1. Astronomy Department, Wesleyan University, Middletown, Connecticut 06459, USA
2. Physics and Astronomy Department, Dickinson College, Carlisle, Pennsylvania 17013, USA
3. Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
4. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA
5. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
6. Ulugh Beg Astronomical Institute of the Uzbek Academy of Sciences, Astronomicheskaya 33, 700052 Tashkent, Uzbekistan

Correspondence to: William Herbst¹ Correspondence and requests for materials should be addressed to W.H. (Email: wherbst@wesleyan.edu).

Abstract

In the standard model of terrestrial planet formation, the first step in the process is for interstellar dust to coagulate within a protoplanetary disk surrounding a young star, forming large grains that settle towards the disk plane¹. Interstellar grains of typical size 0.1 m are expected to grow to millimetre- (sand), centimetre- (pebble) or even metre-sized (boulder) objects rather quickly². Unfortunately, such evolved disks are hard to observe because the ratio of surface area to volume of their constituents is small. We readily detect dust around young objects known as 'classical' T Tauri stars, but there is little or no evidence of it in the slightly more evolved 'weak-line' systems³. Here we report observations of a 3-Myr-old star, which show that grains have grown to about millimetre size or larger in the terrestrial zone (within 3 au) of this star. The fortuitous geometry of the KH 15D binary star system allows us to infer that, when both stars are occulted by the surrounding disk, it appears as a nearly edge-on ring illuminated by one of the central binary components. This work complements the study of terrestrial zones of younger disks that have been recently resolved by interferometry^{4, 5, 6}.

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