In 2024, astronomers using Antarctic observation stations discovered two giant planets orbiting a distant star that are so extraordinarily low in density they weigh less than candy floss, gram for gram. Each planet is roughly the size of Jupiter, one of our solar system's gas giants with a mass 318 times that of Earth, yet these worlds contain so little material inside their massive atmospheres that they defy conventional understanding of how planets form and evolve. The two planets orbit the same parent star as a pair, gravitationally bound siblings that tug on each other as they travel around their star, occasionally pulling each other off their regular orbital schedules in what scientists call orbital perturbations. This discovery raises a profound question that has puzzled astronomers for decades: how can a world grow so enormous while remaining so nearly empty?

The detection of these unusually light worlds came possible through observations conducted during the Antarctic winter, when the atmosphere is extraordinarily stable and clear, allowing telescopes to make precise measurements of how the planets' gravity affects their parent star's motion. Astronomers measure exoplanet masses by observing the tiny "wobbles" a planet's gravity creates in its star's position as the planet orbits. The two sibling planets' gravitational interaction with each other produces additional, measurable disturbances in their orbital timing, creating a double confirmation of their masses. These measurements revealed densities of roughly 0.05 grams per cubic centimeter, making them less dense than candy floss, which has a density around 0.08 to 0.1 grams per cubic centimeter. To put this in perspective, Jupiter's density is about 1.3 grams per cubic centimeter, while Earth's is a much denser 5.5 grams per cubic centimeter.

These planets belong to a class of exoplanets called "puffy Jupiters" or "super-puffed planets," a category that has puzzled astronomers since the first such world was discovered in 1996. Most gas giants, including Jupiter and Saturn in our own solar system, have relatively normal densities for their size because gravity compresses their thick atmospheres inward. The newly discovered planets, by contrast, have enormous, extended atmospheres that barely compress at all, creating a world that is mostly empty space. The leading theories suggest several possible explanations: maybe these planets formed closer to their stars than they currently orbit, where intense radiation from their parent star inflated and puffed up their atmospheres, and they somehow migrated outward. Alternatively, they may have collided with other objects early in their history, losing dense rocky cores while retaining their gas envelopes. Another possibility is that their atmospheres are still being heated by stellar radiation and have not yet had time to cool and contract over the billions of years of the planets' existence.

These discoveries matter because they challenge our understanding of planetary formation and evolution. When astronomers first began discovering exoplanets in the 1990s, they assumed planets would follow similar patterns to those in our solar system: small, rocky worlds close to their stars, and larger gas giants farther out. The existence of hot Jupiters (massive planets orbiting very close to their stars) forced scientists to completely revise their theories, proposing that planets could migrate across their planetary systems. The puffy giants push this revision further, suggesting that planetary formation and evolution are far more complex and varied than once imagined. Each new discovery of an impossibly light world provides clues to how planets grow and change throughout their lifespans, and these Antarctic observations add crucial data to that growing picture of planetary diversity.