Ancient Romans and the Concept of Planet X

Beyond Neptune, numerous celestial bodies, including the dwarf planet Pluto, traverse the frigid expanse of the Kuiper Belt at the outer reaches of the Solar System. Many of these objects possess unusual orbits, hinting at an unseen influence nearby.

Since 2012, a faction of astronomers and planetary scientists has proposed the existence of a substantial planet—potentially five to ten times the size of Earth—concealed beyond this distant belt of icy and rocky entities. This hypothetical world would likely follow a highly eccentric orbit around the Sun, complicating its discovery.

In 2013, the planet HD 106906 b was detected by researchers using the Magellan Telescopes in Chile, located 336 light-years from Earth and orbiting far away from its parent star. This exoplanet is significantly larger than what is theorized for Planet X (often referred to as Planet Nine)—estimated to be around 3,500 times Earth's mass. Nevertheless, HD 106906 b offers a unique opportunity for astronomers to study a planet situated at such a great distance from its star.

Since its discovery, the HD 106906 system has drawn interest due to its dense dust disk encircling the young star. This system is believed to resemble our own during the formative stages of the planets in our solar system, suggesting that new worlds may currently be in the process of formation there.

At just 15 million years old, HD 106906 b orbits its star at a distance 737 times that of Earth's distance from the Sun. This distant planet was initially observed traveling along an orbit tilted 21 degrees from the primary debris disk.

Researchers have debated whether this planet will remain a long-term member of the system or if it might be ejected into space. In 2015, it was discovered that the outer disk of this system is asymmetrical compared to the rest of the disk, potentially resulting from a cataclysmic event that could have propelled HD 106906 b to its remote position.

“The hypothesis is that each time the planet approaches the binary star, it disturbs the material in the disk. Every time it passes through, it truncates the disk and pushes it to one side. Simulations of the system with the planet on a similar orbit were conducted before the planet's actual orbit was known,” explained Robert De Rosa from the European Southern Observatory.

However, analyses indicate that events causing the disk's misalignment would likely have also cast such a planet out of the system. A passing star would have been necessary to prevent it from becoming a rogue planet drifting through interstellar space.

An Unexpected Visitor in the Neighborhood

A recent study published in The Astronomical Journal tracked the position of HD 106906 b over 14 years, concluding that the planet is likely to remain within its system for the foreseeable future.

This discovery raises further questions: How did such a massive planet end up in such an eccentric orbit, so distanced from its star? Did HD 106906 b form in its current remote position, or did it migrate outward?

“The prevailing theory suggests that it formed much closer to its star, approximately three times the distance that Earth is from the Sun. However, interactions within the gas disk caused its orbit to decay, pulling it inward toward the binary stars. The gravitational forces from the spinning stars then cast it into an eccentric orbit that nearly ejected it from the system, until a passing star stabilized its orbit and kept it from leaving,” the Hubble team reported.

The analysis of HD 106906 indicates that massive planets like this one could form in distant orbits during the early stages of solar system formation, while passing stars may significantly influence the evolution of these colossal, icy worlds.

“An early event likely initiates the outward migration of planets and comets, followed by passing stars that stabilize their orbits. We are gradually collecting the evidence needed to comprehend the variety of exoplanets and how this relates to the enigmatic features of our own solar system,” explained Meiji Nguyen, a recent UC Berkeley graduate.

If a Planet Nine does exist in our Kuiper Belt, it may have been displaced there by the dominant force of our solar system, Jupiter. Once positioned in the outer reaches, a passing star might have helped stabilize its orbit within the Kuiper Belt.

A Long Time Ago, in a Star-Studded Sky

Dr. Paul Kalas from UC Berkeley and De Rosa initiated a search for a star that may have passed near the HD 106906 system, aiding in stabilizing the enormous planet's orbit. They identified several stars that could have played this pivotal role.

Combining this information with orbital data collected from 2004 to 2018, the team concluded that HD 106906 is likely to be in a stable, albeit highly elliptical, orbit.

While the Hubble Space Telescope can capture images of this distant exoplanet, astronomers must block the star's light, complicating the precise measurement of the distance between the star and planet. However, using positional data from the European Space Agency's Gaia spacecraft, researchers accurately measured the distance between the two.

“We achieve high astrometric precision by cross-referencing the locations of background stars with the Gaia astrometric catalog, providing the subpixel location of HD 106906 that may be saturated or obscured by coronagraphic optical elements,” the researchers described in an article published in The Astronomical Journal.

This remarkable discovery emerged from just 15 years of data tracking a planet on an orbit that lasts 15,000 years. The study also revealed that HD 106906's tilt is greater than previously thought—between 36 and 44 degrees relative to the planetary disk.

“To illustrate why this is unusual, we can look at our own Solar System, where all planets orbit in a roughly aligned plane. If Jupiter, for example, were to be inclined 30 degrees relative to the plane of the other planets, it would raise numerous questions about how HD 106906 b ended up so distanced on such an inclined orbit,” Nguyen explained.

Similar to the hypothesized Planet Nine in our solar system, HD 106906 b remains too far from its parent star to exert influence on any inner planets.

“What makes HD 106906 particularly unique is that it is the only exoplanet we know of that has been directly imaged, surrounded by a debris disk, misaligned relative to its system, and is widely separated from its star. This distinguishes it as the sole candidate so far with an orbit analogous to the hypothetical Planet Nine,” Nguyen stated.

While the gravitational influence of an unseen planet could account for the peculiar movements of objects in the Kuiper Belt, alternative explanations exist. Instead of a single large planet, several smaller bodies might be responsible, or these movements could simply be a statistical anomaly.

In the upcoming years, astronomers plan to study HD 106906 using the James Webb Space Telescope, the successor to Hubble.

As the search for Planet X continues within our solar system, the investigation of HD 106906 b may offer insights into an unseen world in our own cosmic neighborhood.

James Maynard is the founder and publisher of The Cosmic Companion. A New England native now residing in Tucson, he shares his home with his wife, Nicole, and their cat, Max.

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