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In the central bulge of the Milky Way, about 24 light-years from Earth, a strange pair of objects appear to be hurtling through space at breakneck speed. A new study reports that data suggests the objects are a high-velocity star and its companion exoplanet. If confirmed, it would set a new record for the fastest-moving exoplanet system known to science. Stars travel throughout the Milky Way, typically at speeds of several hundred thousand miles per hour. Our solar system's average speed through the galaxy's Orion Arm is 000 miles per hour, or 450 kilometers per second. These two objects are spinning twice as fast, at at least 000 million miles per hour (200 kilometers per second).
“We think it’s a so-called super-Neptune world orbiting a low-mass star at a distance that would lie between the orbits of Venus and Earth if it were in our solar system,” says astronomer Sean Terry of the University of Maryland and NASA’s Goddard Space Flight Center. “If so, it would be the first planet ever found orbiting a hypervelocity star.”
The two objects were first discovered in 2011 when researchers searched for exoplanets using data from the Microlensing Observations in Astrophysics (MOA), a project based at Canterbury's Mount John Observatory in New Zealand. Gravitational microlensing is a phenomenon that occurs when a massive celestial object is close to the line of sight between a distant observer on one side and a distant star on the other.
Because mass distorts space-time, the star's light is bent as it passes through the object's warped space-time on its way to an observer. If all three points are close enough, the curved space-time around the central object acts as a lens for the observer, magnifying the starlight. Researchers studying the MOA data in 2011 determined the relative masses of the objects—one is 2300 times more massive than the other—but the actual masses of both remained unclear.
"It's easy to figure out the mass ratio," says astronomer David Bennett of the University of Maryland and NASA Goddard, who worked on the 2011 and 2025 studies. "It's much harder to calculate their actual mass."
To find the actual mass of an object, you need to know its distance, much like how moving a magnifying glass closer and further away distorts the apparent size of objects without changing the differences between them. In 2011, Bennett and his colleagues proposed two scenarios for the pair of objects: either it is a star and a planet, where the star is slightly smaller than our Sun and the planet is 29 times more massive than Earth, or it is a less distant super-Jupiter towing a moon smaller than Earth.
For the new study, researchers set out to find out what these two are and what they’re collecting more than a decade later, using data from the Keck Observatory in Hawaii and the European Space Agency’s Gaia satellite. They settled on a star system about 24 light-years from Earth as the most likely candidate. It’s the bright, densely populated central bulge of stars in the Milky Way, the galactic center of the city to our distant suburbs.
Based on the distance from the 2011 signal, the team calculated the star's speed, finding that it was moving at more than twice the speed of our Sun. However, this only accounts for its two-dimensional motion as seen from Earth. It could also be moving towards us or away from us, which is harder to detect from our location, but it means it's moving even faster.
This suggests that this star may be fast enough to exceed the Milky Way's escape velocity, which is thought to be around 550-600 kilometers per second. If so, it is heading for intergalactic space - although not for millions of years, as the Milky Way is huge and still almost in the middle. While this solar system fits the profile of the 2011 objects, only time will tell.
"To be sure that the newly identified star is part of the system that caused the signal in 2011, we would like to look again in a year and see if it is moving at the right magnitude and in the right direction," says Bennett.
If the star simply remains stationary, then we will know that it is not contributing to the system that is creating the signal.
“This would suggest that the rogue planet and exomoon model is superior,” explains astrophysicist Aparna Bhattacharya of the University of Maryland and NASA Goddard. The study was published in The Astronomical Journal .
