What WASP-69b’s Comet-Like Tail Tells Us About Hot Exoplanets

Rebecca Jean T.
5 min readFeb 15, 2024

New research on exoplanet WASP-69b has a comet-like tail of hydrogen and helium. The planet is so close to its host star that stellar winds are blowing off large amounts of its atmosphere. This discovery was conducted using the Keck Observatory’s NIRSPEC instrument, and may help astronomers understand exoplanet formation. The new findings have been outlined in a paper published in The Astrophysical Journal in January.

An illustration of an orange and yellow round star with some wispy filaments. In front of the star to the right is a gas giant exoplanet with some cloud bands.
An artist impression of a hot exoplanet orbiting close to its host star. Gas giants that orbit close to their star are more vulnerable to losing atmospheric mass due to stellar winds. Credit: ESA/ATG medialab, CC BY-SA 3.0 IGO.

Exoplanet WASP-69b, a “Hot Jupiter”

WASP-69b is a gas giant exoplanet with a diameter slightly larger than Jupiter but a mass less than a third of Jupiter. It is often referred to as a “hot Jupiter” because of its compositional similarities to the gas giant and proximity to its host star. WASP-69b is located a mere 0.04525 AU from its star, making its orbit only 3.9 days long. This closeness is what allows the exoplanet to be around the same size as Jupiter, despite only being 0.29 Jupiter masses. The heat from its host star gives more energy to the gases the planet is comprised of, allowing it to expand.

Because the gases that makeup WASP-69b’s atmosphere are not as compressed as the gas giants in our solar system, material from its atmosphere can escape much more easily. Previous observations of the planet suggested that it is losing some atmosphere and may have a small tail of helium gas leaving the planet as it rotates. Now, new research published in January finds that the exoplanet is losing atmospheric mass much faster than expected, allowing planetary scientists the opportunity to study this unique phase of planetary development for gas giants that orbit close to their host star.

Detecting WASP-69b’s Tail

Astronomers from the University of California, Los Angeles (UCLA) studying WASP-69b were given telescope time at Keck Observatory in Hawaii. Using Keck Observatory’s Near-Infrared Spectrograph (NIRSPEC), the team was able to make detailed observations of the planet. Because WASP-69b transits in front of its star, the team was able to use the observed changes in the spectrograph of starlight to determine the composition of the exoplanet. Not only that, but they were able to observe any changes that occurred before, during, and after transit. This allowed the…



Rebecca Jean T.

Published author on NASA’s Radio Jove project. Researching science topics to deliver to you in bite-sized stories.