An article published in the journal “Monthly Notices of the Royal Astronomical Society” reports the identification of candidate gravitational microlens events that could be the traces of free-floating planets, which are planets that do not orbit any star. A team of researchers led by Iain McDonald used data obtained in 2016 during NASA’s Kepler Space Telescope K2 mission while monitoring a star-filled area near the center of the Milky Way. The result is the discovery of 27 signals generated by possible gravitational microlenses which lasted between one hour and 10 days. The four shortest events are consistent with Earth-sized planets.
Free-floating planets are one of the frontiers of astronomical research. They may have been born in orbit around a star and subsequently ejected from their system, in which case they’re also called rogue planets or orphan planets, or they may have formed from an amount of materials insufficient to form a star, in which case the definition of a planet may be considered incorrect.
The definitions are still under discussion and for example, the term planemo, from “planetary mass object”, has been proposed to include both planets orbiting a star and free-floating planets. Another proposal concerns the planemos that formed from a small cloud of materials that defines them as sub-brown dwarfs.
Several possible free-floating planets have been discovered in recent years but their exact nature is difficult to understand. For example, CFBDSIR 2149-0403 was discovered during a search for brown dwarfs and there are quite different estimates for its distance from Earth and mass, a vagueness that makes it difficult to determine whether it’s a rogue planet or a sub-brown dwarf.
Finding more free-floating planets will help improve their study, and to discover them, Iain McDonald’s team searched for their traces in the form of gravitational microlenses. These are the tiny distortions created in space by planets’ gravity that are detected by changes in the trajectories of the electromagnetic emissions that reach the Earth passing close to one of those planets.
The mission of the Kepler space telescope ended in October 2018 but the data collected is available and will be useful for many years to come. In this case, it,s the data collected in 2016 during observations of an area full of stars near the center of the Milky Way. 27 signals generated by possible gravitational microlenses were discovered which lasted between one hour and 10 days. Analysis of the data indicates that the four shortest events are consistent with Earth-sized planets.
The Kepler space telescope was not designed for this type of research and this also complicates the attempts to confirm the events identified in the archive data. However, the results indicate that it’s possible to search for gravitational microlens effects and then conduct follow-up observations in the areas where they have been identified. This is also useful in preparation for upcoming exoplanet-hunting missions with telescopes also designed to search for gravitational microlenses. Discovering free-floating planets and their origin will help to better understand the processes of formation and evolution of planets and planetary systems.