The very remote and frigid dwarf planet Sedna was discovered on November 13, 2003, and it was first observed when it was at a distance more than 90 times greater than that from the Earth to our Sun–about three times further out than the very remote dwarf planet Pluto. In March 2014, more than a decade after Sedna’s discovery, a team of astronomers announced that they had detected yet a second object dwelling in Sedna’s mysterious and remote region of our Solar System that is called the inner Oort cloud. The discovery of a new sister for Sedna prompts a scientific reevaluation in regard to the strange, dark, frigid, and very distant region that exists between the realm of the four giant planets and the reservoir of long-period comets.
Long-period comets take more than 200 years to orbit our Sun, while short-period comets take 200 years or less.
The petite ice-world Sedna was discovered by Dr. Michael Brown (Caltech), Dr. Chad Trujillo (Gemini Observatory), and Dr. David Rabinowitz (Yale University). The discovery was made on the Samuel Ochin Telescope at the Palomar Observatory just east of San Diego, California. In the first discovery images, Sedna shows itself as a mere tiny point of light–and it is one of the most bizarre objects in our Sun’s family. This tiny body composed of ice is on an extremely eccentric (out-of-round) orbit that carries it out to approximately 1,000 astronomical units (AU). One astronomical unit is the average distance between our planet and the Sun, which is about 150,000,000 kilometers. Sedna‘s point of closest approach to our Star (perihelion) is about 76 AU! Its orbit carries it well beyond Neptune, the outermost planet of our Solar System, which twirls around our Sun at a distance of 30 AU–a very long way from the edge of our Solar System where the Oort cloud, the mysterious domain of long-period comets, is situated at about 10,000 AU. Although other potential denizens of this distant domain have been detected, Sedna has remained, up until now, the only known constituent of the inner Oort cloud at beyond 70 AU.
Sedna is one of the reddest objects in our Sun’s family–indeed, it is almost as red as Mars–and it sports a diameter somewhere between 1,200 and 1,600 kilometers. This dark red hue is thought to be the result of a surface coating of hydrocarbon sludge called tholin, that may have formed when simpler organic compounds experienced a sea-change from long exposure to ultraviolet radiation. Its surface is homogenous in both color and spectrum, and this is likely because Sedna was only rarely blasted into by other bodies–in marked contrast to objects closer to our Sun, which have endured showers of tumbling, crashing objects ever since their formation, leaving many of their surfaces heavily pockmarked by craters that tell the tale of such violent events. Impact areas usually display regions of fresh, bright ice, and Sedna’s red surface is believed to be composed of 24% tholins, 33% methane, 26% methanol, 10% nitrogen, and 7% amorphous carbon. When Sedna was first spotted, planetary scientists incorrectly believed that it possessed a bizarre long rotational period of 20 to 50 days. It was then thought that Sedna’s long rotational period was the result of the gravitational tugs of a large moon. However, a search for this elusive moon by the venerable Hubble Space Telescope (HST) turned up nothing, and subsequent measurements suggested that Sedna really has a considerably shorter rotation period of 10 hours. This is typical for an object of its petite size.
The later discovery of a probable dwarf planet that circles our Star beyond Pluto, is reshaping scientific speculations about how our Solar System may have formed. The more recently discovered icy body, dubbed 2012 VP113 sports the most distant trajectory known.
“It goes to show that there’s something we don’t know about our Solar System, and it’s something important. We’re starting to get a taste of what’s out beyond what we consider the edge,” Dr. Trujillo explained in the March 26, 2014 Nature News. Dr. Trujillo, a co-discoverer of 2012 VP113, is also one of the co-discoverers of Sedna.
Dr. Trujillo and Dr. Scott Sheppard, who is an astronomer at the Carnegie Institution for Science in Washington D.C. reported their discovery in the March 26, 2014 issue of the journal Nature.
A New Sister For Sedna
Although the newfound object’s official name is 2012 VP113, the discovery team simply refers to it as VP, for short–or just Biden, in honor of the Vice President of the United States, Joe Biden. Several years from now, after further observations have determined its orbit, the team of astronomers will submit a name for consideration by the International Astronomical Union (IAU), which is the official organization in charge of naming new celestial objects.
“This is a great discovery. We’ve been searching for more objects like Sedna for more than 10 years now,” commented Dr. Michael Brown in the March 26, 2014 Nature News. Discovering another object like Sedna shows that Sedna is not alone in the remote, dark, and frigid “no man’s land” in which it wanders, he continued to note. However, astronomers have now come up with ideas to explain how these objects remain tightly gravitationally bound to our Star when they are so far away.
The realm of our Sun’s familiar family of eight major planets cuts off at Neptune. The mysterious region beyond Neptune is the domain of a multitude of mostly small icy objects termed the Kuiper belt, that includes Pluto, which is one of its largest known members.This remote domain extends approximately 30 to 50 AU–however, even further beyond that resides the Oort cloud with Sedna lying at its inner edge, with long-period comets being even more remote than Sedna. Even though Sedna never gets closer to our Sun than 76 AU, its fellow inner Oort cloud denizen VP is even more remote, and is about 80 AU away at its perihelion.
