Surveying atmospheric escape from gas giants orbiting F-type stars

Why is it important to know about exoplanets that had their atmospheres removed while orbiting F-type stars? This is what recent study presented Astronomical magazine hopes to appeal as an international team of scientists conducts its first study of atmospheric leakage on planets orbiting F-type stars, the latter of which are larger and hotter than our Sun. Atmospheric leakage occurs on planets orbiting very close to their stars, causing extreme temperatures and radiation from the host star to slowly erode the planet's atmosphere.

For the study, scientists analyzed data from ten transits between six exoplanets using the Wide Field Infrared Camera (WIRC) at Palomar Observatory, operated by the California Institute of Technology. The six exoplanets included:

• HAT-P-8 b (~750 light years, 3.08 days) orbit)
• KELT-7 b (~815 light years, orbit 2.73 days)
• WASP-93 b (~1220 light years, 2.73 day orbit)
• WASP-103 b (1250 light years, 0.925 day orbit)
• WASP-12 b (~1400 light years, 1.09 day orbit)
• WASP-180 A b (~1500 light years, 3.41 day orbit)

The goal of the study was to determine the amount of atmospheric venting that each exoplanet experienced as they orbited their host stars in their extremely narrow orbits. Ultimately, the researchers found that WASP-12 b and WASP-180 A b had significant atmospheric egress detections, with WASP-93 b and HAT-P-8 b having potential atmospheric egress detections and WASP-103 b and KELT-7 b having no atmospheric egress detections.

Comparing these results with long-standing computer models, the researchers found that WASP-12 b and WASP-180 A b had atmospheric exit velocities of logarithmic scale approximately 12.4 and 11.85 grams per second, respectively. This effectively means that WASP-12 b and WASP-180 A b are estimated to have an atmospheric escape velocity of approximately 10^12.4 and 10^11.85 grams per second respectively.

The study's conclusions note: “Our mass loss constraints for the other four study objects (HAT-P-8 b, WASP-93 b, WASP-103 b and KELT-7 b) are similar to published measurements for WASP-48 b and WASP-94 A b, which also orbit early-type stars, and are generally consistent with measured mass loss rates for gas giants orbiting cooler stars.” This suggests that the strong outflows reported in the literature for planets orbiting early-type stars are not representative of all early-type systems.”

As noted, this study is the first to analyze atmospheric exit of exoplanets orbiting F-type stars, although the researchers note that atmospheric exit studies have been limited to exoplanets orbiting K- and M-type stars. Although K- and M-type stars are smaller and cooler than our Sun, it is noted above that F-type stars are larger and hotter than our Sun, meaning that exoplanets that orbit close to it, like the ones analyzed in this study, are exposed to more heat and radiation than our Sun and the K- and M-type stars studied previously.

One of the main motives studying the exit of an exoplanet's atmosphere is to better understand the long-term evolution of exoplanets and, in particular, gas giants orbiting their stars, also called “hot” Jupiters and “super-hot” Jupiters.

It is also an excellent method for obtaining information about star-planet interactions, exoplanet atmosphere composition and detection, habitability potential, and testing planetary models, the latter of which was used in this study. Thus, studies such as these demonstrate the importance of using atmospheric escape as a method to study the formation and evolution of exoplanets for several types of stars.