Research has long suggested that stars like our Sun rarely occur individually. Usually, there are one or more sibling stars scattered nearby. The fact that the Sun doesn't have an established twin is problematic to our understanding of how star formation works — beginning with massive molecular clouds chock full of the gas and dust needed for them to collapse under their own weight and begin the internal ignition of nuclear fusion. It is thought that more than four-fifths of star systems may be made up of two or more stars orbiting together. The majority of these systems are likely binary, consisting of two stars orbiting a common center of mass. As many as 85 percent of stars are estimated to belong to such systems.
In research published by the Monthly Notices of the Royal Astronomical Society back in 2007, "Embedded binaries and their dense cores" was one of the first deep dives into whether the Sun ever had a twin. Several research papers have been released in the intervening years, but one particularly interesting story, published in 2020, used updated technology to delve into the core question, and it just so happens to bring the mysterious Planet X into the discussion.
What is Planet X?
Until the late 18th century, our solar system was thought to host just six planets: Mercury, Venus, Earth (naturally), Mars, Jupiter, and Saturn, all of which could easily be seen through the most basic of telescopes, or even with the naked eye in optimal conditions. It wasn't until 1781 that renowned astronomer, Sir Willian Herschel, discovered the existence of an icy blue planet — originally believed to be a star, then a comet — orbiting the Sun from a distance that is roughly 18 times greater than that of Earth.
On average, it lies about 1.8 billion miles (2.9 billion kilometers) away from the Sun, but at their closest approach to one another, they are separated by approximately 1.6 billion miles (2.6 billion kilometers). On the flip side, they can be as far away as 1.98 billion miles (3.2 billion km). It takes Uranus 84 years to complete one trip around the Sun.
As an aside, Sir William Herschel is credited with discovering and cataloging an additional "800 double stars and 2,500 nebulae." Moreover, "he was the first astronomer to correctly describe the spiral structure of our Milky Way Galaxy."
Herschel nearly discovered Neptune as well, which has an extremely interesting history that will lead us further down the Planet X rabbit hole, but for starters, it was ultimately discovered by a trio of astronomers on the evening of September 23-24, 1846. As the story goes:
"The discovery was made based on mathematical calculations of its predicted position due to observed perturbations in the orbit of the planet Uranus. The discovery was made using a telescope since Neptune is too faint to be visible to the naked eye, owing to its great distance from the Sun.
Astronomers soon discovered a moon orbiting Neptune, but it took more than a century to discover a second one. Our knowledge of distant Neptune greatly increased from the scientific observations made during Voyager 2’s flyby in 1989, including the discovery of five additional moons and confirmation of dark rings orbiting the planet."
Something still didn't quite add up though. Even after the discovery of Neptune, the unique orbits of some dwarf planets and other small, icy objects in the Kuiper Belt couldn't be fully explained. These tend to follow orbits that cluster together. Analyzing the orbits has led some astronomers to suggest the possibility that a large, yet-to-be-discovered planet may be lurking far beyond Pluto.
What do we know?
This theoretical planet has been given many names... Nibiru, Tycho, etc., but we'll just go with Planet 9. If this planet does exist, it would be traveling many billions of miles beyond that pesky dwarf planet, Pluto, in a portion of the Kuiper Belt that receives very little sunlight or energy. Like many of the outermost planets, its orbit would be highly elliptical. To what extent? Well, it would take the planet between 7,400 and 18,500 years to complete just one rotation around the Sun. For perspective, Pluto has yet to make one full orbit around the Sun since its discovery — it's estimated to take 248 years to make one full revolution, and it was officially discovered in 1930.
According to NASA, "Caltech researchers have found mathematical evidence suggesting there may be a "Planet X" deep in the solar system. This hypothetical Neptune-sized planet orbits our Sun in a highly elongated orbit far beyond Pluto. The object, which the researchers have nicknamed "Planet Nine," could have a mass about 10 times that of Earth."
There's both evidence that Planet 9 formed alongside the other planets in our solar system billions of years ago, and that it just passed through long enough to shake things up before disappearing once more.
