Astronomers have identified more than 4,000 exoplanets orbiting other stars but just a few “rogue planets” wandering the galaxy without a star to call home. A new study claims to have spotted one of these worlds, and it may be a small, rocky world like Earth. If confirmed, the planet known as OGLE-2016-BLG-1928 would be a major milestone in our efforts to spot these unattached worlds.
While scientists believe rogue planets are common throughout the universe, they’re very difficult to find. We currently lack the technology to directly image exoplanets in most instances, so we can only locate them by observing the stars they orbit. The dearly departed Kepler Space Telescope single-handedly detected more than 2,500 exoplanets, and that number continues to rise as scientists analyze its data. Kepler used the transit method, which involves watching stars for dips in brightness as a planet passes in front of them. Scientists have also used radial velocity measurements of stars to look for small wobbles caused by the mass of planets.
Without a host star, spotting planets gets a lot harder. The Optical Gravitational Lensing Experiment (OGLE) project found the potential rogue planet using gravitational microlensing, which superficially similar to the transit method. This approach monitors the light from a distant star in hopes a massive object like a planet will pass in front of it. While the star and planet may be many light-years away, the planet bends or “lenses” the star’s light from our perspective on Earth. This can reveal the foreground object’s mass and size, but only if you happen to be looking in the right place at the right time.
This light curve indicates a massive object passed in front of the star.
Andrzej Udalski of the OGLE project notes that you could watch a single star for a million years and only see a single lensing event. Luckily, Udalski and his team didn’t have to go one star at a time. They used the Las Campanas Observatory in Chile, which scans millions of stars in the direction of the galactic center on a daily basis. In analyzing this data, the OGLE team spotted a lensing event dubbed OGLE-2016-BLG-1928. At just 42 minutes long, it’s the shortest such detection ever recorded. That suggests the planet, if indeed that’s what it is, would be somewhere between the size of Earth and Mars.
The team believes this object is a rogue planet because there are no known stars to which it could be connected. The data also showed no light sources within eight astronomical units of the lensing event. Other researchers will need to confirm this object is a planet before it goes in the history books, but if current theories are right, there are uncountable millions of similar objects out there just waiting to be discovered.
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