Ephemeral and fragile, comets that come streaking inward towards the melting heat and light of our Star are the wandering refugees from a distant twilight region of ice. There, in our Solar System’s deep freeze, myriad comet nuclei dance around our Sun beyond the orbit of Neptune–the outermost major planet inhabiting our Sun’s family. Pluto–the beloved faraway world with a big heart–was originally categorized as the ninth major planet from our Star. However, poor Pluto was demoted from the pantheon of major planets when others of its distant, frigid kind were found inhabiting the same twilight region known as the Kuiper Belt. For decades, the true identity of Pluto has been a subject of considerable debate within the planetary science community–but now a team of astronomers have announced that they may have finally put an end to the debate. This is because they have found evidence that Pluto is not a planet. Instead, Pluto is a giant comet.
In May 2018, planetary scientists of the Southwest Research Institute (SwRI) in San Antonio, Texas, announced that they have integrated NASA’s New Horizons’ discoveries with data obtained from the European Space Agency’s (ESA’s) Rosetta mission, in order to formulate a new theory explaining how Pluto may have been born in our Solar System’s far suburbs.
NASA’s New Horizons spacecraft ended a decade-long, dangerous, and difficult journey on July 14, 2015, when it successfully made its closest approach to Pluto, at approximately 7,750 miles above its alien and unexplored surface. This closest approach is about the same distance that New York is from Mumbai, India–and this flyby above Pluto made history. That is because New Horizons became the first space mission to successfully explore a world located so far from Earth.
The ESA’s Rosetta mission became another historic first when, in August 2014, it arrived at comet 67P/Churyumov-Gerasimenko (67 P, for short), thus becoming the first spacecraft to land on a comet.
“We’ve developed what we call ‘the giant comet’ cosmochemical model of Pluto formation,” commented Dr. Christopher Glein in a May 23, 2018 SwRI Press Release. Dr. Glein is of SwRI’s Space Science and Engineering Division. The new research is described in a paper published online on May 23, 2018 in the planetary science journal Icarus. Central to the research is the existence of a nitrogen-rich ice located in Sputnik Planitia, a large glacier that forms the left lobe of the shining Tombaugh Regio feature observed on Pluto’s distant surface.”
Sputnik Planetia is a highly reflective, ice-coated basin that is approximately 650 by 500 miles in size. It was named after Sputnik, Earth’s first artificial satellite, that was launched by the Soviet Union on October 4, 1957. Sputnik Planetia composes the western lobe of the big heart-shaped Tombaugh Regio, named after Pluto’s discoverer.
“We found an intriguing consistency between the estimated amount of nitrogen inside the glacier and the amount that would be expected if Pluto was formed by the agglomeration of roughly a billion comets or other Kuiper Belt Objects similar in chemical composition to 67P, the comet explored by Rosetta,” Dr. Glein added.
In other words, Pluto is really a giant comet with a big heart that formed very differently from the planets of our Solar System. According to this new study, Pluto was born as the result of myriad mergers between icy comets and other KBOs in the turbulence of what was our primeval Solar System.
Along with the comet scenario, the scientists also studied a still-viable solar model. According to the solar model, Pluto was born from the agglomeration of extremely cold ices that would have had a chemical composition that is closely akin to that of our Sun.
Our Solar System’s Outer Suburbs
The distant Kuiper Belt is a dimly lit domain; a region of perpetual twilight far from our Star. Here, in the deep-freeze of our Sun’s region of gravitational influence, a multitude of icy objects, both large and small, perform a dazzling dance within their murky swath of space beyond the blue-and-white banded ice-giant planet Neptune. Astronomers are only now first beginning to explore this faraway domain. New Horizons has left Pluto and its quintet of moons behind, and it is now soaring deeper into the Kuiper Belt to explore a second object inhabiting this very alien realm of the comets.
Pluto is a relatively large denizen of the Kuiper Belt, as is its largest moon Charon, that is almost half the size of Pluto itself. After Pluto’s discovery in 1930, it was classified as the ninth major planet of our Sun’s family. However, the growing understanding among planetary scientists that this frozen little “oddball” is really only one of a number of large icy worldlets inhabiting the Kuiper Belt, forced many astronomers to change their minds. In 2006, the International Astronomical Union (IAU) was left with the task of formally defining what is meant by a “planet”, and Pluto lost its lofty designation as the ninth major planet from our Star. Currently, Pluto has been reclassified as a mere dwarf planet, but the scientific struggle to come to terms with exactly how it should be classified remains elusive.
The Pluto saga commenced less than a century ago when a young farmer’s son from Kansas, the astronomer Clyde Tombaugh (1906-1997), was given the difficult task of hunting for the elusive, and possibly non-existent, so-called Planet X. According to theory, Planet X is a well-hidden giant planet that haunts the frigid darkness beyond Neptune. Using a telescope in Flagstaff, Arizona, Tombaugh did, indeed, detect a faint pinpoint of intriguing light that turned out to be the beloved “oddball” Pluto. In a classic example of scientific serendipity, Tombaugh did not find what he was looking for–he found something else instead.
