NASA scientists have detected a star and trailing exoplanet that may be sailing through the Milky Way with unprecedented speed. The star-and-planet duo, if confirmed, would set a record for the fastest-known exoplanet system – moving, by one estimate, at a blistering 1.2 million miles per hour, nearly double our solar system’s speed.

Exoplanet goes whoosh!

The system burst onto astronomers’ radar in 2011 when the Microlensing Observations in Astrophysics (MOA) project first spotted light signatures that hinted at two objects crossing paths with a background star’s light. Now, after analyzing archival data and turning to the telescopes of Keck Observatory in Hawaii and ESA’s Gaia satellite, a team led of astronomers believes they’ve pinned down the system’s true identity.

The team is led by Sean Terry, a postdoctoral researcher at the University of Maryland, College Park and NASA’s Goddard Space Flight Center, and the study was published in The Astronomical Journal.

“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,” Terry said. “If so, it will be the first planet ever found orbiting a hypervelocity star.”

A super-Neptune is essentially a planet larger and more massive than our familiar Neptune, but smaller than a gas giant like Jupiter.

It wasn’t even clear this is possible

Hypervelocity stars, so-called for their extraordinary speeds, are believed to be propelled by powerful gravitational interactions in the galactic center or potentially ejected from stellar collisions.

Researchers have spotted several such stars before, but it was unclear what happens to their planets. Do they just come along for the ride, bracing extreme speeds and chaotic origins, or are they catapulted someplace else? If the research team is correct, it suggest that in some cases, the planet can come along for the ride.

At the heart of this discovery is microlensing, a curious quirk of Einstein’s theory of general relativity. When an object passes in front of a background star, its gravity warps space-time, acting like a natural magnifying lens and brightening the star’s light from our perspective. This brightening betrays the presence of the intervening object. This is what allowed researchers to characterize the fast objects.

“Determining the mass ratio is easy,” said David Bennett, a senior research scientist at the University of Maryland, College Park and Goddard, who co-authored the new paper and led the original study in 2011. “It’s much more difficult to calculate their actual masses.”

The 2011 discovery team suspected the microlensed objects were either a star about 20% as massive as our Sun and a planet roughly 29 times heavier than Earth, or a nearer “rogue” planet about four times Jupiter’s mass with a moon smaller than Earth. Follow-up observations have tipped the scale in favor of the star-planet combination, especially after the researchers spotted what could well be the star in question.

What we know about this system

Measurements suggest it lies some 24,000 light-years away, nestled in the Milky Way’s densely populated galactic bulge, and streaking through the cosmos at a minimum of 1.2 million miles per hour. However, in their paper, the authors state that if it’s also moving toward or away from us, it must be moving even faster. Its true speed may even be high enough to exceed the galaxy’s escape velocity of 1.2 million miles per hour.

But uncertainties remain.

“To be certain the newly identified star is part of the system that caused the 2011 signal, we’d like to look again in another year and see if it moves the right amount and in the right direction to confirm it came from the point where we detected the signal,” Bennett said.

If the star does not move as expected, then the data may favor the “rogue planet” scenario.

More definitive answers may soon arrive with NASA’s upcoming Nancy Grace Roman Space Telescope.

“In this case we used MOA for its broad field of view and then followed up with Keck and Gaia for their sharper resolution, but thanks to Roman’s powerful view and planned survey strategy, we won’t need to rely on additional telescopes,” Terry said. “Roman will do it all.”