The atmosphere of a gas giant disappears under the influence of its parent star
At a distance of about 164 light years from Earth lies the giant planet WASP-69b, which is so close to its star that it completes one revolution in less than four days. Due to the strong radiation from the star, the planet loses its atmosphere, which leads to the formation of a trail, which, like a «tail» comets, extends over vast distances into outer space.
Scientists had previously had an idea that stars could «take away» the mass of planets that are too close to them. This is due to the extreme energy of the star as well as the stellar wind. Although this phenomenon is common, researchers have not yet fully studied it.
The observation of the gas trail emanating from WASP-69b presents a unique opportunity to study the planet's mass loss. Previous studies have already found a slight trace extending from this planet, which was barely noticeable. However, new research shows that the length of the «tail» significantly more than previously thought.
Dakota Tyler of the University of California Department of Physics and Astronomy, lead author of the new study, says: «Earlier studies had shown that the planet was losing some of its atmosphere, but the trace was either barely noticeable or there was none at all. In our study, we determined the presence of a trace and showed that its length is seven times the size of the planet itself».
With the increasing number of exoplanet discoveries thanks to NASA's Kepler and TESS missions, it has become clear that gaps exist in the exoplanet population. The concept of «Neptune Desert» refers to the absence of Neptune-sized planets in orbits of two to four days around the star. The concept of «interval of the small radius of a planet» refers to the absence of exoplanets with radii between 1.5 and 2 Earth radii. Scientists believe that mass loss plays a role in both of these phenomena, and it is unlikely that these «omissions» and «deserts» exoplanets were indeed absent.
However, the details of the atmospheric loss process are still not fully understood. WASP-69b and its extensive trail of gas provide astronomers with a unique opportunity to study this process in more detail. Eric Petigura, co-author and professor of physics and astronomy at the University of California, says: «Studying the disappearance of the atmospheres of highly irradiated exoplanets plays an important role in understanding the physical mechanisms that determine the demography of planets orbiting close to their stars.».
For their observations, Tyler and his team used the Keck Observatory's 10-meter telescope and its high-resolution spectrograph NIRSPEC.
«Over the past decade, we have learned that most stars have planets that orbit them much closer than Mercury does around the Sun, and that the loss of atmosphere plays a key role in the formation of modern planet types. However, for most known exoplanets, the period of atmospheric loss is assumed to have already ended. The WASP-69b system is a godsend because it provides a rare opportunity to study atmospheric mass loss in real time and understand the fundamental principles that shape thousands of other planets», — Petigura explains.
The study showed that the wake formation process is influenced by two different forces: radiation from the star and stellar wind. The interaction of these forces leads to the loss of WASP-69b's atmosphere and the formation of its gaseous trace.
To study the planet's mass loss, researchers measured helium content in the “tail”. They also compared the current study, conducted using a larger telescope, with previous observations to identify patterns of change.
WASP-69b, loses approximately one Earth mass of material every 100 million years. However, according to the authors, WASP-69b will never completely lose its atmosphere: «With such a large mass (about 90 Earth masses), WASP-69 has a sufficient supply of material that even a significant mass loss will have little effect on its condition& ;raquo;, — Tyler clarifies.
Exoplanets can also stabilize when they reach a certain mass. Some studies suggest that exoplanets with atmospheres twice the radius of their core are the most stable and resistant to atmospheric loss. In case the atmosphere exceeds this size, the planet undergoes atmospheric erosion and eventually reaches a more stable state.
Despite the significance of this study, the authors note that the data obtained from it are based on limited observations. They also indicate that variability likely exists in the WASP-69b system, which may alter the rate of mass loss over time. To more fully understand the mass loss process, repeated observations using different instruments will be necessary to study any changes in the planet's characteristics.