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Astronomers may have discovered a skinny star hurtling through the middle of our galaxy with a planet in tow. If confirmed, the pair would set a new record for the fastest exoplanet system, nearly double the speed of our solar system through the Milky Way. The planetary system is thought to be moving at at least 1,2 million miles per hour, or 540 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,” said Sean Terry, a doctoral student at the University of Maryland, College Park, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Because the star is so faint, it is far outside its habitable zone. “If so, this would be the first planet ever found orbiting a hypervelocity star.”
A paper describing the results under Terry's supervision was published in The Astronomical Journal.
A star in motion
The pair of objects were first spotted indirectly in 2011 through a chance alignment. The team of scientists analyzed archival data from the Microlensing Observations in Astrophysics (MOA) — a collaborative project focused on microlensing studies conducted using the University of Canterbury's Mount John Observatory in New Zealand — in search of light signals that would indicate the presence of exoplanets, or planets outside our solar system.
Microlensing occurs because the presence of mass warps the fabric of space-time. Whenever an intervening object appears to drift near a background star, the light from the star is bent as it travels through the warped space-time around the nearest object. If the alignment is particularly close, the warping around the object can act as a natural lens, magnifying the light from the background star.
In this case, the microlensing signals revealed a pair of celestial bodies. Scientists have determined their relative masses (one is about 2300 times heavier than the other), but their exact mass depends on how far they are from Earth. It's a bit like how the magnification changes if you hold a magnifying glass over a page and move it up and down.
"It's easy to figure out the mass ratio," said David Bennett, a senior research scientist at the University of Maryland, College Park, and NASA Goddard, who is a co-author of the new paper and led the original study in 2011. "It's much more difficult to calculate their actual mass."
A team of researchers in 2011 suspected that the microlensed objects were either a star about 20 percent more massive than our Sun and a planet about 29 times heavier than Earth, or a closer "unworthy" planet about four times the mass of Jupiter with a moon smaller than Earth.
To figure out which explanation is more likely, astronomers examined data from the Keck Observatory in Hawaii and the European Space Agency's (ESA) Gaia satellite. If the pair were a rogue planet and moon, they would be virtually invisible — dark objects lost in the inky void of space. But scientists could identify the star if the alternative explanation were correct (although the orbiting planet would be too faint to see). They found a strong suspect about 24 light-years away, placing it in the Milky Way's galactic bulge — the central hub where stars are most densely packed. By comparing the star's location in 000 and 2011, the team calculated its high velocity.
But this is only its two-dimensional motion; if it is also moving towards or away from us, it must be moving even faster. Its actual speed may even be high enough to exceed the galaxy's escape velocity of just over 1,3 million miles per hour, or about 600 kilometers per second. If so, the planetary system is destined to cross intergalactic space many millions of years in the future.
"To be sure that the newly identified star is part of the system that caused the 2011 signal, we would like to look again in a year and see if it is moving at the right magnitude and in the right direction to confirm that it is coming from the point where we detected the signal," Bennett said.
“If high-resolution observations show that the star simply stays in the same position, then we can say for sure that it is not part of the system that caused the signal,” said Aparna Bhattacharya, a research scientist at the University of Maryland, College Park and NASA Goddard, who is a co-author of the new paper. “That would mean that the rogue planet and exomoon model is superior.”
The upcoming Nancy Grace Rome Space Telescope will help us learn how common planets are around such fast stars, and could provide clues to how these systems accelerate. The mission will survey the galactic bulge, combining a wide view of space with sharp resolution.
“In this case, we used MOA for its wide field of view, and then continued with Keck and Gaia for their sharper resolution, but with Roman’s powerful view and the planned view strategy, we won’t need to rely on additional telescopes,” Terry said. “Roman will do it all.”
