A small star with a planet companion appears to be on a high-speed trip around the center of the Milky Way and could be destined to escape the galaxy. If confirmed, the system would be the first “hypervelocity” star-and-exoplanet combo yet seen, raising questions about how a star could maintain its grip on a planet as they’re both booted from the galaxy’s core.
Known as MB11262, the system appears to consist of a red dwarf star about 20% the mass of the Sun and a super-Neptune 29 times the mass of Earth orbiting less than 1 astronomical unit (AU, the average Earth–Sun distance) from the star. The system is about 24,400 light-years from Earth, in the Milky Way’s central bulge about 1,500 light-years from its center.
“This is a great discovery—it’s really going to get people thinking about whether a hypervelocity star could have a planet.”
It’s zipping along at about 541 kilometers per hour, which is just slightly below the speed needed to escape the bulge and possibly the entire galaxy. Adding its yet-unmeasured radial motion toward or away from Earth could show that the system exceeds escape velocity, making it a hypervelocity system.
“This target is quite exotic,” said Sean Terry, an astrophysicist at the University of Maryland and NASA’s Goddard Space Flight Center and lead author of a recent analysis of the system published in the Astronomical Journal. “We’re pretty certain that this is consistent with a high-velocity, low-mass star with a planet, but not 100%.”
“This is a great discovery—it’s really going to get people thinking about whether a hypervelocity star could have a planet,” said Scott Kenyon, a senior astrophysicist at the Smithsonian Astrophysical Observatory who was not involved in the study.
Choosing the Right Model
The system was discovered in 2011 by Microlensing Observations in Astrophysics, an automated search for microlensing events, in which a star or planet passes between Earth and a background star. The gravity of the intervening object acts as a lens, making the background star appear much brighter. The length, intensity, and other parameters of the lensing event, along with observations made before and after, allow scientists to calculate the details of the magnifying object.
Other microlensing searches, alerted to the discovery, turned their sights to the lensing event, which lasted a few days. High-resolution follow-up observations with the Keck II Telescope in Hawaii in 2012 showed no evidence of the lensing object. At that time, the object had moved far enough away from the background star to no longer lens it, but not far enough away to be distinguished from it.
Analysis of the light showed that the lensing was caused by two bodies. Researchers developed two possible explanations for the system: a distant star and exoplanet, or a free-floating planet 3.2 times the mass of Jupiter and a moon roughly half the mass of Earth just 1,800 light-years away.
To test the two hypotheses, Terry and colleagues observed the area with the Keck I Telescope in 2021, 9 years after the last observation. If the planet-moon model was correct, they would see only the background star, they surmised. If the star-planet model was correct, the lensing star should have moved far enough from the background star for both to be seen.
The observations revealed a faint star where none had been seen before.
With the lensing star’s distance from Earth and the gap between it and the background star in hand, the team then calculated the system’s velocity—just below hypervelocity status.
Picking Up Speed
The system could have accelerated when it and a binary companion passed too close to the 4-million-solar-mass black hole in our galaxy’s core. “The gravitational interaction can cause one star to be captured by the black hole and one to be ejected at a high velocity,” said study coauthor Macy Huston, an astronomer at the University of California, Berkeley.
The boost could also have come from a random encounter with one of the many stellar-mass black holes thought to populate the Milky Way’s bulge, Terry said.
MB11262 could have achieved its speed through a series of less dramatic encounters with stars, too, Kenyon said. “This star could have been around forever, and orbited the center of the galaxy many times and had many [stellar] encounters,” he said. “Those perturbers wouldn’t have to be especially close. If a star passes, say, 100 or 200 AU away, it would give the system a small extra velocity.”
Keeping a Grip
Assuming the system is confirmed, Terry said, “we have one data point of a planet in a system going this fast,” and more examples are needed to begin to understand how these objects come to be. “Everything we can say now is highly speculative,” he said.
The Nancy Grace Roman Space Telescope, scheduled for launch in 2027, is expected to discover hundreds of microlensing events, including some produced by stars with planets and others by free-floating planets.
The researchers have scheduled follow-up observations of MB11262 with Keck in July. The extra 4 years since the last measurement should reveal whether the star has moved as predicted, confirming the star-plus-planet scenario. The observations won’t provide its 3D space velocity, though, which is needed to check its true speed.
—Damond Benningfield, Science Writer