A Mysterious World Is Hidden Beyond Pluto
By Judith E Braffman-Miller
Far from our Sun’s welcoming heat and brilliant light, there is a dark and distant domain inhabited by frigid, frozen bodies that are well-hidden from the prying eyes of curious observers. Indeed, astronomers are just beginning to explore this weird region of everlasting twilight that has, up until now, been so far away that it has not yielded its many enticing and bewitching secrets easily. This distant domain, in our Solar System’s outer limits, is called the Kuiper Belt, and the ice dwarf planet Pluto and its quintet of moons have–so far–been the only denizens of this region to have been visited by spacecraft. In June 2017, planetary scientists from the University of Arizona in Tucson, announced their new finding that the plane of our Solar System is warped in the outer fringes of the Kuiper Belt, revealing the possible presence of a Mars-to-Earth-mass world far beyond Pluto. This unknown, unseen “planetary mass object” may lurk far, far away, according to this new research on the orbits of minor planets to be published in the Astronomical Journal. This hidden, mysterious, and icy world would be different from, and much closer than, the so-called Planet Nine, whose existence yet awaits confirmation.
In the new research paper, Dr. Kat Volk and Dr. Renu Malhotra of the University of Arizona’s Lunar and Planetary Laboratory (LPL), present strong evidence of a yet-to-be-discovered world sporting a mass somewhere between that of our Earth and Mars. The authors show that this mysterious mass has given away its existence–for now–only by controlling the orbital planes of a population of icy space rocks known as Kuiper Belt Objects (KBOs), that do their mysterious, distant dance in the dark and frozen outskirts of our Solar System.
Our Solar System was born about 4.56 billion years ago, and the frozen denizens of the distant Kuiper Belt are the lingering leftovers of that ancient era. While most KBOs circle our Star with orbital inclinations (tilts) that average out to what planetary scientists term the invariable plane of the Solar System, the most distant KBOs do not. Indeed, Dr. Volk and Dr. Malhotra discovered that the most remote of these frozen objects show an average plane that is tilted away from the invariable plane by approximately eight degrees. This means that there is something unknown–something mysterious–haunting this frigid distant shadow-region that is warping the average orbital plane of the outer limits of our Solar System.
“The most likely explanation for our results is that there is some unseen mass. According to our calculations, something as massive as Mars would be needed to cause the warp that we measured,” explained Dr. Volk in a June 20, 2017 University of Arizona Press Release. Dr. Volk is a postdoctoral fellow at LPL and the lead author of the study.
The Kuiper Belt is situated beyond the orbit of Neptune, and it extends out to a few hundred Astronomical Units (AU). One AU is the average distance between Earth and Sun, which is about 93,000,000 miles. In a way that is analogous to the inner Solar System’s Main Asteroid Belt, situated between Mars and Jupiter, the Kuiper Belt plays host to a myriad of relatively small objects.
There are also a few frozen dwarf planets hiding in the secretive shadows of the Kuiper Belt. In this remote realm of twilight, our Sun shines with only a feeble stellar fire that can barely reach this frigid, remote shadow-region. Indeed, in this distant realm of darkness, our Sun appears to be only a particularly large star, swimming within a sparkling sea, swarming with countless other stars.
Our Solar System’s Deep Freeze
Although the Kuiper Belt bears a family resemblance to the Main Asteroid Belt, it is considerably more vast–it is about 20 times as wide and 20 to 200 times as massive as the Asteroid Belt. However, like the Astroid Belt, the Kuiper Belt is the home of a host of relatively small objects, that are tattle-tale primordial remnants of our Solar System’s birth. While many asteroids are made up of rock and metal, the majority of KBOs are composed mostly of frozen volatiles (“ices”), such as water, ammonia, and methane. The Kuiper Belt is also home to a trio of officially confirmed dwarf planets: Pluto, Haumea, and Makemake. Some of our Solar System’s most mysterious moons–such as Triton of Neptune and Saturn’s Phoebe–are frequently considered to be bodies that originated in the Kuiper Belt, but escaped from their birthplace long ago. Triton circles Neptune backwards, a clear clue that it is really a captured moon that was snared by its beautiful, blue, banded planet’s powerful gravity, only to become an adopted member of its family. Triton circles its adopted parent-planet in the wrong direction, and it is probably doomed to plunge, one day, into the waiting thick blanket of clouds that veil the blue planet that it has circled since it wandered away from its frozen birthplace in the Kuiper Belt.
