years ago the outer solar system was a lonely place. Pluto was a singular planet with an unexplainable orbit and an uncertain formation history. The discovery of the Kuiper belt in 1992, however, began to give us a true glimpse into the architecture of the outer solar system. The millions of objects, big and small, traveling with Pluto in the Kuiper belt record an early history of planet migration, collision, and ejection that was previously unknowable. In the next 25 years a similar revolution will take place. The discovery last year of Sedna in a distant eccentric orbit, well beyond the outer edge of the Kuiper belt, presages a vast, unexpected population beyond what was thought to be the edge of the solar system. We don’t know what this population will tell us, but these objects, isolated from the giant planets, might record a fossil record of even earlier and larger-scale events: the formation of the sun within a cluster of stars, the early presence of additional outer planets, the passage of nearby stars.

MICHAEL BROWN, PROFESSOR OF PLANETARY ASTRONOMY, CALTECH

is fun when a simple question such as

“How does the sun shine?” brings a deep answer. In the last quarter century, high-resolution observations from the ground and from space have greatly improved our understanding of the motions and the magnetic field in the sun, allowing us to find out in detail just what solar gases are doing. At the same time, lower-resolution observations have permitted us to find out what the inside of the sun is like and even to track sunspot groups when they are on the sun’s far side. The ability to make simultaneous observations of the corona from the ground at total eclipses and from space with ultraviolet, X-ray, and other telescopes has shown solar physicists not only what our star’s outer layers are like but also what is underlying the X-ray emission from sunlike stars. Further, studies of the sun during the recent transit of Venus may allow us to understand exoplanets and their parent stars better during exoplanet transits, soon to be observed by the dozen with new spacecraft.