First author

Many stars are surrounded by a swirling mass of interstellar dust, known as a protoplanetary disk. Over millennia, the dust coagulates to form pebbles, then boulders and, eventually, planets. Thousands of such disks exist, but most are too far away for us to study the details and timing of planet formation. On page 194, William Herbst at Wesleyan University in Middletown, Connecticut, and his colleagues use the unique geometry of the system surrounding a star called KH 15D to observe the growth of grain-sized particles from interstellar dust — the first step in the evolution of protoplanetary material. Herbst talks to Nature about the project.

How did this star's discovery change your research?

Wesleyan students discovered the KH 15D star in 1995, using a small telescope, as part of a training project I started almost 30 years ago. Originally, we were looking for stars with odd behaviours. KH 15D was known as the winking star because it goes from normal brightness to almost undetectable during the course of a single day. The winking occurs because the KH 15D system contains not one but two stars that orbit one another in a 48-day cycle. The geometry of these stars' orbits caused the disk to be illuminated in a way that allowed us to see the particles in the disk.

Will this system continue to be productive?

Definitely. So far, we have only scratched the surface by analysing light that is reflected by the particles in the disk. We still have high-resolution spectroscopy data to explore that include light transmitted from the stars through the disk. Our work is complicated by the fact that the disk's orientation is changing at the moment, and will block the stars in future observations.

Will this compromise your work?

We won't be able to replicate this work. We don't have any way to see around the blockage, and we don't know how long we'll have to wait for conditions to change — it could be 30 years, or just until tomorrow. In the meantime, we hope to use different techniques to get further clues from the stars' reflections as the system shifts.

What was the biggest challenge?

It is very difficult for researchers, especially those at small institutions, to get time on the top telescopes around the world. With collaborators in Germany and Uzbekistan, we were able to continuously monitor the stars and gather preliminary data. This, in turn, allowed us to successfully compete for limited telescope time at the W. M. Keck Observatory in Hawaii and at the Chile-based Very Large and Magellan telescopes.