Many more Jupiters than Stars

In this very exciting paper (http://arxiv.org/abs/1105.3544), based on microlensing experiments which monitor the sudden increase in the luminosity of the Galactic halo stars due to gravitational magnification by a foreground source, the authors have obtained the statistics of lens masses. The duration of the amplification even is ~ (M/M_J)^0.5 days, where M_J is the Jupiter mass. Thus, you need higher cadence  (shorter sampling of the stellar lightcurve) to see smaller mass lenses. The authors find an excess of events at ~ day timescale, corresponding to the Jupiter mass objects. They claim that these Jupiter mass objects are either free-floating, or are farther by >10 AU from the central star, and are more common than main sequence stars by a factor of ~ 2. Out of their 474 evens only 10 have an Einstein time (duration of magnification event) < 2 days; although their number is small the probability of lensing by smaller masses is also lower (proportional to the square of the lens mass), so their real abundance is larger.

The lensing statistics reproduce a good fit to the stellar mass function but requires a bump at the Jupiter mass. The argue that these Jupiter mass planets are kicked out from their stars by dynamical interactions. Another possibility is that  they are Jupiters formed at large distances from their stars, but this is less likely because such systems would have been detected very commonly in nearby directly-imaged planetary systems. With even better cadences they may be able to constraint the number of free-floating lower mass planets. Let wait and watch!