– January 7, 2026 (London time) – One of the biggest recent surprises in astronomy is the discovery that most stars like the Sun harbor a planet between the size of Earth and Neptune within the orbit of Mercury — sizes and orbits absent from our solar system. These 'super-Earths and sub-Neptunes' are the galaxy's most common planets, but their formation has been shrouded in mystery. Now, an international team of astronomers has found a crucial missing link. By weighing four newborn planets in the V1298 Tau system, they've captured a rare snapshot of worlds in the process of transforming into the galaxy's most common planetary types.
"What's so exciting is that we're seeing a preview of what will become a very normal planetary system," says John Livingston, the study's lead author from the Astrobiology Center in Tokyo, Japan. "The four planets we studied will likely contract into 'super-Earths' and 'sub-Neptunes'—the most common types of planets in our galaxy, but we've never had such a clear picture of them in their formative years."
The study focused on V1298 Tau, a star only about 20 million years old—a blink of an eye in cosmic time compared to our 4.5-billion-year-old Sun. Orbiting this young, active star are four giant planets, all between the sizes of Neptune and Jupiter, caught in a fleeting and turbulent phase of rapid evolution. This system appears to be a direct ancestor of the compact, multi-planet systems found throughout the galaxy. Like the Rosetta Stone that helped scholars decipher Egyptian hieroglyphics, V1298 Tau helps us decode how the galaxy's most common planets came to be.
For a decade, the team used an arsenal of ground- and space-based telescopes to precisely measure when each planet passed in front of the star, an event known as a transit. By timing these transits, astronomers detected that the planets' orbits were not perfectly regular. Their orbital configuration and gravity cause them to tug on each other, slightly speeding up or slowing down their celestial dance. These tiny shifts in timing, called Transit-Timing Variations (TTVs), allowed the team to robustly measure the planets' masses for the first time.
"For astronomers, our go-to 'Doppler' method for weighing planets involves making careful measurements of the star's velocity as it's tugged by its retinue of planets." said Erik Petigura, a co-author from UCLA. "But young stars are so extremely spotty, active, and temperamental, that the Doppler method is a non-starter." By using TTVs, we essentially used the planets' own gravity against each other. Precisely timing how they tug on their neighbors allowed us to calculate their masses, and sidestep the issues with this young star."
The results were remarkable. The planets, despite being 5 to 10 times the radius of Earth, were found to have masses of only 5 to 15 times that of our own world. This makes them incredibly low-density—more like planetary-sized cotton candy than rocky worlds.
"The unusually large radii of young planets led to the hypothesis that they have very low densities, but this had never been measured," said Trevor David, a co-author from the Flatiron Institute who led the initial discovery of the system in 2019. "By weighing these planets for the first time, we have provided the first observational proof. They are indeed exceptionally 'puffy,' which gives us a crucial, long-awaited benchmark for theories of planet evolution."
This puffiness helps solve a long-standing puzzle in planet formation. A planet that simply forms and cools down over time would be much more compact. The team's analysis reveals that these planets must have undergone a dramatic transformation early in their lives, rapidly shedding much of their initial atmospheres and cooling dramatically when the gas-rich disk around their young star disappeared.
"These planets have already undergone a dramatic transformation, rapidly losing much of their original atmospheres and cooling faster than what we'd expect from standard models," explains James Owen, a co-author from Imperial College London who led the theoretical modeling. "But they're still evolving. Over the next few billion years, they will continue to lose their atmosphere and shrink significantly, transforming into the compact worlds we see throughout the galaxy."
"I'm reminded of the famous 'Lucy' fossil, one of our hominid ancestors that lived 3 million years ago and was one of the key 'missing links' between apes and humans," added Petigura. "V1298Tau is a critical link between the star/planet forming nebulae we see all over the sky, and the mature planetary systems that we have now discovered by the thousands."
The V1298 Tau system now serves as a crucial laboratory for understanding the origins of the most abundant planets in the Milky Way, giving scientists an unprecedented glimpse into the turbulent and transformative lives of young worlds. Understanding systems like V1298 Tau may also help explain why our own solar system lacks the super-Earths and sub-Neptunes that are so abundant elsewhere in the galaxy.
"This discovery fundamentally changes how we think about planetary systems," adds Livingston. "V1298 Tau shows us that today's super-Earths and sub-Neptunes start out as giant, puffy worlds that contract over time. We're essentially watching the universe's most successful planetary architecture in the making."