Indeed VP is so far away that when Dr. Sheppard first detected it, it proved to be the slowest-moving astronomical body he had ever observed. The more distant a celestial object is, the slower it appears to be traveling across the sky. Dr. Trujillo and Dr. Sheppard have been searching for remote bodies with the Dark Energy Camera, a 520-megapixel camera on the 4-meter Blanco Telescope located at the Cerro Tololo American Observatory in Chile. The two astronomers spotted VP in November 2012 during their first observing run, where it showed up on only the fifth image out of hundreds that they would eventually take. “We got lucky right away,” Dr. Sheppard commented in the March 26, 2014 Nature News.
The astronomers studied the object for months, until its full orbit became more obvious. But while Sedna can wander as far as 1,000 AU from our Star, VP can travel no further that approximately 452 AU. With its farthest distance from our Star at 452 AU and its perihelion no closer than 80 AU, VP is situated well within the expected inner Oort cloud outside the Kuiper belt. VP seems to be closely bound by the Sun’s gravity–and this is something of a mystery, Dr. Sheppard added.
VP is about 450 kilometers across, making it only about 50% the size of Sedna, Planetary scientists suspect it is composed primarily of ice, and that its gravity likely can pull it into a round shape. This would make VP qualify as a dwarf planet, under the current revised rules of planethood devised by the IAU in 2006.
The discovery of VP solidifies the likelihood of the existence of a vast population of tumbling icy objects–ranging in size from a few to thousands of kilometers–in the proposed domain of the inner Oort cloud. But, for all intents and purposes–based on the present construction of our Solar System–Sedna and VP should not be there! These two small icy objects exist in a “no man’s land” between the four giant denizens of the outer Solar System–Jupiter, Saturn, Uranus, and Neptune–and the Oort cloud, where nothing in the known architecture of today’s Solar System could have deposited them. The two sister bodies appear to have been frozen in place and have remained untouched as our Sun and its family evolved to its current state. Therefore, the orbits of both Sedna and VP preserve the ancient dynamics of whatever event chased these bodies to such extreme distances and severed them off from the domain of the giant planets.
With Sedna’s discovery in 2003, several explanations for its formation were proposed, each of them predicting different orbital distributions of inner Oort cloud denizens. However, with only one lone object known at the time, it was virtually impossible to definitely distinguish between the competing theories.
At present, the preferred theory suggests that the gravitational scattering of planetesimals, which were the building blocks of the major planets in the primordial Solar System, occurred as a result of close encounters between the young Sun and other sister stars that were members of its birth star cluster. Our Sun’s natal stellar cluster would probably have harbored between 10 to 1,000 sisters of the Sun. Most stars are born in just such stellar nurseries that dissipate after a few million years. At the present age of our Solar System, the Sun and its long lost sisters would have traveled throughout our entire Milky Way Galaxy, never to meet again. However, there is no direct way to discover whether or not our Sun was ever a sparkling member of just such a stellar nursery–and there is also no way to find its vanished sisters. However, indirectly, the fingerprints of such ancient stellar turbulence could provide evidence for the sculpted distribution of orbits in the inner Oort cloud.
But with only two known objects, it still remains impossible to unambiguously identify the origin of the inner Oort cloud. However Dr. Trujillo and Dr. Sheppard discovered that the orbits of both VP and Sedna are consistent with theories involving models of our Sun’s stellar birth cluster, as well as with constraints derived from earlier ground-based, wide-field suveys. This finding indicates that VP and Sedna are likely just the tip of the iceberg–a very, very big iceberg–for this heavy population of remote, icy inner Oort cloud objects. Most of the distant icy denizens dwelling in the frigid darkness of this population would be situated at distances farther out than either Sedna or VP, and for only an extremely small fraction of their distant orbits around our Star would be brilliant enought to be spotted in current ground-based surveys.
In addition, Dr. Trujillo and Dr. Sheppard noticed a peculiarity with the orbits of both VP and Sedna. These two frozen inhabitants of our Solar System’s outer limits show similar values for one of their orbital parameters: the angle between the point of perihelion and where the orbit crosses the plane of our Solar System. This peculiarity may be the first clue astronomers have of an identifiable signature of the inner Oort cloud’s formation mechanism on the orbits of closer-in members of our Sun’s family. It is fascinating that the most remote icy denizens of the Kuiper belt, with orbital axes greater than 150 AU and perihelia beyond Neptune, also apparently have values for such angles similar to those of Sedna and VP. This sort of clustering of orbital angles cannot be explained by the gravitational influence of Neptune alone. If this is true, any formation mechanism suggested for the origin of both VP and Sedna will need to explain this orbital architecture. More detections of objects inhabiting both the Kuiper belt and inner Oort cloud will be necessary in order to confirm this result.
Astronomers now know that Sedna is not alone in the darkness of that cold and mysterious “no man’s land” beyond Neptune. Dr. Megan Schwamb, an astronomer at the Academia Sinica in Taipei, Taiwan, searches for Sedna-type objects, and has modeled how much material could be out there. She thinks that the inner Oort cloud could host between 10 and 100 times the mass of the entire Kuiper belt.
Dr. Trujillo and Dr. Sheppard calculate that there are hundreds more inner Oort objects waiting to be spotted. They are currently tracking six more candidates that potentially could belong to extreme regions of our Solar System.