Runaway star may be dragging a planet along with it

Astronomers may have discovered a star hurtling through the Milky Way with a planet in tow. If confirmed, this could become the fastest-moving exoplanet system ever recorded.

Moving at nearly twice the speed of our solar system, this phenomenon challenges our understanding of how planets remain bound to their host stars under extreme conditions.

High-speed stars are rare, and even rarer are those dragging a planetary companion along for the ride.

The system in question is estimated to move at a staggering 1.2 million miles per hour (1.9 million kilometers per hour). This revelation raises new questions about planetary survival and migration in the cosmos.

High-speed star with an orbiting planet

The planet orbiting this star is believed to be a gas giant that is larger than Neptune but smaller than Jupiter. Scientists estimate that it is orbiting relatively close to its central 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 postdoctoral researcher at the University of Maryland, College Park and NASA’s Goddard Space Flight Center.

However, the star’s weak brightness places the planet well outside the habitable zone. This means it is unlikely to support life as we know it.

Despite that, its presence could provide valuable insight into how planetary systems behave when subjected to extreme speeds. If confirmed, this would mark the first time a planet has been found orbiting a hypervelocity star.

Detection using microlensing

This extraordinary system was first detected in 2011. Scientists working on the MOA (Microlensing Observations in Astrophysics) project, a collaborative survey based in New Zealand, were searching for distant exoplanets.

Instead, the team stumbled upon an unusual microlensing event that hinted at a pair of celestial objects.

Microlensing is a phenomenon where gravity bends and magnifies the light from a background star. When an object passes in front of a more distant star, its gravitational pull can act as a natural lens, briefly amplifying the star’s brightness.

By studying these fluctuations in light, astronomers can infer the presence of unseen objects. In this case, the microlensing event suggested two objects, one significantly more massive than the other.

Objects with unknown masses

The scientists calculated their relative masses, but could not determine the exact sizes of the objects without knowing their distance from Earth.

“Determining the mass ratio is easy. It’s much more difficult to calculate their actual masses,” said David Bennett, a senior research scientist at NASA Goddard.

The 2011 data suggested two possibilities. Either the system consisted of a low-mass star with a Neptune-like planet in orbit around it, or it was a rogue planet accompanied by a smaller moon. Determining which scenario was correct required further investigation.

Is it a star or a moon?

To solve this mystery, astronomers turned to data from the Keck Observatory in Hawaii and the European Space Agency’s Gaia satellite. If the objects were a rogue planet and moon, they would be extremely difficult to detect. But if a small star were present, its light could be identified.

The search led to a promising candidate located about 24,000 light-years away. This placed it within the Milky Way’s galactic bulge, the densely packed central region of the galaxy. By comparing the star’s position in 2011 and 2021, the researchers determined it was moving at incredible speeds.

However, the team needed to confirm whether this star was indeed part of the system detected in 2011. Its measured speed already suggested it was among the fastest-moving stars known, but its full velocity remained uncertain.

“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 explained.

Planet-star system may escape the galaxy

If this system includes a hypervelocity star, it could eventually escape the Milky Way. Its true velocity might exceed 1.3 million miles per hour (2 million kilometers per hour), which is faster than the galaxy’s escape velocity.

If this is the case, the planet and star will drift into intergalactic space over millions of years, joining the ranks of stars that have already been lost to the void.

Scientists are eager to verify this possibility. Observations over the next few years will determine if the identified star is indeed linked to the 2011 microlensing event.

“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. That would mean the rogue planet and exomoon model is favored,” said Aparna Bhattacharya, a research scientist at NASA Goddard.

The role of future telescopes

Upcoming missions will help astronomers find more of these high-speed planetary systems. NASA’s Nancy Grace Roman Space Telescope, set to launch in the near future, will play a crucial role.

Its advanced capabilities will allow scientists to study the galactic bulge in unprecedented detail, and help them understand how planets form around stars that are moving at 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, but thanks to Roman’s powerful view and planned survey strategy, we won’t need to rely on additional telescopes,” said Terry. “Roman will do it all.”

With technology advancing, astronomers are closer than ever to unraveling the mysteries of fast-moving stars and their planets. Whether this system turns out to be a star with a planet or a rogue planet with a moon, it will deepen our knowledge of how celestial bodies travel across the vast reaches of space.

The study is published in The Astronomical Journal.

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