For most of the 20th century, astronomers considered Pluto to be an isolated small world, circling our Star in its remote and lonely orbit located in our Solar System’s distant suburbs. However, all this changed in 1992 when the first KBO–other than Pluto and Charon–was detected, and astronomers were forced to come to the realization that Pluto has company–plenty of company. Since 1992, a multitude of icy, frozen little worldlets similar to Pluto–that also have eccentric orbits–have been discovered. The most important of these scattered disc objects, Eris, was discovered in 2005. The realization that Pluto is just one of many others of its frozen kind resulted in its demotion and re-classification.
Pluto has a quintet of known moons: Charon, Nix, Hydra, Kerberos, and Styx. Charon is by far the largest of the five moons, and it was discovered in 1978 by the American astronomer James Christy. Some astronomers think that Charon is really just a big chunk of Pluto that was torn from it when some unknown object rampaged through the Kuiper Belt and crashed into Pluto.
Like its comet-neighbors in the Kuiper Belt, Pluto is likely made up of ice and rock. It is also very small–only about 1/6 the mass of Earth’s Moon and about 1/3 its volume. Pluto also displays a highly inclined and eccentric (out of round) orbit that takes it from 20 to 49 astronomical Units (AU) from our Star. One AU is equivalent to the Earth-Sun separation of about 93,000,000 miles. Pluto periodically wanders towards our Sun at a closer distance than Neptune. However, a colossal smash-up between the two is unlikely to occur because an orbital resonance with Neptune fortunately prevents the two from crossing paths and blasting into each other–with catastrophic results.
The data gathered by the New Horizons mission will help astronomers understand the distant, mysterious frigid worlds, that circle our Sun in a dimly lit swath of space, located at our Solar System’s faraway fringe.
Currently, both Pluto and Charon are designated members of yet a third category of frigid objects called ice dwarfs. Both small worlds possess solid surfaces but, in dramatic contrast to the rocky terrestrial planets, a large portion of the duo’s mass is composed of ice.
In August 2014, the ESA’s Rosetta spacecraft arrived at comet 67P and entered orbit around this odd little object that has been likened to a child’s “rubber ducky.” Rosetta is a robotic space probe that, along with its attached Philae lander module, successfully performed a detailed analysis of its comet quarry. It was on November 12, 2014, that Rosetta made history by becoming the first spacecraft to land on a comet.
Launched on March 2, 2004 from the Guiana Space Center in French Guiana on an Ariane 5 rocket, Rosetta studied comet 67P, and provided evidence that it is really composed of two separate chunks–a small “ducky” head and large “body”–rather than a single object. Exactly how the two chunks managed to meet up and merge is still unknown. It has been proposed that the two chunks are really fragments of what was a larger comet that was blasted apart in a horrific collision with another object. However, an alternative explanation exists that proposes a more peaceful formation–that two small proto-comets, that formed a few million years after our Solar System’s birth, simply bumped into one another gently, thus forming the interesting “ducky” shape.
The rather weird way that Rosetta’s comet acquired its unusual shape could offer some important insight into the way our Solar System was born and evolved. Our ancient Solar System is thought to have been a turbulent place, where primordial bodies violently blasted into one another. The ancient environment surrounding Rosetta’s comet was probably no exception. However, objects can merge without undergoing a violent type of collision, and can instead attach themselves to one another more peacefully. For this reason, many planetary scientists think that the “rubber ducky” shape of 67P is the result of a merger of a duo of small proto-comets that may not have experienced a violent collision. This viewpoint suggests that such assumptions about our ancient Solar System’s “cosmic shooting-gallery” environment may not be exactly on target.
The two lobes that form comet 67P are of similar composition. This suggests that they likely formed in the same environment in our primordial Solar System. Because of the combination of its unusual two-lobed shape and the inclination of its rotation axis, Rosetta’s comet is subjected to some bizarre seasonal changes over the course of its 6.5-year-long-orbit. Indeed, this comet’s seasons are very uneven between the two hemispheres. Each hemisphere sports areas that contain regions belonging to both the comet lobes and the “neck”.
For most of the duration of 67P’s orbit, the northern hemisphere enjoys a long summer that lasts for 5.5 years, while the southern hemisphere receives little in the way of sunlight. Indeed, a large area of the region near the comet’s south pole, is stuck in the sinister dark grip of a dismal polar night, and it experiences complete blackness for almost five years.
When Rosetta finally reached its target, the northern hemisphere of 67P was still in the toasty grip of a long, hot summer, while the southern hemisphere was receiving little sunlight.
A Billion Comets
Planetary scientists wanted to solve the intriguing puzzle of why nitrogen is present at Pluto now, as well as how much of this volatile element–that currently exists in Pluto’s glaciers and atmosphere–potentially could have escaped out of its atmosphere and into space over the passage of time. The scientists then needed to reconcile the proportion of carbon monoxide to nitrogen in order to attain a better picture of what had actually occurred on this frigid little world since its mysterious birth. At last they reached the conclusion that the scanty amount of carbon monoxide at Pluto indicates either a burial of surface ices or destruction from liquid water.
“Our research suggests that Pluto’s initial chemical makeup, inherited from cometary building blocks, was chemically modified by liquid water, perhaps even in a subsurface ocean,” Dr. Glein explained in the May 23, 2018 SwRI Press Release.
Nevertheless, the solar model also agrees with certain constraints. Even though this new study points to some fascinating possibilities, a number of questions remain to be answered.
The paper describing this new research is published under the title: Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto. The paper is coauthored by Dr. Glein and Dr. J. Hunter Waite Jr., who is a SwRI program director.