescopes, which tilt up and down, HET’s 10 by 11-meter mirror is always tilted at an angle of 55 degrees above the horizon. This can lead to high-resolution, long-duration observations of the same patch of sky every night.

“Because we can observe the system every night for several days in a row, we can detect physically large structures like this,” Zhang said. “Other planets may also have had runaway atmospheres waiting to be discovered by similar observations.”