‘Cotton-candy’ planets around young star offer rare glimpse of how worlds shrink with age

Around a young, tempestuous star 350 light-years from Earth, four planets so swollen and lightweight that astronomers liken them to cotton candy are offering an unusually clear look at how planetary systems grow up.

A study published Jan. 7 in Nature reports that all four known planets orbiting the star V1298 Tau are enormous—ranging from Neptune-sized to nearly Jupiter-sized—yet weigh only a few to a few dozen times as much as Earth. That combination makes them among the least dense planets ever measured and, researchers say, a long-sought “missing link” between newborn planetary systems and the mature worlds that populate the Milky Way.

“What’s so exciting is that we’re seeing a preview of what will become a very normal planetary system,” lead author John Livingston of Japan’s Astrobiology Center and the National Astronomical Observatory of Japan said in a statement.

The team argues the planets will likely contract over time into the compact “super-Earths” and “sub-Neptunes” that dominate exoplanet surveys—a dramatic early-life evolution that helps explain why so many older systems are packed with small worlds on tight orbits.

A cosmic toddler with tightly packed planets

V1298 Tau, in the constellation Taurus, is a K-type pre-main-sequence star roughly 20 million to 25 million years old—a cosmic toddler compared with the Sun’s 4.6 billion years. All four known planets orbit closer to their star than Mercury orbits the Sun, completing orbits in about 8, 12, 24 and 49 days.

Their near-resonant periods—close to simple numerical ratios—mean the planets repeatedly line up in ways that amplify their gravitational pulls on one another. Those interactions, researchers found, were key to weighing the planets.

Why the usual weighing method failed

Under typical conditions, astronomers estimate an exoplanet’s mass by measuring the slight “wobble” it induces in its star using the radial-velocity (Doppler) method. But V1298 Tau is highly active, with spots and flares that can mimic or overwhelm the planet signal.

In this system, the star’s activity-driven velocity shifts are about 100 times larger than the subtle Doppler signatures the planets would produce, making conventional mass measurements unreliable.

Using transit timing to weigh ‘super-puffs’

Instead, the team relied on transit-timing variations (TTVs): tiny deviations in when each planet crosses the face of the star as seen from Earth. Over nine years—beginning with NASA’s repurposed Kepler mission (K2) in 2015 and continuing through 2024 using NASA’s Transiting Exoplanet Survey Satellite (TESS), the retired Spitzer Space Telescope, and ground-based observatories including NAOJ’s 188-centimeter telescope at Okayama—astronomers tracked dozens of transits and measured timing shifts.

Some transits arrived 50 to 100 minutes early or late, the result of gravitational tugs among the planets. By fitting those timing deviations with dynamical models, the researchers inferred the planets’ masses.

“By using TTVs, we essentially used the planets’ own gravity against each other,” co-author Erik Petigura of the University of California, Los Angeles, said in an interview distributed by Caltech’s IPAC center.

Big in size, light in mass

The mass-and-radius results were striking:

• V1298 Tau c: just over 5 Earth radii, about 4.7 Earth masses
• V1298 Tau d: about 6.5 Earth radii, about 6 Earth masses
• V1298 Tau b: about 9.4 Earth radii, about 13.1 Earth masses
• V1298 Tau e: about 10.2 Earth radii, about 15.3 Earth masses

Despite their large sizes, all four planets fall in the super-Earth to sub-Neptune mass range. Their inferred densities are extraordinarily low—several below one-third the density of water, comparable to Styrofoam or spun sugar.

The new analysis also helps reconcile conflicting earlier work. A 2021 radial-velocity study suggested some planets might be far more massive—approaching Saturn or Jupiter—but the Nature authors argue those estimates were likely skewed by the star’s magnetic activity.

For at least one world, independent evidence supports the lower masses: Hubble and James Webb Space Telescope observations of V1298 Tau b suggest an unusually extended atmosphere, consistent with a lightweight planet.

A benchmark for planet evolution

Planet-formation theories have long predicted that young planets close to their stars should start out larger and puffier, still hot from formation and wrapped in thick hydrogen-helium envelopes. But direct mass measurements at such early ages have been rare.

“By weighing these planets for the first time, we have provided the first observational proof,” said Trevor David of the Flatiron Institute’s Center for Computational Astrophysics, who led the 2019 discovery paper identifying the planets.

Based on the new measurements, the team modeled the planets’ likely evolution. They estimate each planet has a rocky core roughly 4 to 6 Earth masses, surrounded by a hydrogen-helium envelope initially comprising about 10% to 20% of the planet’s mass.

As the system’s protoplanetary disk dissipated, the loss of external pressure could have triggered “boil-off,” a phase in which hot gas escapes rapidly. Over longer timescales—tens to hundreds of millions of years—intense X-ray and ultraviolet radiation from the young star can further erode atmospheres.

The study projects that by the time V1298 Tau reaches an age comparable to the Sun’s, its planets may shrink to roughly 1.5 to 4 Earth radii—a mix of rocky super-Earths and sub-Neptunes retaining thinner gas layers.

Connecting to the ‘radius valley’

NASA’s Kepler survey found that short-period planets cluster into two size groups—about 1 to 1.5 Earth radii and 2 to 3 Earth radii—with fewer planets in between, a feature known as the “radius valley.” Many researchers attribute it to atmospheric loss: some planets are stripped down to bare rocky cores while others retain enough gas to remain larger.

The V1298 Tau planets currently sit well above that valley in the super-puff regime, and the new results suggest they are on a trajectory that would carry them through it—linking youthful, bloated worlds to the compact planets seen around older stars.

A ‘missing link’—and a reminder our solar system may be unusual

Petigura compared the system’s significance to a famous transitional fossil in human evolution.

“I’m reminded of the famous ‘Lucy’ fossil … one of the ‘missing links’ between apes and humans,” he said. “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.”

Exoplanet surveys indicate that planets between one and four times Earth’s radius on tight orbits are the most common type around Sun-like stars—yet the solar system has no close-in super-Earths or sub-Neptunes. If V1298 Tau evolves as predicted, its mature architecture may resemble what appears to be the galaxy’s “standard” outcome rather than our own.

Researchers expect V1298 Tau to remain a prime target for the James Webb Space Telescope and future missions, particularly to study how young atmospheres escape and how quickly planets cool and contract. For now, the system provides what planetary scientists have sought for years: a quantitative snapshot of worlds caught in the act of slimming down.