Astronomers have announced the discovery of smallest gravitation-free planet of a star, the size of the Earth, found to date.
So far they have been discovered more than four thousand extrasolar planets. Although many of the known ones do not look like those in our solar system, they have one thing in common: they all orbit a star. However, theories of planet formation and evolution predict the existence of free-floating (solitary) planets, gravitationally not attached to any stars.
In fact, a few years ago, Polish astronomers from the Optical Gravitational Lens Experiment (OGLE) team at the Astronomical Observatory of the University of Warsaw, provided the first evidence for the existence of such planets in the Milky Way. Now they have announced the discovery of the smallest solitary planet found to date. They publish results in ‘Astrophysical Journal Letters’.
Exoplanets can only be observed directly on rare occasions. Astronomers generally find planets by observing the light from the planet’s host star. For example, if a planet crosses in front of the disk of its parent star, the star’s observed brightness periodically drops by a small amount, causing so-called transits. They can also measure the motion of the star caused by the planet.
Free-floating planets emit virtually no radiation, and by definition do not orbit any host stars, so they cannot be discovered using traditional astrophysical detection methods. However, planets can be detected by an astronomical phenomenon called gravitational microlensing.
The microlensing is the result of Einstein’s theory of general relativity: A massive object (the lens) can deflect light from a bright background object (the source). The gravity of the lens acts like a huge magnifying glass that bends and magnifies the light from distant stars.
“If a massive object (a star or planet) passes between a terrestrial observer and a distant source star, its gravity can deflect and focus the light from the source. The observer will measure a brief brightness of the source star“Dr. Przemek Mroz, a postdoctoral researcher at the California Institute of Technology and lead author of the study, explains in a statement.
“The chances of observing microlenses are extremely slim because three objects (source, lens, and observer) should be almost perfectly aligned. If we were to observe a single source star, we would have to wait almost a million years to see the microlensing source, “he adds.
This is why modern studies looking for gravitational microlensing events are monitoring hundreds of millions of stars in the center of the Milky Way, where the chances of microlenses are highest.
The OGLE study, led by astronomers from the University of Warsaw, carries out one such experiment. OGLE is one of the largest and longest sky studios, it started operating more than 28 years ago. Astronomers are currently using a 1.3-meter Warsaw telescope located at the Las Campanas Observatory in Chile. Every clear night they point their telescope at the central regions of the galaxy and observe hundreds of millions of stars., looking for those that change their brightness.
Gravitational microlensing does not depend on the brightness of the lens, so it allows the study of dim or dark objects such as planets. The duration of microlensing events depends on the mass of the lens object: the less massive the lens, the shorter the microlensing event. Most of the observed events, which usually last several days, are caused by stars.
Microlensing events attributed to free-floating planets have timescales of just a few hours. By measuring the duration of a microlensing event (and the shape of its light curve) we can estimate the mass of the lens object.
Scientists announced the discovery of the smallest timescale microlensing event ever found, called OGLE-2016-BLG-1928, which has a timescale of just 42 minutes. “When we first saw this event, it was clear that it must have been caused by an extremely small object,” explains Dr. Radoslaw Poleski from the Astronomical Observatory of the University of Warsaw, a co-author of the study.
In fact, models of the event indicate that the lens must have been less massive than Earth, probably a mass object from Mars. Furthermore, it is likely that the lens is a rogue planet. “If the lens were orbiting a star we would detect its presence in the light curve of the event
Or, ” Dr. Poleski adds. We can rule out that the planet has a star within about 8 astronomical units (the astronomical unit is the distance between the Earth and the Sun) “.
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 ever found among the so-called loners.
“Our discovery shows that low-mass floating planets can be detected and characterized using ground-based telescopes “, says Professor Andrzej Udalski, principal investigator of the OGLE project.
Astronomers suspect that the free-floating planets actually formed in protoplanetary disks around stars (like “ordinary” planets) and they have been expelled from their parent planetary systems after gravitational interactions with other bodies, for example with other planets in the system.
Planet formation theories predict that ejected planets should typically be smaller than Earth. Therefore, the study of free-floating planets allows us to understand the turbulent past of young planetary systems, like our solar system.
The search for free-floating planets is one of the scientific engines of the Roman Nancy Grace Space Telescope, which is currently being built by NASA. The observatory is scheduled to begin operating in the mid-2020s.
Due to the brevity of the event, additional observations collected by the Korea Microlens Telescope Network (KMTNet) were needed to characterize the event. KMTNet operates a network of three telescopes: in Chile, Australia and South Africa.