A Super-Puffed Giant: WASP-193b’s Surprisingly Low Density and Well-Aligned Orbit
Exoplanets—planets that orbit stars outside our solar system—come in a variety of sizes and densities. Some are rocky, like Earth, while others are gas giants, like Jupiter. Among these, a rare category exists: super-puffs. These planets have a size similar to Jupiter but a mass comparable to Neptune, making them incredibly low-density and difficult to explain. In their latest study, Samuel Yee and collaborators examine one such planet, WASP-193b, and find that it is not only extremely low in density but also has an orbit that is surprisingly aligned with its host star.
Super-Puffs: A Planetary Mystery
Planets like WASP-193b challenge what scientists expect from planet formation. The core accretion model, which explains how planets grow by pulling in gas and dust from their surroundings, suggests that planets with large gas envelopes should also have much more mass. Yet, WASP-193b defies this expectation with a density of just 0.060 g/cm³—far lower than any planet in our solar system. To put this into perspective, it's almost as fluffy as cotton candy! Several ideas have been proposed to explain these low-density planets, including gas loss from strong stellar winds, expansion due to internal heat, or stalled growth during their formation.
Observing a Puffy Planet
To better understand WASP-193b, the researchers collected data using the Planet Finder Spectrograph (PFS) on the Magellan Clay Telescope in Chile. By measuring changes in the star's light as the planet passed in front of it, the team refined previous measurements of the planet’s size and mass. They also used the Rossiter-McLaughlin effect, a phenomenon where a planet’s transit distorts the star’s spectral lines, allowing astronomers to determine if the planet's orbit is aligned with the star’s rotation.
WASP-193b: A Unique Case
The findings confirmed that WASP-193b is one of the least dense planets known, with a radius 1.3 times that of Jupiter but only about 11% of its mass. Interestingly, while other known super-puffs have orbits that are significantly tilted compared to their star’s rotation, WASP-193b is relatively well-aligned, with an orbital tilt of only about 17 degrees. This suggests that it may not have undergone the dramatic gravitational disturbances that tilted other super-puffs’ orbits, pointing to a different formation history.
What’s Next?
WASP-193b remains an enigma. One possibility is that it is currently losing mass due to radiation from its host star, causing its atmosphere to expand. Future observations, particularly with the James Webb Space Telescope (JWST), could help confirm whether WASP-193b is undergoing atmospheric escape. Additionally, further radial velocity studies could determine if there are other planets in the system that might have influenced its orbit. In short, WASP-193b is one of the puffiest planets we’ve found, yet it doesn’t fit neatly into the current theories of how planets like this form. Its discovery adds a fascinating new piece to the puzzle of exoplanetary science, reminding us that the universe is always full of surprises.
Source: Yee
