Puffy young exoplanets reveal origin of super Earths

by Riko Seibo
Tokyo, Japan (SPX) Jan 08, 2026

Astronomers have identified a young planetary system that links newborn giant worlds to the compact super Earths and sub Neptunes that dominate the Milky Way. V1298 Tau, a star about 20 million years old, hosts four planets between the sizes of Neptune and Jupiter in close-in orbits, offering a snapshot of planets during a brief and dynamic stage of evolution.

Over ten years, researchers combined observations from ground and space telescopes to track when each planet crossed the face of V1298 Tau. By measuring small deviations in the timing of these transits, known as Transit Timing Variations, they determined that the planets gravitationally perturb one another, which allowed precise mass estimates.

The team found that although each planet has a radius 5 to 10 times that of Earth, their masses range from only 5 to 15 Earth masses. This combination implies very low densities and extended atmospheres, resembling large, diffuse envelopes rather than compact rocky bodies.

Lead author John Livingston of the Astrobiology Center in Tokyo said, "What's so exciting is that we're seeing a preview of what will become a very normal planetary system. 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."

Because simple cooling cannot keep such planets inflated, the results indicate that V1298 Tau's planets have already lost a significant fraction of their primordial atmospheres. The analysis suggests that once the gas-rich protoplanetary disk dispersed, intense radiation and changing conditions around the young star drove rapid atmospheric escape and accelerated cooling.

Co-author Erik Petigura of UCLA noted that the usual Doppler radial-velocity technique is not practical for such an active young star. "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. 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 and radius measurements confirm that the planets are extremely low density, consistent with earlier suggestions that young close-in worlds begin as large, tenuous objects. These values provide a benchmark for models that track how planetary atmospheres cool and contract after formation.

Trevor David of the Flatiron Institute, who led the initial discovery of the system, emphasized that the new measurements confirm long-standing theoretical expectations about young planets. "The unusually large radii of young planets led to the hypothesis that they have very low densities, but this had never been measured. 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."

Theoretical modeling led by James Owen at Imperial College London shows that the V1298 Tau planets will keep evolving over billions of years. "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," he explained. "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."

Erik Petigura compared the system's role in planetary science to a key fossil in human evolution. "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 added. "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 acts as a benchmark laboratory for studying how abundant close-in planets form and evolve. Its properties may also illuminate why the Solar System lacks super Earths and sub Neptunes, even though these planets are common around other stars.

Livingston stressed the broader implications for exoplanet demographics. "This discovery fundamentally changes how we think about planetary systems," he said. "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."

Research Report:A young progenitor for the most common planetary systems in the Galaxy