Discovery of free–floating planets in the Galaxy

Astronomers conducting long term surveys of the Galactic center searching for the so called microlensing phenomena have discovered a new class of Jupiter–sized planets floating alone in the Galaxy. It is believed that these lone worlds were probably ejected from developing planetary systems.

The discovery results from a dedicated search by two teams of observers: the Japan–New Zealand Microlensing Observations in Astrophysics (MOA) collaboration and the Polish, University of Warsaw Astronomical Observatory based Optical Gravitational Lensing Experiment (OGLE) collaboration. Results of the study have been published in Nature, 473, pp. 349-352.

The discovery is based on the results of search for short–lived microlensing phenomena of the background stars from the Galactic center. The characteristic time scale of such phenomena, that can be precisely measured, is a function of the lensing object mass. In the case of regular stars microlensing lasts for several days – in the case of much less massive planets: 1–2 days for Jupiter–mass planet.

In the photometric data collected in the years 2006–2007 astronomers have found ten cases of short–lived microlensing events caused most likely by Jupiter–mass objects. Surprisingly, in all these cases there were no corresponding lensing signal to betray the presence of nearby stars. Thus, the discovered planets must be either free drifting across the interstellar space or planets on very wide orbits. However, other studies suggest that giant planets can rarely exist in such wide orbits.

Based on these discovery statistics it is estimated that big planets must be far more common than believed and in fact they must outnumber all the Galaxy’s normal stars by a factor of two.

The discovery of ten free–floating Jupiters supports the ejection scenario of planets from their original planetary systems where they were born. Theories predicted that some planets are ejected from their early, turbulent solar systems, due to close gravitational encounters with other planets or stars. Without a star to circle, these planets would move alone through the Galaxy as our sun and other stars do in stable orbits around the Galaxy’s center.

Present study is only the first step toward precise characterization of the free–floating planets. Much larger samples of these object are needed. Also more precise data could allow the discovery of less massive free floating planets. It is believed that the population of the latter should be much more numerous because the recent observations clearly show that small mass planets are more common than Jupiter–mass objects and such light bodies can be ejected from planetary systems more frequently.

Precise characterization of the free–floating planets will be possible in the time scale of a few years. In March 2010 the OGLE project started its fourth phase – the new generation sky survey – and it is currently discovering an order of magnitude larger number of microlensing phenomena than previous surveys. In the more distant time scale of a decade a space mission WFIRST is planned with planetary microlensing program as one of its main goals.

Discovery of free-floating planets in the Galaxy is one of the Science Magazine's 9 runners-up for Breakthrough of the Year 2011 [more]