A strange population of “free-floating” planets has been discovered, planets that may be alone in deep space, unfettered by any home star. The findings, which were published today in Monthly Notices of the Royal Astronomical Society, include four new finds that are compatible with planets of similar masses to Earth.
The study, led by Iain McDonald of the University of Manchester in the United Kingdom (now at the Open University in the United Kingdom), used data collected in 2016 during NASA’s Kepler Space Telescope’s K2 mission phase. During this two-month mission, Kepler observed a dense field of millions of stars around the galactic center every 30 minutes in the hopes of detecting rare gravitational microlensing occurrences.
The researchers discovered 27 potential microlensing signals with short durations ranging from an hour to ten days. Many of these have previously been detected in data collected from the ground at the same time. The four shortest occurrences, on the other hand, are new finds that are compatible with planets with masses close to Earth’s.
These new events do not have a longer signal that would be anticipated from a host star, indicating that they might be free-floating planets. Such planets might have developed around a host star before being expelled by the gravitational pull of the system’s other, heavier planets.
Microlensing shows how the light from a background star can be momentarily amplified by the presence of additional stars in the foreground, as predicted by Albert Einstein 85 years ago as a result of his General Theory of Relativity. This results in a brief burst of brilliance that can last anywhere from a few hours to a few days. Microlensing affects around one out of every million stars in our Galaxy at any one time, although only a small percentage of them are thought to be produced by planets.
Kepler was not built to discover planets via microlensing or to investigate the core Galaxy’s incredibly dense star fields. To hunt for signals inside the Kepler dataset, new data reduction techniques have to be created.
“These signals are extremely difficult to find. Our observations pointed an elderly, ailing telescope with blurred vision at once the most densely crowded parts of the sky, where there are already thousands of bright stars that vary in brightness and thousands of asteroids that skim across our field. From that cacophony, we try to extract tiny, characteristic brightenings caused by planets, and we only have one chance to see a signal before it’s gone. It’s about as simple as using a portable phone to look for a single flicker of a firefly in the midst of a motorway.”
Co-author Eamonn Kerins of the University of Manchester also comments, “Kepler has accomplished something it was never intended to do: it has added to the growing body of evidence indicating the presence of a population of Earth-mass, free-floating planets. It now hands the baton to subsequent missions that will be tasked with locating such signals, signals so elusive that Einstein himself believed they would never be discovered. I am very excited that the upcoming ESA Euclid mission could also join this effort as an additional science activity to its main mission.”
The NASA Nancy Grace Roman Space Telescope and potentially the ESA Euclid mission, both of which will be optimized to hunt for microlensing signals, will be focused on confirming the presence and nature of free-floating planets.
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