On December 8, 2024, an international team announced the discovery of an extraordinarily light planet in orbit around a distant star in the Milky Way. Named WASP-193b, this planet is 50% larger than Jupiter but has a density 25 times lower than that of the gas giant, suggesting it is as ethereal as cotton candy.
This finding, led by the Instituto Andaluz de Astrofísica (IAA-CSIC) in Spain and published in 'Nature Astronomy', challenges existing understanding of the formation of giant and ultra-light planets.
WASP-193b is the second lightest planet discovered to date, surpassed only by Kepler 51d, which is similar in size to Neptune. According to researcher Francisco J. Pozuelos from IAA-CSIC, the planet's dimensions and extremely low density make it a rarity among the over 5,000 exoplanets identified so far.
Julien de Wit, a co-author of the study and assistant professor at the Massachusetts Institute of Technology, described WASP-193b as an extreme case of a class of planets known as 'puffed-up' or 'sponge-like' Jupiters. Although these types of planets have been known for 15 years, they remain a genuine mystery.
Pozuelos noted that this planet challenges current theories of planetary formation, stating, 'We cannot explain how this planet formed. We need detailed observations of its atmosphere to understand its evolution.'
The discovery was made by the Wide Angle Search for Planets (WASP), an international collaboration operating two robotic observatories in both hemispheres. These observatories utilize wide-field cameras to measure the brightness of thousands of individual stars across the sky.
Observations collected between 2006 and 2008, and again from 2011 to 2012, revealed periodic decreases in brightness of WASP-193, a Sun-like star located approximately 1,200 light-years from Earth. The analysis of these transits indicated that a massive 'super-Jupiter' passes in front of the star every 6.25 days.
To determine the planet's mass, density, and potential composition, the team employed the radial velocity method, which analyzes small oscillations in the star's movement due to the gravitational pull of an orbiting planet. These variations are reflected in shifts in the star's spectral wavelength: the more massive the planet, the greater the observed shift.
In the case of WASP-193b, significant changes in the star's radial velocity were barely detected. 'Despite its enormous size, this planet is so light that it exerts almost no detectable pull on its star,' Pozuelos explained.
Collecting the necessary data to determine the new planet's mass took nearly four years.
The calculations confirm that WASP-193b has a mass approximately 0.14 times that of Jupiter and a density of 0.059 grams per cubic centimeter, significantly lower than that of Jupiter and Earth, yet comparable to the 0.05 grams per cubic centimeter of cotton candy.
'The planet is so light that it's hard to imagine a solid material equivalent,' said Julien De Wit, adding that the similarity to cotton candy arises because both are practically air. 'The planet is essentially super-spongy.'
The authors suggest that WASP-193b may possess an atmosphere predominantly made up of hydrogen and helium, potentially extending several tens of thousands of kilometers beyond Jupiter's atmosphere. Currently, no planetary formation model can explain a planet with such extensive atmospheric proportions.
Pozuelos indicated that among the few known ultra-light planets, WASP-193b is the best candidate for study by the James Webb Space Telescope, which could help to understand 'how a planet as lightweight as cotton candy can form.'