Scientists have discovered a lonely orphan planet wandering the Milky Way without a parent star to guide it – a “rogue” planet, stuck in endless darkness with no days, nights, or gravitational siblings to keep it company.
It is possible that our galaxy is filled to the brim with these, but this one is particularly unusual for one particular reason: It is the smallest found to date – even smaller than Earth – with a mass similar to that of Mars.
Scientists have discovered more than 4,000 “extrasolar” planets, also known as, which are planets orbiting a star other than the sun. Many exoplanets, for example one where it – are nothing like the planets in our solar system, but they all have one thing in common: they all orbit around a star.
But just a few years ago, Polish astronomers found evidence of free-floating planets, not gravitationally attached to a star, in the Milky Way galaxy. In a new study, the same astronomers have now found the smallest such planet to date.
Exoplanets are difficult to spot, typically found only by observing the light from their host stars. Because the floating planets have no parent star and hardly emit any radiation, astronomers have to take a different approach to find them.
Rogue planets are spotted using gravitational microlenses, the result of Albert Einstein’s general theory of relativity. In this case, the planet’s gravity (lens) acts like a kind of magnifying glass, able to bend light from a bright star (source) behind it so that an observer on Earth can detect its presence.
“The observer will measure a brief brightening of the source star,” lead author Dr. Przemek Mroz, postdoctoral researcher at the California Institute of Technology, said Thursday. “The chances of observing the microlens are extremely slim because three objects – the source, the objective and the observer – must be almost perfectly aligned. If we were to observe a single source star, we would have to wait close to a million. years to see the source be micro-lensed. ”
Researchers on the lookout for these events monitor hundreds of millions of stars in the center of the galaxy, offering the greatest chance of microlensing.
The Optical Gravitational Lensing Experiment, led by astronomers from the University of Warsaw, is one of the largest and longest studies of the sky, operating for more than 28 years. Currently using a telescope at the Las Campanas Observatory in Chile, astronomers look to the center of the galaxy on a clear night, looking for changes in the brightness of the stars.
Because this technique relies only on the brightness of the source and not the lens, it allows astronomers to spot weak or dark objects, like rogue planets.
Measuring the duration of such an event, in addition to the shape of its light curve, can provide an estimate of the mass of the object that astronomers are looking for. While most observed events, caused by stars, last for several days, small planets only offer a window of a few hours.
In this case, OGLE-2016-BLG-1928, the shortest microlens event on record, only lasted 42 minutes. Based on the event, astronomers estimated that the planet has a mass similar to that of Mars and found it to be uncontrollable.
“When we first spotted this event, it was clear that it must have been caused by an extremely small object,” said co-author Dr Radoslaw Poleski of the Warsaw University Astronomical Observatory. . “If the lens were orbiting a star, we would detect its presence in the light curve of the event. We can rule out that the planet has a star about 8 astronomical units away (the astronomical unit is the distance between the Earth and the Sun). “
It’s not entirely clear why these rogue planets don’t have mother stars, but scientists don’t think the planets had a say in the matter. Rather, they may have originally formed as “ordinary” planets – only to be kicked out of their parent systems after gravitational interactions with other planets.
NASA is currently building the Nancy Grace Roman Space Telescope, which is expected to begin operations in the mid-2020s. Studying these floating planets can help astronomers better understand the unstable histories of young planetary systems, including our own solar system. .