Astronomers were surprised to learn in recent years that most Sun-like stars host at least one planet that falls between Earth and Neptune in size and orbits closer than Mercury does in our own solar system -- sizes and orbits absent from our solar system. These worlds, known as super-Earths and sub-Neptunes, turn out to be the most abundant type of planet in the Milky Way. Yet for all their prevalence, how they form has remained unclear. An international research team has now identified a long-missing piece of that puzzle by directly measuring four extremely young planets as they evolve toward these common planetary forms.
By studying the V1298 Tau system, the researchers captured an unusually early snapshot of planetary development. Their measurements reveal planets caught in the act of changing into the super-Earths and sub-Neptunes seen throughout the galaxy.
"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."
V1298 Tau is remarkably young by astronomical standards, at just about 20 million years old -- a blink of an eye compared to the Sun's 4.5-billion-year history. Four large planets circle this energetic star, each ranging in size from Neptune to Jupiter. These worlds appear to be in a short-lived and chaotic stage of rapid change, offering a glimpse of what many mature planetary systems once looked like.
Astronomers believe this system represents an early version of the tightly packed, multi-planet systems commonly found across the galaxy. Much like the Rosetta Stone helped scientists interpret ancient hieroglyphics, V1298 Tau provides a key reference for understanding how the galaxy's most common planets take shape.
Over a ten-year period, the team relied on a combination of space-based and ground-based telescopes to monitor the system. They tracked the precise moments when each planet crossed in front of its star, events called transits. These observations revealed that the planets' orbits were not perfectly steady. Instead, the planets subtly pulled on one another, causing small but measurable changes in their transit timing.
These variations, known as Transit-Timing Variations (TTVs), allowed scientists to calculate the planets' masses directly 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 mass measurements revealed a striking result. Even though the planets are five to ten times larger than Earth, their masses are only five to fifteen times greater. This combination makes them extraordinarily low in density -- more like planetary-sized cotton candy than solid, 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 system's original discovery 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 extreme puffiness helps resolve a long-standing question in planet formation. If planets simply formed and cooled slowly, they would be far more compact. Instead, the analysis shows that these young worlds must have changed dramatically early on, quickly losing large portions of their thick atmospheres as the surrounding disk of gas around their 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."
Petigura compared the system's importance to a famous fossil discovery. "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," he said. "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."
Today, V1298 Tau stands out as a natural laboratory for studying how the most common planets in the Milky Way come into existence. Observations of this system provide rare insight into the chaotic and transformative early lives of planets and may help explain why our own solar system lacks the super-Earths and sub-Neptunes that dominate elsewhere.
"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."
