Thousands of exoplanets have been discovered beyond our solar system, showcasing a vast diversity in size, mass, and orbital configurations. This ongoing exploration continually broadens our understanding of planetary systems and their evolution.
Unveiling Kelt 9b
Among discovered exoplanets, Kelt 9b stands out as an “ultra-hot Jupiter,” a gas giant orbiting extremely close to its parent star. This colossal planet is located approximately 670 light-years from Earth in the constellation Cygnus. Kelt 9b is significantly larger than Jupiter, with a mass around 2.88 times and a radius nearly twice Jupiter’s. Its low density, less than half of Jupiter’s, results from the extreme radiation causing its atmosphere to expand.
Kelt 9b orbits its host star, Kelt-9, at only about 0.03462 astronomical units, roughly ten times closer than Mercury is to our Sun. This tight orbit means a “year” on Kelt 9b lasts less than 1.5 Earth days. The host star, Kelt-9, is a rapidly rotating B-type/A-type star, significantly more massive, larger, and hotter than our Sun, with a surface temperature around 10,170 Kelvin.
A World of Extremes
Kelt 9b holds the record as the hottest known exoplanet, with dayside temperatures reaching approximately 4,600 Kelvin (over 4,300 degrees Celsius or 7,800 degrees Fahrenheit). This temperature is hotter than the surfaces of many stars and only about 1,200 Kelvin cooler than our Sun’s surface. The planet is tidally locked to its star, with one side perpetually facing intense stellar radiation, leading to a dramatic temperature difference between its day and night sides.
The extreme heat on Kelt 9b causes molecules to be ripped apart on its dayside, preventing the formation of compounds like water, carbon dioxide, or methane. This intense radiation also causes rapid atmospheric escape, effectively boiling off the planet’s atmosphere. The escaping gas is then captured by the host star, suggesting the planet may be losing mass at a significant rate. Kelt 9b’s unusual orbital path, which is nearly perpendicular to the spin axis of its rapidly rotating star, creates unique “seasons” with two summers and two winters every 36 hours.
Peering into its Atmosphere
Scientists have detected heavy elements such as atomic iron, ionized iron, and ionized titanium in Kelt 9b’s atmosphere. At such high temperatures, these metals exist as individual charged atoms rather than being sequestered in molecules or cloud particles. Hydrogen, a primary component of gas giants, is also present, but its molecules are continually torn apart on the dayside and can only temporarily re-form on the cooler nightside before being ripped apart again as they cycle back to the dayside.
Observations using instruments like the CARMENES spectrograph have revealed an extended hydrogen envelope around Kelt 9b. This envelope is so spread out that it reaches more than half the planet’s radius and is being pulled toward the host star by its gravity. The study of Kelt 9b’s atmosphere helps astronomers understand how elements behave under conditions far more extreme than those found in our solar system. Detecting these atmospheric components involves analyzing how the planet’s atmosphere absorbs specific wavelengths of light as it passes in front of its star.
Its Place in Exoplanet Research
Kelt 9b was discovered using the transit method, which observes slight dips in a star’s brightness as a planet passes in front of it. The Kilodegree Extremely Little Telescope (KELT) project, which consists of two robotic telescopes, first identified Kelt 9b in 2017.
Kelt 9b provides an unparalleled natural laboratory for studying planetary formation and atmospheric physics under extreme conditions. Its intense environment challenges existing models of how planets form and interact with their host stars. Understanding the processes occurring on Kelt 9b, such as atmospheric escape and the behavior of elements at high temperatures, offers insights into the boundaries of planetary survival. This research contributes to the broader search for life beyond Earth by defining the limits of habitability.