The Kuiper Belt was named after the Dutch-American astronomer Gerard Kuiper, even though he did not predict its existence. In 1992, 1992 QB1 was detected, and it became the first KBO to be discovered since Pluto was spotted back in 1930 by the American astronomer Clyde Tombaugh. Since the discovery of 1992 QB1, the number of known KBO’s has skyrocketed to over a thousand, and more than 100,000 KBOs over 62 miles in diameter are believed to exist. Originally, astronomers thought that the Kuiper Belt was the main home for periodic comets, which are those with orbits lasting less that 200 years (short-period comets). However, more recent studies, dating from the mid-1990s, have shown that the Kuiper Belt is really dynamically stable, and that short-period comets’ true birthplace is in the scattered disc. The scattered disc is a dynamically active region created by the outward migration of Neptune 4.5 billion years ago. Scattered disc objects, such as Eris, sport extremely eccentric orbits that carry them as far as 100 AU from our Sun.
Another distant home for Solar System comets is the still hypothetical Oort Cloud. The Oort Cloud is thought to be approximately a thousand times more remote than the Kuiper Belt and is mostly spherical. The distant denizens of the Kuiper Belt, along with the frozen inhabitants of the scattered disc, are together termed trans-Neptunian objects (TNOs). The ice dwarf planet Pluto is the largest known and most massive constituent of the Kuiper Belt. It is also the largest and second-most-massive known TNO–after Eris–to dwell in the scattered disc. Pluto was originally classified as a major planet, but it was unceremoniously booted out of the pantheon of major Solar System planets–only to be reclassified as a dwarf planet in 2006. In composition, Pluto is similar to many other KBOs, and its orbital period is characteristic of a particular class of KBOs called “plutinos”. Plutinos are objects that share the same 2:3 resonance with Neptune.
After Pluto’s discovery in 1930, many astronomers suspected that it might not really be a solitary world in our Solar System’s dark and cold outer limits. The first astronomer to propose the existence of a trans-Neptunian population was Dr. Frederick C. Leonard. Shortly after Clyde Tombaugh spotted Pluto, Dr. Leonard considered whether it was “not likely that in Pluto there has come to light the first of a series of trans-Neptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected”. That same year, the astronomer Dr. Armin O. Leuschner proposed that Pluto “may be one of many long-period planetary objects yet to be discovered.”
In 1943, Dr. Kenneth Edgeworth proposed that in the outskirts of our Solar System, beyond Neptune, the material of the primeval solar nebula was too thin and dispersed to coagulate into planets. Instead, this frigid place evolved into the home of comparatively small bodies that sometimes were disturbed by the antics of other objects. These small, icy, dirty bodies, as a result, would occasionally wander far from their home, to become occasional invaders screeching into the inner Solar System–becoming brilliant comets with flashing, thrashing tails cutting brilliant capers in the sky.
In 1951, Gerard Kuiper published a paper in the journal Astrophysics: A Topical Symposium, where he speculated that a disc had developed early in our Solar System’s ancient formation. However, he did not think that such a belt could still exist. Kuiper was devising his theory on the erroneous assumption, common in his time, that Pluto was the same size as Earth and had, as a result, scattered these small frozen bodies out toward the Oort Cloud–or even out of our Solar System altogether. If Kuiper’s theory had been correct, there would not be a Kuiper Belt today.
In 1977, Charles Kowal discovered 2060 Chiron, a frozen planetoid with an orbit situated between the giant planets, Saturn and Uranus. In 1992, another icy object, 5145 Pholus, was detected in a similar orbit. Currently, a sizeable population of comet-like objects, named centaurs, do their distant dance in the region between Jupiter and Neptune. However, the orbits of the centaurs are not stable and, as a result, have dynamical lifetimes of only a few million years. From the time of Chiron’s discovery, astronomers have contemplated on the possibility that centaurs must be frequently replenished by some objects inhabiting an outer reservoir.
Additional evidence for the existence of the Kuiper Belt ultimately emerged from the study of comets. Comets are delicate, fragile, ephemeral objects with finite lifespans. As they wander towards the glaring light and melting heat of our Star, its stellar fires cause their volatile surfaces to sublimate into interplanetary space–slowly scattering them. In order for comets to continue to be visible over the 4.56 billion-year-existence of our Sun and its family, they must be replenished frequently. One such reservoir of icy replacements is the Oort Cloud, composed of a spherical swarm of frozen comets that reach out beyond 50,000 AU from our Sun. The existence of the Oort Cloud was first proposed by the Dutch astronomer Jan Oort in 1950. The still-hypothetical Oort Cloud is generally thought to be the origin of long-period comets. Long-period comets travel on orbits that can last thousands of years.
In 1987, astronomer Dr. David Jewitt, who was then at the Massachusetts Institute of Technoogy (MIT) in Cambridge, started to wonder about the strange emptiness of the outer Solar System. Because he was fascinated by this intriguing mystery, he encouraged his then-graduate student Dr. Jane Luu to help him hunt for the location of another object beyond Pluto’s orbit. Finally, after five years of searching, Dr. Jewitt and Dr. Luu announced on August 30, 1992 that they had discovered a candidate KBO (15760) 1992 QB 1. Six months later they found a second object in this distant, and previously unexplored, region of our Solar System.
