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We examine the implications for the distribution of extrasolar planets basedon the null results from two of the largest direct imaging surveys published todate. Combining the measured contrast curves from 22 of the stars observed withthe VLT NACO adaptive optics system by Masciadri et al. (2005), and 48 of thestars observed with the VLT NACO SDI and MMT SDI devices by Biller et al.(2007) (for a total of 60 unique stars; the median star for our survey is a 30Myr K2 star at 25 pc), we consider what distributions of planet masses andsemi-major axes can be ruled out by these data, based on Monte Carlosimulations of planet populations. We can set this upper limit with 95%confidence: the fraction of stars with planets with semi-major axis from 20 to100 AU, and mass >4 M_Jup, is 20% or less. Also, with a distribution of planetmass of dN/dM ~ M^-1.16 between 0.5-13 M_Jup, we can rule out a power-lawdistribution for semi-major axis (dN/da ~ a^alpha) with index 0 and uppercut-off of 18 AU, and index -0.5 with an upper cut-off of 48 AU. For thedistribution suggested by Cumming et al. (2007), a power-law of index -0.61, wecan place an upper limit of 75 AU on the semi-major axis distribution. At the68% confidence level, these upper limits state that fewer than 8% of stars havea planet of mass >4 M_Jup between 20 and 100 AU, and a power-law distributionfor semi-major axis with index 0, -0.5, and -0.61 cannot have giant planetsbeyond 12, 23, and 29 AU, respectively. In general, we find that even nullresults from direct imaging surveys are very powerful in constraining thedistributions of giant planets (0.5-13 M_Jup) at large separations, but morework needs to be done to close the gap between planets that can be detected bydirect imaging, and those to which the radial velocity method is sensitive.

astrophysical journal/the astrophysical journal

Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT  SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at  Large Separations