In a new blind survey of gravitational microlensing, an international team of astronomers has detected likely evidence for four Earth-sized planets wandering freely through interstellar space. Using observations from the aging Kepler Space Telescope, researchers led by Iain McDonald at the University of Manchester picked out key signs of microlensing by the planets in a crowded and noisy field of stars. Their success in the face of challenging circumstances clearly demonstrates the feasibility of blind, space-based microlensing surveys in future missions.
In some star systems, astronomers predict that the strong gravitational tug of large planets could have thrown their smaller planetary neighbours out into interstellar space. Without any host star, these roughly Earth-sized “free-floating planets” (FFPs) would be virtually impossible to detect using conventional exoplanet searching techniques – but should be detectable through the effect of gravitational microlensing.
First predicted by Einstein as part of his theory of general relativity, this effect occurs when a massive object passes in front of a more distant star in our line of sight. As the object’s gravitational field bends more of the star’s light towards us, it causes a brief burst in its observed brightness. In the case of Earth-sized FFPs, these bursts would be extremely faint, and last for little more than an hour.
To search for these signals, McDonald’s team analysed observations taken by NASA’s Kepler Space Telescope as part of its later K2 mission. Between April and July 2016, the telescope surveyed a region of densely packed stars, close to the centre of the Milky Way. Since Kepler was designed to detect exoplanets transiting their host stars, and was also reaching the end of its lifetime, it was far from optimized for the task of microlensing detection. In addition, many stars in the crowded field had far more variable brightness than the subtle signals associated with microlensing.
Despite these challenging factors, the latest advances in photometric techniques allowed McDonald’s team to identify 27 short-duration microlensing candidate signals in a blind survey of Kepler’s data. Out of these, four events were not only entirely new, they also had durations lasting less than 0.1 days, and weren’t accompanied by any stronger signal, which might indicate a host star. Both of these factors were key attributes of microlensing by an Earth-sized FFP.
Unbound planets could abound in the universe
With such a poorly-optimized setup, the success of the team’s results provide reassurance that future missions, which are actually intended to search for microlensing signals, will be a resounding success. McDonald and colleagues will now await the first observations of two upcoming missions: the ESA’s Euclid mission, and NASA’s Nancy Grace Roman Space Telescope – now scheduled for launch in 2022 and 2025, respectively. Through these observations, astronomers could finally determine how common these wandering interstellar exoplanets really are, and shed new light on their turbulent origins.
The research is described in Monthly Notices of the Royal Astronomical Society.