A bright yellow circle on a purple background. The circle is surrounded by wispy tails of orange-red clouds coming from a small yellow circle on the left of the image.
Simulated image showing gigantic streams of helium escaping from the atmosphere of planet HAT-P-32b as it orbits its host star. Credit: M. MacLeod (Harvard-Smithsonian Center for Astrophysics) and A. Oklopčić (Anton Pannekoek Institute for Astronomy, University of Amsterdam)

Earth’s atmosphere is just a thin veneer of gas. Despite its seemingly ephemeral nature, however, our atmosphere isn’t, for the most part, going anywhere. That’s not always the case for planets.

Scientists observing a distant exoplanet recently found vast quantities of helium rapidly streaming away from the planet’s upper atmosphere. Filaments of gas several million kilometers long stretch away from the distant world, a finding that highlights the dynamic nature of planetary systems.

About 900 light-years away, a Jupiter-sized gas planet orbits HAT-P-32, a run-of-the-mill star just slightly hotter than the Sun. But the HAT-P-32 planetary system looks nothing like our own: There’s just one planet, HAT-P-32b, and it orbits far closer to its host star than even Mercury does the Sun. (HAT-P-32b completes an orbit—that is, a planetary year—in just over 50 hours, or more than 40 times faster than Mercury.)

Follow the Gas

Because HAT-P-32b orbits so close to its host star, it’s constantly bombarded with high levels of radiation. All that energy dumping onto the planet has a striking outcome: Hydrogen and helium gases are streaming away from HAT-P-32b’s upper atmosphere. To better understand where all that gas is going, Zhoujian Zhang, an astronomer at the University of California, Santa Cruz, and his colleagues trained the University of Texas at Austin’s Hobby-Eberly Telescope on the HAT-P-32 system.

“It’s rare for people to cover the full orbital period.”

The researchers opted to observe the planet over its full orbit. In other words, Zhang and his colleagues timed their observations precisely so that their data captured views of HAT-P-32b when it was not only directly in front of its host star but also off to both sides and behind it, too.

Previous observations of the HAT-P-32 system have typically focused on only recording transits, when the planet is directly in front of the star, as seen from Earth, Zhang said. “It’s rare for people to cover the full orbital period.”

Transit observations reveal information about only a small portion of HAT-P-32b’s orbit. To look for evidence of material at different points in the planet’s orbit, it’s obviously necessary to look over the entire 360°, Zhang said. “If we only focus on the planet’s transit, then we would miss the chance to determine whether the planet’s escaping atmosphere is extended.”

A Two-Tailed Planet

The researchers’ persistence paid off—the spectrographic data they amassed confirmed that not only was helium streaming away from HAT-P-32b but also the gas extended even farther in space than previously believed. (The instrument the researchers used was sensitive to only helium, not hydrogen.) The new observations were most consistent with simulations that modeled two tails of helium gas extending from HAT-P-32b—one ahead of the planet in its orbit and one behind.

“It’s a huge structure.”

Other planets with trailing tails of gas have been spotted, but HAT-P-32b is the first extrasolar planet known to have both leading and trailing tails. “It’s pretty special,” said Zhang, who, along with his colleagues, reported the new results in June in Science Advances.

In total, those two tails have a projected length of more than 6 million kilometers—more than 50 times the radius of HAT-P-32b, the team calculated. That’s astonishingly large, said Stefan Czesla, an astronomer at the Karl Schwarzschild Observatory in Tautenburg, Germany, who was not involved in the research but has extensively studied the HAT-P-32 system. “It’s a huge structure.”

One Trillion Kilograms per Second

Zhang and his team also used their computer simulations to estimate the rate at which helium is escaping from HAT-P-32b’s upper atmosphere. They calculated that more than 1 trillion kilograms of helium are streaming away from the planet every second. That’s orders of magnitude more than the paltry 50 grams or so of helium being lost each second by Earth’s atmosphere.

Despite the prodigious rate of escape, it’d still take well over a billion years for HAT-P-32b to lose its atmosphere entirely, the researchers calculated. And even that’s not likely to happen, Czesla pointed out, because the star in the HAT-P-32 system will probably die before then. The gas’s rate of escape is simply “longer than the star will live,” he said.

HAT-P-32b is a dramatic example of a planet losing its atmosphere. By studying more worlds like this one, astronomers might be able to solve a long-standing mystery, Zhang said. Scientists have long been baffled by a relative dearth of Neptune-mass planets with close orbits around their stars. One idea is that these planets are literally disappearing, molecule by molecule, as their atmospheres are stripped away by high-energy radiation. “Mass loss is one of the scenarios that can explain the observations,” Zhang said.

—Katherine Kornei (@KatherineKornei), Contributing Writer

Citation: Kornei, K. (2023), A planet is dramatically losing its atmosphere, Eos, 104, https://doi.org/10.1029/2023EO230266. Published on 24 July 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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