Astronomers have observed four young planets in the V1298 Tau system, providing new insight into how the most common types of planets in the galaxy—super-Earths and sub-Neptunes—are formed. This research, led by an international team including astrophysicists from UCLA, was published in the journal Nature.
Planets larger than Earth but smaller than Neptune are found around many stars, yet our own solar system does not contain one. This absence has limited scientists’ understanding of their formation. “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 Erik Petigura, UCLA professor of physics and astronomy and second author on the study. “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.”
The process begins with a nebula—a cloud of gas and dust—that contracts under gravity to form a young star surrounded by a protoplanetary disk. Planets form from this disk during a turbulent period lasting hundreds of millions of years. Scientists have questioned why so many mature planets are between Earth’s size and Neptune’s.
V1298 Tau is about 20 million years old—much younger than our 4.5-billion-year-old sun—and hosts four large planets ranging in size from Neptune to Jupiter. The new research shows these planets are shrinking and steadily losing their atmospheres as they evolve. Petigura and co-author Trevor David first discovered these 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, lead author from the Astrobiology Center in Tokyo. “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 discovery involved nearly ten years of observing planetary transits—when a planet crosses its star—from both ground- and space-based telescopes. Tracking these events allowed scientists to determine each planet’s orbit and interactions within the system.
“We had two transits for the outermost planet separated by several years, and we knew that we had missed many in between. There were hundreds of possibilities which we whittled down by running computer models and making educated guesses,” said Petigura.
Livingston recovered another transit using ground-based observation: “Hey, we got it from the ground!” he messaged Petigura via Slack after pinning down its orbital period.
“I couldn’t believe it! The timing was so uncertain that I thought we would have to try half a dozen times at least. It was like getting a hole-in-one in golf,” said Petigura.
“The effort became more and more tantalizing as we went along,” Livingston added. “We nearly had to resort to brute force to crack the mystery of the outer planet, only to get it right on our first try.”
By measuring how each planet tugged on its neighbors through gravitational interactions affecting transit timing, researchers could calculate their masses for the first time—a step likened to weighing baby planets.
Despite being five to ten times Earth’s radius, each planet’s mass was only five to fifteen times greater than Earth’s, making them extremely low-density compared to rocky Earth-like 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 from Flatiron Institute. “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,” Livingston stated.
As these V1298 Tau planets lose more atmosphere over time, astronomers expect them eventually to become compact super-Earths or sub-Neptunes similar to those found throughout our galaxy.
“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,” explained James Owen from Imperial College London who led theoretical modeling efforts. “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.”
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