A Mysterious World Is Hidden Beyond Pluto
For their recent paper, Dr. Volk and Dr. Malhotra analyzed the tilt angles of the orbital planes of over 600 remote, icy objects inhabiting the Kuiper Belt. The two planetary scientists did this in order to determine the common direction about which these frozen worldlets all precess. Precession is the term used to show change or “wobble” in the orientation of a rotating body.
“Imagine you have lots and lots of fast-spinning tops, and you give each one a slight nudge. If you then take a snapshot of them, you will find that their spin axes will be at different orientations, but on average, they will be pointing to the local gravitational field of Earth,” Dr. Malhotra explained in the June 20, 2017 University of Arizona Press Release. Dr. Malhotra is a Louise Foucar Marshall Science Research Professor and Regent’s Professor of Planetary Sciences at LPL.
“We expect each of the KBOs orbital tilt angle to be at a different orientation, but on average, they will be pointing perpendicular to the plane determined by the Sun and the big planets,” she added.
Imagine the average orbital plane of objects in the outer Solar System as a sheet. It should be very flat past 50 AU, according to Dr. Volk.
“But going further out from 50 to 80 AU, we found that the average plane actually warps away from the invariable plane. There is a range of uncertainties for the measured warp, but there is not more than a 1 or 2 percent chance that this warp is merely a statistical fluke of the limited observational sample of KBOs,” Dr. Volk explained in the June 20, 2017 University of Arizona Press Release.
This means that the effect is probably a real signal and not merely a statistical fluke. According to the new calculations, an object with the same mass as Mars orbiting approximately 60 AU from our Sun on an orbit tilted approximately eight degrees–to the average plane of the known planets–can exert a sufficiently powerful gravitational influence to warp the orbital plane of the remote KBOs within about 10 AU to either side.
“The observed distant KBOs are concentrated in a ring about 30 AU wide and would feel the gravity of such a planetary mass object over time. So hypothesizing one planetary mass to cause the observed warp is not unreasonable across that distance,” Dr. Volk continued to note.
This observation rules out the possibility that the proposed object could be the hypothetical Planet Nine. The possible existence of a Planet Nine is based on other observations. That planet is predicted to be about 10 times more massive than Earth, and much farther out at 500 to 700 AU.
“That is too far away to influence these KBOs. It certainly has to be much closer than 100 AU to substantially affect the KBOs in that range,” Dr. Volk added.
According to the International Astronomical Union’s (IAU’s) definition of a planet, in order for an object to be classified as a planet, it must have swept its orbit clean of minor planets–such as icy little KBOs. This is the reason why the authors refer to a “hypothetical planetary mass object” in their paper. In addition, the data also fail to rule out the possibility that the warp could really be the result of the presence of more than merely one planetary mass object.
So why haven’t they seen this elusive object yet? The most likely answer, according to Drs. Malhotra and Volk, is because they haven’t yet searched the entire sky for remote Solar System objects. The most probable place a planetary mass object could be lurking in secret would be in the galactic plane. The galactic plane is a region so densely filled with a myriad of stars that Solar System surveys have come to avoid it.
“The chance that we have not found such an object of the right brightness and distance simply because of the limitations of the surveys is estimated to be about 30 percent,” Dr. Volk commented in the June 20, 2017 University of Arizona Press Release.
One potential alternative to an unseen object, that could have rudely ruffled the plane of outer KBOs, would be a star that wandered too close to our Solar System in recent history–at least “recent” by astronomical standards.
“A passing star would draw all the ‘spinning tops’ in one direction. Once the star is gone, all the KBOs will go back tp precessing around their previous plane. That would have required an extremely close passage of about 100 AU, and the warp would be erased within 10 million years, so we don’t consider this a likely scenario,” Dr. Malhotra noted in the University of Arizona Press Release.
But humanity’s chance to see this elusive and mysterious object might come soon. Once the construction of the Large Synoptic Survey Telescope (LSST) is at last finished, it might provide astronomers with a precious peek into this shadowy, frigid, and faraway region of our Solar System’s outer limits. The LSST is run by a consortium, and is scheduled for first light in 2020. At that time the telescope will take unprecedented, real-time surveys of the sky–night after night after night.
According to Dr. Malhotra: “We expect LSST to bring the number of observed KBOs from currently about 2000 to 40,000. There are just a lot more KBOs out there–we just have not seen them yet. Some of them are too far and dim even for LSST to spot, but because the telescope will cover the sky much more comprehensively than current surveys, it should be able to detect this object, if it’s out there.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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