Per NASA, "In January 2015, Caltech astronomers Konstantin Batygin and Mike Brown [at the California Institute of Technology in Pasadena] announced new research that provides evidence of a giant planet tracing an unusual, elongated orbit in the outer solar system. The prediction is based on detailed mathematical modeling and computer simulations, not direct observation. This large object could explain the unique orbits of at least five smaller objects discovered in the distant Kuiper Belt."
Brown and Batygin observed that the orbits of six trans-Neptunian objects in the Kuiper belt seemed to be clustered together. This clustering, they theorized, was due to the gravitational influence of a huge planet hiding somewhere in the far reaches of the outer Solar System, at least 400 AU away.
"The possibility of a new planet is certainly an exciting one for me as a planetary scientist and for all of us," said Jim Green, director of NASA's Planetary Science Division. "This is not, however, the detection or discovery of a new planet. It's too early to say with certainty there's a so-called Planet X. What we're seeing is an early prediction based on modeling from limited observations. It's the start of a process that could lead to an exciting result."
Is there a link?
In 2020, a new theory was published in The Astrophysical Journal Letters by scientists from Harvard University which suggested that not only might Planet 9 actually exist but that there may once have been a binary companion to our Sun.
"Dr. Avi Loeb, Frank B. Baird Jr. Professor of Science at Harvard, and Amir Siraj, a Harvard undergraduate student, have postulated that the existence of a long-lost stellar binary companion in the sun's birth cluster—the collection of stars that formed together with the sun from the same dense cloud of molecular gas—could explain the formation of the Oort cloud as we observe it today."
The Oort Cloud is believed to be a giant spherical shell surrounding the Sun, planets, and Kuiper Belt Objects containing billions, or trillions of icy pieces of space debris left over from the formation of the solar system. It is theorized to be the source of long-period comets.
The Oort Cloud is also incredibly distant, with the inner edge between 2,000 and 5,000 AU from the Sun and the outer edge possibly up to 100,000 AU from the Sun (one AU is the distance separating Earth from the Sun). For comparison, Pluto’s elliptical orbit carries it between about 30 and 50 AU from the Sun.
The theory proposes that the objects in the Oort cloud may have been captured with the help of a binary companion to our Sun. "Binary systems are far more efficient at capturing objects than are single stars," said Loeb. "If the Oort cloud formed as observed, it would imply that the Sun did in fact have a companion of similar mass that was lost before the Sun left its birth cluster."
A news release from the Harvard Center for Astrophysics expands, "Popular theory associates the formation of the Oort cloud with debris left over from the formation of the solar system and its neighbors, where objects were scattered by the planets to great distances and some were exchanged amongst stars. But a binary model could be the missing piece in the puzzle.
Previous models have had difficulty producing the expected ratio between scattered disk objects and outer Oort cloud objects. The binary capture model offers significant improvement and refinement, which is seemingly obvious in retrospect: most sun-like stars are born with binary companions."
This theory could also explain the existence of Planet 9. Previous models were unclear on where trans-Neptunian objects like the theoretical Planet 9 could have come from. This new model, however, increases the odds.
Assuming there were once two stars in our solar system, separated by about 1,500 AU, increases the odds of the pair capturing a large, trans-Neptunian object like Planet 9 by a factor of 20. That is until a neighboring star passed through and separated the three objects — leaving behind only the Sun.
The existence of Planet 9 was dealt a blow last year, when a team led by physicist Kevin Napier, at the University of Michigan in Ann Arbor, conducted a meta-analysis of surveys examining the orbits of ‘extreme’ trans-Neptunian objects. The researchers found that the objects’ perturbed orbits could actually be explained without the presence of a nearby planet.
The team concluded that the observed objects only seem to be clustered because of selection bias. However, others disagree, claiming more data is needed. And this may soon be coming.
Both the Oort cloud and the proposed location of Planet 9 are far too distant for direct observation. However, the Vera C. Rubin Observatory (VRO) in Chili, which came online in 2021, will begin a ten-year survey of the sky in 2022. This is expected to detect thousands more Kuiper belt objects. A close examination of their orbits may be able to confirm or deny the existence of Planet 9 and provide clues as to its origin and location.
Says Loeb, "If the VRO verifies the existence of Planet Nine, and a captured origin, and also finds a population of similarly captured dwarf planets, then the binary model will be favored over the lone stellar history that has been long-assumed."