Roger Blandford’s Reference Frame titled “Exploring the Universe” (Physics Today, April 2005, page 10) summarizes many of his concerns regarding NASA’s plans for astronomy and space science in the context of President Bush’s vision for space exploration. As Blandford notes, given the long list of ambitious space telescopes in NASA’s plans, clearly priorities need to be set, and some astronomers worry that “programs with a connection to life will be favored over fundamental investigations in the inanimate, physical sciences.” The president’s vision explicitly calls for NASA to “conduct advanced telescope searches for Earth-like planets and habitable environments around other stars” 1 and categorizes future NASA missions such as the Space Interferometry Mission and the Terrestrial Planet Finder as high-priority and life-oriented. Blandford states, “The discovery of extrasolar planets, 150 and counting, demonstrates that our solar system is unrepresentative with immediate consequences for the quest for extraterrestrial life.” This statement would seem to weaken the case for placing high priority on SIM and the TPF. However, Blandford draws the incorrect conclusion that the more than 160 current extrasolar planet candidates 2 imply that our solar system is unrepresentative and hence that the search for habitable planets may be extraordinarily difficult.
Finding an exact analogue of our planetary system is highly unlikely, given the chaotic processes involved in planet formation, yet scientists have no reason to believe that planetary systems similar to our own are not commonplace. The 160 known extrasolar planetary systems were nearly all discovered by Doppler spectroscopy and photometric transits, methods that strongly favor the detection of gas-giant planets with short-period orbits. Such planets are indeed likely to prevent the formation and stability of habitable, Earth-like planets, but they orbit only about 10% of nearby Sun-like stars. For the remaining 90% or so of such stars, the planetary census takers have not been collecting high-precision data long enough to detect Jupiter-like planets on the 12-year-period orbits that would herald the presence of exact solar-system analogues. In fact, the recent discovery of more than half a dozen super-Earths, planets in the range of 5 to 15 Earth masses, implies that Earth-mass planets are commonplace.
NASA’s Kepler mission, slated for launch around 2008, will determine the frequency of Earth-like planets through an exhaustive transit survey of 100 000 stars. 3
