Sub-Neptunes in Pasadena

We just wrapped up a wonderful week in California at the Silver Jubilee of the Summer Sagan School. The school explored exoplanet demographics with many exciting lectures and several hands-on activities and group projects. Many of the lectures and much of the discussion focused on sub-neptunes, an exciting, new type of planets that is absent in the Solar System but very common around other planets. The school was organized by IPAC and NEXSci and held at the beautiful Caltech campus in Pasadena.

Daniel Apai at the Sagan Summer School in the Caltech campus.

The Sagan Summer Schools provide excellent introduction to the fundamentals of exoplanet science as while also reviewing cutting-edge results. It has been a pleasure to participate and give an invited lecture in the series that has been running for an incredible 25 years!

My lecture focused on the future sub-neptune studies. This was an exciting topic to cover – I always enjoyed thinking about what the future can bring and how we can make sure that we make the most of the scientific opportunities!

Watch my lecture on the Sagan Summer School YouTube Channel!

The realization that a distinct population of exoplanets exists – and even common – that is absent from the Solar System emerged first gradually, with the discoveries of an increasing number of small planets from NASA’s Kepler mission. It became clear that there is a large number of planets with sizes larger than Earth but smaller than Neptune – a size range that is empty in our own planetary system. While Kepler provided radius measurements for these transiting planets, the measurements were always relative (compared to the host star) and carried uncertainties. Through a multi-year effort, a team in California (the California-Kepler Survey) used the powerful Keck telescopes to carefully characterize the host stars of the planets. These studies enabled refining the stellar radii which, in turn, improved the precision on the radii of their planets.

These results (published in Fulton et al. 2017) led to a surprising discovery: Planets in the size range between Earth and Neptune seemed to fall into two categories: Lower-density, larger planets and higher-density, smaller planets. The two groups are separated by a very clear boundary in size, a range in which very few planets fall.

The nature, interior structure, and formation process of sub-neptunes, and their connections to super-Earths remain exciting questions at the forefront of exoplanet research. Slide from Daniel Apai’s Sagan Summer School 2025 presentation.

The distribution strongly suggests that there is a population of rocky worlds larger than Earth (often called super-Earths) and a population of planets larger than super-Earts but smaller than Neptune (often called sub-neptunes). While Super-Earths may in many ways be larger versions of Earth, it is likely that many sub-neptunes are neither a miniature version of Neptune nor a larger versions of super-Earths. They may have very different interior structures and, perhaps, origins.

In my Sagan presentation, I argued that the key to understanding the nature and origins of sub-neptunes is to understand the processes that shape them – and to do so, we can combine information and knowledge from a range of planets.

A slowly increasing number of sub-neptunes is being characterized by the astronomical community – mostly through expensive James Webb Space Telescope observations – which revealed diversity in their atmospheric composition. In one case, my colleague Dr. Nikku Madhusudhan (Cambridge University) reported potential signatures of life in K2-18b, one of the coolest studied sub-neptune. (I wrote about this interesting but controversial claim here.)

With the existing data, we are far from having a clear picture of the nature and origins of sub-neptunes. In my talk at the Sagan Summer School, I offered a path forward: Rather than trying to fully understand individual planets in isolation (probably not possible), we should focus on understanding the processes that shape sub-neptunes. Processes act on many planets and thus shape populations. By characterizing population-level impact of processes such as atmospheric loss, we can verify our models of those processes, narrow down the potential outcomes of those processes, and provide a firm footing for understanding the individual planets. This approach uses all the data that is available on sub-neptunes — but also benefits from all the data available on smaller and larger planets.

My presentation included a brief discussion of the key processes that are candidates for shaping the sub-neptune population and proposed approaches for recognizing and testing their impact on the exoplanet demographics.

The sub-neptune population is shaped by processes (left column). I argued that we should focus on understanding the processes as the path to understand sub-neptunes. Most processes should have observable fingerprints in the exoplanet demographics, i.e., the processes often lead to predictable correlations between observables.

The school provided a great venue for hundreds of early career scientists and some of us, working on this field for two decades, to get together and explore results and ideas about exoplanet demographics and sub-neptunes. It was exciting to see the different groups bringing separate, sometimes surprising puzzle pieces and try to figure out how they fit together!