A young progenitor for the most common planetary systems in the Galaxy

A lot of my work has been characterizing and understanding trends in big samples of planets. I tend to try to not get too attached to any particular system. But over the last seven years, I've grown to love the four planets orbiting V1298Tau — a young star in the constellation Taurus. They feel like dear friends to me. Let me tell you why.

Artist’s illustration of the V1298 Tau planetary system. The gaseous envelopes of the planets are being eroded away by the young star's intense radiation. R. Hurt, K. Miller (Caltech/IPAC).

One of the big questions in planetary science is how newborn stars — encircled by disks of gas and dust — turn into the mature planetary systems we've now detected by the thousands. Another mystery is why most stars like the Sun harbor a planet between the size of Earth and Neptune within the orbit of Mercury. These 'super-Earths' and 'sub-Neptunes' weigh between 1 and 20 Earth masses and are the most common type of planet known, yet our solar system lacks a single example.

One way to make progress would be to find planets around stars of different ages and study how their properties evolve with time, effectively making a movie of planet formation and evolution. Theory tells us that lots of interesting things should happen in the first 100 million years of a planet's life, and we should test these theories by finding and characterizing young planetary systems.

But a key challenge is that most planets we know about orbit mature stars that are ~1–10 billion years old. There are two main reasons for this. The first is that the Milky Way has been producing stars over billions of years, so if you just randomly sample a star in the Milky Way, there's a small chance it formed recently. The second is that young stars are so extremely spotty, active, and temperamental. Around young stars, our workhorse techniques of finding and characterizing planets (the Doppler and transit methods) are severely limited.

I was thrilled when my colleague Trevor David discovered a system of four planets transiting the 20 million-year-old Sun-like star, V1298 Tau. The Sun is about halfway through its 10-billion-year lifespan, so in human terms, V1298 Tau is like a ten-week-old infant. The plot below shows the discovery data from NASA's K2 mission and shows why it's so hard to find planets around young stars.

Figure adapted from David, Petigura, Luger et al. (2019).

The star itself changes in brightness by ~0.5% due to larger starspots rotating in and out of view and stellar flares (explosions on the star's surface). Amazingly, Trevor spotted by eye the transits of four separate planets. Each is between 5 and 10 times Earth’s size (Saturn is 9.4 times Earth’s size) and the inner three have orbital periods of 8, 12, or 24 days. We only caught one transit of planet 'e' in the K2 data, so we just had a lower limit on the period (more on this planet later).

Just the sizes and orbital periods of the planets are unusual. Most close-in planets have sizes between 1 and 3 times the size of the Earth, between the size of Earth and Neptune (4x Earth). Neptune-to-Saturn size planets are rare, and it was even more surprising to find four in the same system. Our hunch was these planets were extremely puffy because they were young, but were in the process of contracting into super-Earths and sub-Neptunes. But the most exciting thing was that the planets were near resonance, meaning the orbital periods followed a very specific pattern. The periods of planets d and c were in a near-perfect 3:2 ratio, and the periods of b and d were nearly 2:1. When planets are arranged like this, their gravitational tugs on one another add up. And this raised the prospect of measuring the planet masses by watching how they tugged on one another to see if they really were proto-super-Earths/sub-Neptunes.

So between 2019 and 2025 our team, led by John Livingston, embarked on a campaign to observe more transits of the planets in this system and measure their masses. Our findings were published in the journal Nature (Livingston, Petigura, David et al. 2026). The observations involved over forty collaborators from around the world who used a mix of ground- and space-based telescopes to try to catch more transits of these planets.

One particular challenge was planet e. How can you observe another transit if you don't know its period? We basically made an educated guess that planet e was also in a near-resonance with planet b (period = 24 days). A 2:1 period ratio seemed reasonable, and John proposed trying to catch it using a ground-based telescope. This seemed like a pretty wild idea to me. The transit lasts around 6 hours and from most sites, we would only have 6–8 hours to observe the star, depending on the weather. This was 6 hours out of 48 days, so if we were off by even ~1% of the orbit, we would miss it entirely. I figured we'd need to try half a dozen times just to rule out the 48 day period as an option. Just scheduling all these observations could take over a year. John went for it anyway... and got it on the first try! When John sent me and Trevor this Slack message, I nearly fell out of my chair.

Planet e is recovered!

Once we had assembled nine years of transit observations, we had enough data to constrain the masses of these planets. The plots below show the transit-timing variations (TTVs) of the four V1298 Tau planets. If the planets had perfectly regular periods, the points would fall on a flat line. Instead, we see the transits arrive either early or late as they tug on one another due to gravity.

Transit-timing variations for the V1298 Tau planets. Figure adapted from Livingston, Petigura, David et al. 2026.

We then "inverted" this dataset to constrain the planet masses and the shapes of their orbits. I used an analytic theory developed by David Nesvorný to show that masses were indeed low (less than Neptune). But extracting all the information in the dataset involved using an N-body code developed by Kento Masuda that leverages automatic differentiation, a tool central to the training of AI models. In the end, we found the planets had masses of between 5-15 Earth masses and densities similar to Styrofoam. Saturn is the least dense planet in our solar system and has a density around that of water.

What this means is that these planets really are the precursors to super-Earths and sub-Neptunes. They are still hot and puffy because they formed recently. A significant amount of their envelopes will be eroded away as the planets are bombarded with high-energy radiation from their young host star. The plot below shows the future evolution of these planets.

Red points show the current size and mass of the V1298 Tau planets. The shaded region shows where the masses and sizes of other known planets cluster in this diagram. Over time, much of the planetary envelopes will be eroded away. What is left will contract as it cools. Figure adapted from Livingston, Petigura, David et al. 2026.

After a billion years or so, these planets will have joined the population of super-Earths and sub-Neptunes that the universe produces in such abundance. With precise masses, radii, and ages, the planets provide valuable benchmarks for how planets evolve with time. 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. Or the Tiktaalik fossil, which revealed a transitional animal between fish and amphibians that lived 375 million years ago.

Tiktaalik roseae. Credit: Zina Deretsky, NSF.

What's next? For the V1298 Tau system, there is much more we can learn. I'm excited to see what molecules JWST can detect in the planets' atmospheres, which point to where and how they formed in their star's protoplanetary disk. More broadly, I'm looking forward to the prospect of finding more planets around stars of all ages to build a more complete picture of how planets form.