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Four baby planets show how super-Earths and sub-Neptunes form [ucla.edu]:

Thanks to the discovery of thousands of exoplanets to date, we know that planets bigger than Earth but smaller than Neptune orbit most stars. Oddly, our sun lacks such a planet. That's been a source of frustration for planetary scientists, who can't study them in as much detail as they'd like, leaving one big question: How did these planets form?

Now we know the answer.

An international team of astrophysicists from UCLA and elsewhere has witnessed four baby planets in the V1298 Tau system in the process of becoming super-Earths and sub-Neptunes. The findings are published in the journal Nature [doi.org].

"I'm reminded of the famous 'Lucy' fossil, one of our hominid ancestors that lived 3 million years ago and was one of the 'missing links' between apes and humans," said UCLA professor of physics and astronomy and second author Erik Petigura. "V1298 Tau is a critical link between the star- and planet-forming nebulae we see all over the sky, and the mature planetary systems that we have now discovered by the thousands."

Planets form when a cloud of gas and dust, called a nebula, contracts under the force of gravity into a young star and a swirling disk of matter called a protoplanetary disk. Planets form from this disk of gas, but it's a messy process. There are many ways a planet can grow or shrink in size during its infancy --- a period of a few hundred million years. This led to major questions about why so many mature planets were between the sizes of Earth and Neptune.

The star V1298 Tau is only about 20 million years old compared to our 4.5-billion-year-old sun. Expressed in human terms, it's equivalent to a 5-month-old baby. Four giant, rapidly evolving planets between the sizes of Neptune and Jupiter orbit the star, but unlike growing babies, the new research shows that these planets are contracting in size and are steadily losing their atmospheres. Petigura and co-author Trevor David at the Flatiron Institute led the team that first discovered the planets in 2019.

"What's so exciting is that we're seeing a preview of what will become a very normal planetary system," said 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."

[...] Once they sorted out the shapes and timing of the orbits of the four planets, the researchers could make sense of how the planets tugged on each other due to gravity, sometimes slowing down and sometimes speeding up, and leading to transits, sometimes occurring early and other times late. These transit and timing variations allowed the team to measure the masses of all four planets for the first time, which is akin to weighing them.

The shocking result? Despite being 5 to 10 times the radius of Earth, the planets had masses only 5 to 15 times larger than Earth. This means they are very low-density, comparable to Styrofoam, whereas the Earth has the density of rock.

"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."

"Our measurements reveal they are incredibly lightweight — some of the least dense planets ever found. It's a critical step that turns a long-standing theory about how planets mature into an observed reality," said Livingston.

[...] "These planets have already undergone a dramatic transformation, rapidly losing much of their original atmospheres and cooled faster than what we'd expect from standard models," said 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 systems of super-Earths and sub-Neptunes we see throughout the galaxy."

Journal Reference: Livingston, J.H., Petigura, E.A., David, T.J. et al. A young progenitor for the most common planetary systems in the Galaxy. Nature 649, 310–314 (2026). https://doi.org/10.1038/s41586-025-09840-z [doi.org]

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