Astronomers have spotted an extraordinary cosmic phenomenon—a tiny star racing through the Milky Way at an astonishing 1.2 million miles per hour (1.9 million km/h). Even more remarkably, this star appears to be dragging along a Neptune-like planet, making it the fastest-known planetary system ever discovered.
The discovery, made by NASA scientists, was first hinted at in data collected back in 2011 using a technique called gravitational microlensing. If confirmed, this will be the first exoplanet ever observed orbiting a hypervelocity star.
“We think this is 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,” said Sean Terry, a researcher at NASA’s Goddard Space Flight Center and lead author of the study.
What Is Gravitational Microlensing?
Unlike traditional methods of exoplanet detection, which rely on visible light, gravitational microlensing exploits a quirk of Einstein’s general relativity. When a celestial body passes in front of a background star, its gravity bends and magnifies the starlight, revealing details about otherwise invisible objects.
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Back in 2011, astronomers detected a microlensing signal suggesting the presence of two cosmic bodies. While they could determine that one was roughly 2,300 times more massive than the other, they lacked enough data to confirm what these objects actually were.
Now, a decade later, a team has identified a fast-moving star around 24,000 light-years away, near the densely packed central bulge of the Milky Way. Given its position relative to the 2011 signal, this star is a strong candidate for the mysterious object detected back then.
A Star and a Planet or a Rogue Planet and a Moon?
While the leading theory suggests a small star dragging a super-Neptune, astronomers cannot yet rule out an alternative explanation: that the detected pair is actually a rogue planet four times the mass of Jupiter, accompanied by a massive exomoon.
To resolve the mystery, NASA scientists turned to the Keck Observatory in Hawaii and the European Space Agency’s Gaia space telescope. Their goal was to track the motion of the suspected star over time.
“If high-resolution observations show that the star just stays in the same position, then we can tell for sure that it is not part of the system that caused the signal,” explained Aparna Bhattacharya, a researcher at the University of Maryland and NASA Goddard. “That would mean the rogue planet and exomoon model is favored.”
Could This System Escape the Milky Way?
The newly discovered system is already moving through the galaxy at twice the speed of our solar system. However, its true velocity might be even higher. Scientists have only measured its speed in two dimensions—if it is also moving toward or away from Earth, its actual velocity could exceed 1.3 million mph (2.1 million km/h).
At that speed, the star and its planet (if it is indeed a planetary system) could be on the verge of escaping the Milky Way entirely. This would make it an intergalactic traveler, doomed to roam the void between galaxies. However, such an event would take millions of years.
Next Steps in the Investigation
To confirm whether this is a runaway star with a planetary companion, scientists will continue monitoring its position. If the suspect star moves exactly as expected, it will solidify the theory that it is indeed the fast-moving object detected in 2011.
The upcoming Nancy Grace Roman Space Telescope, set to launch in 2027, will play a crucial role in finding more hypervelocity stars and their possible planets. Unlike current methods, Roman will have the capability to detect these fast-moving systems in a single survey, providing a much clearer picture of how such objects are ejected at such extreme speeds.
“In this case, we used MOA for its broad field of view and then followed up with Keck and Gaia for their sharper resolution,” said Terry. “But thanks to Roman’s powerful view and planned survey strategy, we won’t need to rely on additional telescopes. Roman will do it all.”
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If confirmed, this discovery would mark a major milestone in planetary science. Not only would it be the first exoplanet ever detected around a hypervelocity star, but it could also provide crucial insights into how such stars get launched at incredible speeds. For now, astronomers are watching closely to determine the fate of this star and its potential planetary companion. Whether it remains within the Milky Way or eventually drifts into intergalactic space, one thing is certain—it’s a cosmic speedster unlike anything ever seen before.