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Sun-like stars have stellar, brown dwarf, and planetary companions. To help constrain their formation and migration scenarios, we analyze the close companions (orbital period <5 yr) of nearby Sun-like stars. By using the same sample to extract the relative numbers of stellar, brown dwarf, and planetary companions, we verify the existence of a very dry brown dwarf desert and describe it quantitatively. With decreasing mass, the companion mass function drops by almost 2 orders of magnitude from 1 M stellar companions to the brown dwarf desert and then rises by more than an order of magnitude from brown dwarfs to Jupiter-mass planets. The slopes of the planetary and stellar companion mass functions are of opposite sign and are incompatible at the 3 level, thus yielding a brown dwarf desert. The minimum number of companions per unit interval in log mass (the driest part of the desert) is at M ¼ 31 þ25 À18 M J . Approximately 16% of Sun-like stars have close (P < 5 yr) companions more massive than Jupiter: 11% AE 3% are stellar, <1% are brown dwarf, and 5% AE 2% are giant planets. The steep decline in the number of companions in the brown dwarf regime, compared to the initial mass function of individual stars and free-floating brown dwarfs, suggests either a different spectrum of gravitational fragmentation in the formation environment or post-formation migratory processes disinclined to leave brown dwarfs in close orbits.

We modeled the evolution of the Milky Way to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution and an environment free of life-extinguishing supernovae. We identified the Galactic habitable zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago. This GHZ yields an age distribution for the complex life that may inhabit our Galaxy. We found that 75% of the stars in the GHZ are older than the Sun.As we learn more about the Milky Way Galaxy, extrasolar planets and the evolution of life on Earth, qualitative discussions of the prerequisites for life in a Galactic context can become more quantitative (1-3). The Galactic habitable zone (GHZ) (4), analogous to the concept of the circumstellar habitable zone (5), is an annular region lying in the plane of the Galactic disk possessing the heavy elements necessary to form terrestrial planets and a sufficiently clement environment over several billion years to allow the biological evolution of complex multicellular life. In order to more quantitatively estimate the position, size and time evolution of the GHZ, we combined an updated model of the evolution of the Galaxy (6) with metallicity constraints derived from extrasolar planet data (7).Of the factors that determine the location of the GHZ, the abundance of elements heavier than hydrogen and helium (metallicity) is particularly crucial because these 1