by Michael C. Liu
The Quest for Other Worlds:
The Gemini NICI Planet-finding Campaign The famous novelist George Eliot once wrote, “Men, like planets, have both a visible and an invisible history.” Today, this line could easily refer to one of the most exciting developments in contemporary astrophysics: the discovery and characterization of extrasolar planets (a.k.a. “exoplanets”), or planets in orbit around stars other than our Sun. Through a stream of innovations in search methods, our understanding of these objects has advanced in leaps and bounds over the last 14 years, starting with the unambiguous discovery of the first extrasolar planet around the Sun-like star 51 Pegasi in 1995, and up to the announcement of images of planets around the massive stars HR 8799 (see article page 44) and Fomalhaut in November 2008. Uncovering both the “invisible” and “visible” history of extrasolar planets has been a long-sought goal. This is true not only for modern-day astronomers, but for philosophers dating back to early civilizations who speculated on the possibility of life-bearing worlds other than our own. We now are in a rich and special time for such studies, as the number of known exoplanets is rapidly increasing along with our abilities to study their physical properties. We are learning how and why planets form, and ultimately whether our own solar system represents a common occurrence elsewhere. The current census of exoplanets now exceeds 300 objects, largely with masses comparable to the gas-giant planet Jupiter, which has a mass about 1/1000 that of the Sun or 300 times that of the Earth. Most exoplanets discovered to date have been identified using the radial velocity (RV) technique, which gathers very precise measurements of the motion of nearby stars (where “radial” means the motion along the line-of-sight from the observer on Earth to the star). Unseen exoplanets around these stars induce a periodic signal in the radial velocity as the planets orbit their host stars, leading to a characteristic increase and decrease in the star’s velocity relative to the observer. Given the very large number of exoplanet discoveries so far, one might naturally ask what remains to be learned. In fact, while radial velocity discoveries have led the way in exoplanet research, there is still much that remains unanswered.