Four Baby Planets Reveal How Super-Earths and Sub-Neptunes Actually Form

Astronomers have finally managed to answer one of the most frustrating questions in modern planet science: how do super-Earths and sub-Neptunes form? These planets, which are bigger than Earth but smaller than Neptune, turn out to be the most common planets in the galaxy, yet our own solar system doesn’t have a single one. That gap has made them difficult to study in detail. Now, a young star system called V1298 Tau has provided the clearest evidence yet of how these worlds are born and how they change over time.

An international team of researchers led by scientists from UCLA, along with collaborators from institutions across the world, has observed four extremely young planets caught in the middle of their transformation. These planets are essentially planetary infants, still evolving, shrinking, and shedding their atmospheres. The findings were published in the journal Nature, marking a major milestone in exoplanet research.

Why Super-Earths and Sub-Neptunes Matter

Thanks to thousands of exoplanet discoveries over the last two decades, astronomers now know that planets between Earth and Neptune in size dominate planetary systems across the Milky Way. These planets fall into two broad categories: super-Earths, which are typically rocky and larger than Earth, and sub-Neptunes, which have thick gas envelopes but are smaller than Neptune.

Oddly enough, our solar system jumps straight from Earth-sized planets to gas giants like Jupiter and Saturn. This absence has raised a fundamental question: are super-Earths and sub-Neptunes born that way, or do they evolve into those forms over time? Until now, scientists lacked direct observations of planets young enough to answer that question.

The V1298 Tau System: A Planetary Nursery

The star V1298 Tau is located about 350 light-years away in the Taurus constellation. It is only about 20 million years old, making it extraordinarily young by cosmic standards. For comparison, our Sun is roughly 4.5 billion years old. In human terms, astronomers describe V1298 Tau as the equivalent of a five-month-old baby.

Orbiting this young star are four giant planets, all discovered through the transit method, where a planet passes in front of its star and causes a slight dip in brightness. These planets were first identified in 2019, but recent observations have finally allowed scientists to measure their masses and densities with precision.

What makes these planets special is their size and composition. Each one has a radius between five and ten times that of Earth, placing them closer in size to Neptune or even Jupiter. However, their masses are surprisingly low—only five to fifteen times Earth’s mass. This means they are extremely low-density, among the fluffiest planets ever measured.

Puffy Worlds That Are Shrinking

These young planets are not stable, finished worlds. Instead, they are in the middle of a dramatic transformation. The research shows that the planets are rapidly contracting and losing their atmospheres. Despite their large sizes, they are lightweight and filled with gas, making them comparable in density to Styrofoam rather than rock.

This confirms a long-standing hypothesis in planetary science: young planets start out puffed up and bloated, then gradually cool, shrink, and lose gas over time. Until now, this idea was based mostly on theory. V1298 Tau provides the first direct observational proof that this process actually happens.

Over the next few billion years, these planets are expected to evolve into compact super-Earths and sub-Neptunes, matching the types of planets astronomers commonly detect around mature stars.

How Scientists “Weighed” Baby Planets

Measuring the mass of planets around young stars is notoriously difficult. Young stars are active, turbulent, and prone to stellar noise that interferes with traditional methods like radial velocity measurements. Instead, the team relied on transit timing variations, or TTVs.

Because the four planets orbit close to one another, their gravitational pulls cause slight changes in the timing of their transits. Sometimes a planet crosses its star earlier than expected; other times, it arrives late. By carefully tracking these variations over nearly a decade of observations, researchers were able to calculate the masses of all four planets.

This was a challenging task, especially for the outermost planet, whose orbit was initially uncertain. The team had only two observed transits separated by several years, leaving hundreds of possible orbital solutions. A fortunate ground-based observation eventually captured another transit, locking in the planet’s orbital period and making the mass calculations possible.

What This Means for Planet Formation

The V1298 Tau system represents a missing evolutionary link between newborn planets and the mature planetary systems astronomers see today. It shows that many super-Earths and sub-Neptunes likely begin life as much larger, gas-rich planets, then lose their atmospheres due to heat, radiation from their star, and internal cooling.

This process explains why planets between Earth and Neptune are so common and why they come in such a wide range of densities. Some lose most of their gas and become rocky super-Earths, while others retain enough atmosphere to remain sub-Neptunes.

The study also helps explain why our solar system is unusual. The absence of super-Earths around the Sun may reflect differences in early disk conditions, timing, or how quickly gas was removed during planet formation.

Why Young Planet Systems Are Rare

Finding systems like V1298 Tau is difficult because the infant phase of planets is short-lived. Planets spend only a few hundred million years in this transitional state, which is a blink of an eye on cosmic timescales. Catching four planets in the act of shrinking and shedding atmosphere is extraordinarily rare.

This makes V1298 Tau a crucial benchmark system. It allows scientists to test models of atmospheric loss, planetary cooling, and orbital evolution using real data instead of assumptions.

Expanding Our Understanding of Exoplanets

Beyond this specific system, the findings have broad implications for future exoplanet studies. They help refine models used to interpret data from missions like TESS and upcoming observatories such as the James Webb Space Telescope. Understanding how planet sizes and atmospheres evolve over time is essential for determining which planets might eventually become habitable.

By measuring planets at a critical moment in their development, astronomers can now trace a clearer path from birth to maturity, something that was impossible just a few years ago.

A Clearer Picture of Planetary Evolution

The discovery of these four baby planets offers a rare and convincing look at how the most common planets in the galaxy come to be. Instead of being born as super-Earths or sub-Neptunes, many planets appear to grow big, stay puffy, and then shrink into their final forms.

With V1298 Tau, astronomers finally have a system that connects theory with observation. It is a reminder that planetary systems, including our own, are shaped by complex and dynamic processes that unfold over billions of years.

Research paper: https://www.nature.com/articles/s41586-025-09840-z