Planetary science is a broad field that covers every object in the solar system, except the Sun itself, and all of the processes that have shaped and reshaped these bodies for billions of years. So, as an editorial board we had to ask ourselves: as a scientific journal what would be useful to the planetary science community if we want to focus some attention on fundamental questions and discoveries in the field? Ultimately, our decision was to solicit a number of “review” papers that capture some of the most notable topics and discoveries of the past 25 years.

A review paper differs from a typical scientific article in that it focuses on collecting knowledge from many more typical papers and describing and assessing what that collective wisdom tells us about some “big picture” issue. Review papers are valuable because they condense or distill understanding of a scientific problem to its essential parts yet also direct readers to the relevant regular papers for more details. Review papers are a great way to catch up on, or start learning about, a particular scientific problem.

Planetary science has been on a steady diet of exploration and study the last 25 years and covering all of that in a single journal issue just isn’t practical. We made conscious choices to select topics from a range of bodies (from tiny chondrules to giant planets like Jupiter), and parts of planetary bodies (deep interiors to atmospheres), and the disciplines and techniques used to study the solar system. In the end, sixteen different topics were selected and authors were invited to write on these topics.

Identifying potential lead authors for the articles followed a similar pattern as for the topics themselves. Our goals were to identify knowledgeable scientists with appropriate expertise who were clear writers. We also aimed to include a variety of voices and approaches to the questions we posed. Review papers are often written by scientists with many years of experience. Yet in a period of expanding exploration of the solar system, the voices of scientists early in their careers provide a potentially different glimpse of what questions and approaches will also be important in the future. We also made sure to develop a list of potential authors that had a significant representation of women with the final list of authors influenced by who accepted our invitation to add writing a review paper to their workload this year. Ultimately, we have a list of authors spanning from early career scientists to some of the most experienced researchers in the field, from several countries, and six of the sixteen papers have women as first authors.

Among the enduring topics in planetary science are questions about the origin of the planets. We have three reviews on origins. The first is a discussion of competing ideas for the origin of chondrules (Connolly and Jones, 2016). Chondrules are small, spherical rocks that in the most common meteorite class: chondrites. They may have been one of the most abundant components of the early solar system that wasn’t gas – but we still don’t completely understand how or why they formed. Understanding how planets are assembled continues to be fundamentally challenging (Morbidelli and Raymond, 2016).

Figuring out how objects the size of chondrules eventually get put together into planets is an intense area of study and a lot of progress has been made in the past 25 years. While recent reviews of that work already exist, this paper takes a look forward and what questions remain. There are several, from understanding how the first large bodies are built to how our solar system ended up with its particular arrangement of planets. Then there is the origin of the Moon (Barr, 2016), which continues to fascinate us. While the idea of a giant impact between an approximately Mars-sized object with the Earth is the leading idea, the details necessary to explain the properties of the Moon have been the focus of study for decades.