Astronomers have discovered a new planet the size of Earth in the Milky Way, which does not orbit any host star
Polish astronomers from the OGLE team from the Astronomical Observatory of the University of Warsaw found the initial evidence for the existence of free-floating planets in the Milky Way. Writing in Astrophysical Journal Letters, OGLE astronomers announced the discovery of the smallest rogue planet found to date.
What is a free-floating or rogue planet?
Free-floating planets usually emit little to no radiation. They also don’t orbit a host star, so traditional methods fail when attempting to find them. However, they can be seen via gravitational microlensing. Microlensing results from Einstein’s theory of general relativity – a massive object (the lens) may bend the light of a bright background object (the source). The lens’ gravity acts as a huge magnifying glass which bends and magnifies the light of distant stars.
Gravitational microlensing does not depend on the lens’ brightness, so it enables the study of faint or dark objects such as planets. Duration of microlensing events depends on the mass of the lensing object – the less massive the lens, the shorter the microlensing event. Most of the observed events, which typically last several days, are caused by stars. Microlensing events attributed to free-floating planets have timescales of barely a few hours. By measuring the duration of a microlensing event (and shape of its light curve) scientists can estimate the mass of the lensing object.
“If a massive object (a star or a planet) passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source. The observer will measure a short brightening of the source star,” explains dr Przemek Mroz, a postdoctoral scholar at the California Institute of Technology and a lead author of the study.
“Chances of observing microlensing are extremely slim because three objects – source, lens, and observer – must be nearly perfectly aligned. If we observed only one source star, we would have to wait almost a million year to see the source being microlensed.”
On the new planet the size of Earth
OGLE astronomers provided the first evidence for a large population of rogue planets in the Milky Way a few years ago. However, the newly-detected planet is the smallest rogue world ever found. The scientists announced the discovery of the shortest-timescale microlensing event ever found, called OGLE-2016-BLG-1928, which has the timescale of just 42 minutes.
“When we first spotted this event, it was clear that it must have been caused by an extremely tiny object,” said Dr Radoslaw Poleski from the Astronomical Observatory of the University of Warsaw, a co-author of the study.
“Our discovery demonstrates that low-mass free-floating planets can be detected and characterized using ground-based telescopes,” says Prof. Andrzej Udalski, the PI of the OGLE project.
Astronomers suspect that free-floating planets actually formed in protoplanetary disks around stars (as “ordinary” planets) and they have been ejected from their parent planetary systems after gravitational interactions with other bodies, for example, with other planets in the system. Theories of planet formation predict that the ejected planets should be typically smaller than Earth. This means that studying free-floating planets enables us to understand the turbulent past of young planetary systems, such as our solar system.
The search for free-floating planets is one of the science drivers of the Nancy Grace Roman Space Telescope, which is currently being constructed by NASA. The observatory is scheduled to start operations in the mid-2020s.