Super-fluffy celestial bodies: Lighter than cotton candy: Planet with extremely low density

Astronomers have discovered two "super-fluffy" planets orbiting the distant star TOI-791, boasting a density lower than cotton candy. These gas giants, located 1,100 light-years from Earth, are comparable in size to Jupiter but contain merely one-thirtieth of its mass. This discovery, reported in "Monthly Notices of the Royal Astronomical Society," marks a significant find as such planets are exceptionally rare.

"Finding two such planets in one system is an even greater rarity," said lead author Georgina Dransfield of the University of Oxford. "Their extremely low density makes them fascinating testbeds for our understanding of planet system formation and evolution."

The planets were initially detected in archival data from NASA's Transiting Exoplanet Survey Satellite (TESS) by amateur astronomers in 2019 and 2023. TESS monitors the brightness of millions of stars to identify potential planets. When a planet passes in front of its star from Earth's perspective, it causes a slight, regular dimming of the star's light, revealing its presence.

By analyzing the dimming during these transits, scientists determined the planets' sizes. Further observations of slight irregularities in the transits allowed researchers to calculate the planets' masses. The surprisingly low mass led to the calculation of an average density of about 0.04 grams per cubic centimeter, even less than the typical density of cotton candy (0.05 g/cm³). For comparison, Earth's average density is 5.5 g/cm³, and Jupiter's is 1.3 g/cm³.

Scientists do not believe these planets are literally fluffy. Instead, they hypothesize that the planets' large size and low density are due to extensive gas or dust envelopes. These envelopes might surround a relatively small, dense core of rock and metal, creating the illusion of extreme lightness. Further study is needed to understand the precise mechanisms behind their formation and evolution.