James Webb Space Telescope Reveals Unexplained Helium Tails from WASP-121b

Astronomers using the James Webb Space Telescope (JWST) have made a groundbreaking discovery regarding a distant exoplanet, known as WASP-121b or “Tylos.” This “hot Jupiter,” located approximately 858 light-years from Earth, exhibits two extraordinarily long tails made of helium, a phenomenon that current scientific models struggle to explain. The findings mark the first comprehensive study of gases leaking from an exoplanet during its complete orbit, significantly advancing our understanding of atmospheric escape.

Located in a close orbit around its parent star, WASP-121b exemplifies an “ultrahot Jupiter.” It completes an orbit in just 30 hours while enduring extreme temperatures of around 4,200 degrees Fahrenheit (2,300 degrees Celsius). This intense heat causes lighter elements like hydrogen and helium to escape into space, a gradual process that can take millions of years and significantly alters the planet’s atmosphere.

Traditionally, scientists have observed atmospheric escape during a phenomenon known as a “transit,” where an exoplanet passes in front of its star. However, this method left gaps in understanding whether atmospheric gases continued to leak outside of these limited observational periods. The recent JWST observations, which utilized the telescope’s Near-Infrared Spectrograph (NIRSpec) over 37 consecutive hours, provide the first continuous evidence of helium presence and its escape during a full orbit.

Discovering Dual Helium Tails

The research team, led by Romain Allart from the University of Montreal, expressed their surprise at the duration of the helium escape. “This discovery reveals the complexity of the physical processes that sculpt exoplanetary atmospheres and their interaction with their stellar environment,” Allart stated. The findings illustrate the intricate dynamics at play in exoplanetary atmospheres, suggesting that scientists are only beginning to uncover the full complexity of these distant worlds.

Helium serves as a crucial indicator of atmospheric escape from exoplanets, and the sensitivity of the JWST allows for observations at great distances. By analyzing the light absorbed by helium atoms, researchers discovered that the gas envelope surrounding WASP-121b extends far beyond the planet itself. The helium signal persisted for over half of the planet’s orbit, establishing the longest continuous detection of atmospheric escape recorded to date.

One intriguing aspect of the study is the formation of two distinct helium tails. The first tail is pushed backward by radiation and stellar winds from the parent star, while the second tail leads the planet in its orbit, likely influenced by its gravity. Together, these tails measure over 100 times the width of WASP-121b and extend three times the distance from the planet to its star. The presence of dual tails poses a challenge to existing scientific models.

Implications for Future Research

Despite the significant findings, the research team acknowledges that these observations reveal limitations in current numerical models of exoplanetary atmospheres. As Vincent Bourrier from the University of Geneva noted, “Very often, new observations reveal the limitations of our numerical models and push us to explore new physical mechanisms to further our understanding of these distant worlds.”

The team’s research was published on December 8, 2023, in the journal Nature Communications. As scientists continue to analyze these unique characteristics of WASP-121b, this discovery opens new avenues for exploration in exoplanetary science and enhances our understanding of how these distant worlds evolve over time.