Our galaxy may be full of rogue planets that are not tied to any star by gravity. An international team of scientists led by Polish astronomers announced the discovery of the smallest such free-floating planet discovered to date. It turned out to be the size of the Earth. The results of the study are published by The Astrophysical Journal.
To date, more than 4,000 extrasolar planets have been discovered. While many of them do not look like exoplanets in our solar system, they all have one thing in common – they all orbit a star. However, theories of planetary formation and evolution predict the existence of free-floating planets that are not gravitationally associated with any star.
Indeed, a few years ago, Polish astronomers from the OGLE group at the Astronomical Observatory of Warsaw University provided the first evidence for the existence of such planets in the Milky Way. In an article in the Astrophysical Journal Letters, OGLE astronomers announced the discovery of the smallest rogue planet discovered to date.
Exoplanets are rarely directly observed. Astronomers usually find planets by observing the light from the planet’s host star. For example, if a planet crosses the disk of its parent star, then its observed brightness periodically drops by a small amount, causing so-called transits.
Free-floating planets emit virtually no radiation and – by definition – do not orbit a parent star, so they cannot be detected using traditional astrophysical detection methods. However, rogue planets can be detected using gravitational microlensing. Microlensing is the result of Einstein’s general theory of relativity. A massive object (lens) can deflect light from a bright background object (source). The lens’s gravity acts like a huge magnifying glass that bends and magnifies the light of distant stars.
This is why current research on gravitational microlensing phenomena is tracking hundreds of millions of stars in the center of the Milky Way. The OGLE survey conducted by astronomers at the University of Warsaw is conducting one such experiment. OGLE is one of the largest and longest-running surveys of the sky, begun over 28 years ago. OGLE astronomers are currently using the 1.3-meter Warsaw Telescope located at the Las Campanas Observatory, Chile. Every clear night, they point their telescope at the central regions of the galaxy and observe hundreds of millions of stars in search of those that change their brightness.
And today, scientists announced the discovery of the shortest microlensing event ever discovered, called OGLE-2016-BLG-1928, which takes only 42 minutes. “When we first noticed this event, it was clear that it must have been caused by an extremely tiny object,” explains Dr. Radoslaw Polesski of the Astronomical Observatory at Warsaw University, co-author of the study.
Several years ago, OGLE astronomers provided the first evidence of a large number of rogue planets in the Milky Way. However, the newly discovered planet is the smallest rogue ever discovered. “Our discovery demonstrates that low-mass, free-floating planets can be detected and characterized by ground-based telescopes,” says Professor Andrzej Udalski, OGLE project leader.
Astronomers suspect that free-floating planets actually formed in protoplanetary disks around stars (like “normal” planets) and were later ejected from their parent planetary systems after gravitational interactions with other bodies, such as other planets in the system. Planet formation theories predict that ejected planets should usually be smaller than Earth. Thus, the study of free-floating planets allows scientists to understand the turbulent past of young planetary systems, such as the solar system.