Two Solar System Surprises
Sometimes we learn more from surprises than predictions.
Rings belong around giant planets, not asteroids. Tell that to Chariklo, an asteroid between Saturn and Uranus that boasts not just one ring, but two! This first-of-its-kind discovery came as a “complete surprise,” the Brazilian discoverer said in Science Magazine’s news column; they were not even looking for rings. New Scientist posted a video clip about how the rings were found. Since an asteroid has far less gravity than a planet to hold onto ring particles, questions immediately arise: (1) how did the rings form, and (2) how are they sustained? Partial answers, with disclaimers, were offered in the article:
At present, it’s not clear how Chariklo’s ring system formed. One possibility is that a slow-motion collision with a smaller asteroid or comet once blasted debris into orbit, and the gravitational pull of the largest bits of debris have shepherded the smaller bits into a sharply defined ring. (The presence of such “shepherd moons” could help explain the distinct, 9-kilometer-wide gap between Chariklo’s two rings as well, Braga-Ribas notes.)
Even if this “possibility” could be upgraded to a probability, it would seem that the rings would be very delicate and easily perturbed. The asteroid, Science Daily notes, has such little gravity you could accelerate a sports car to escape velocity on its surface and drive off into space. Wouldn’t the slightest tug from a planet or another asteroid make the ring particles fly away? How long have they been in this configuration?
Planetary scientists generally dislike assuming we live in a “special time” to observe a phenomenon. Saturn’s rings are being eroded by multiple destructive processes (sunlight pressure, collisional spreading, impacts among them), leading scientists to conclude the rings are young, despite Saturn’s much greater holding power. Unless little Chariklo’s rings formed very recently, 4.5 billion years (the assumed age of the solar system) is a long time to wipe them out.
It seems a stretch to believe the scientists’ suggestion that the rings might be accreting into a moonlet. Even if that could happen, a moonlet would just as easily be tugged out of orbit. And it’s hardly a solution to multiply one’s surprises: Space.com quotes the discoverer saying, “Rings may be a much more common property than we thought.”
New minor planet
Far out! Another sizable minor planet to join Sedna has been detected at the edge of the solar system – or, at least, where the hard bodies end and the assumed Inner Oort Cloud of comets (never observed) theoretically begins. We should note at the outset that, contrary to USA Today‘s titillations, a super-Earth has NOT been discovered out there (more on that later). The new body, with its temporary name 2012 VP113, is 83 A.U. (astronomical units, 1 AU = earth-sun distance) away from the sun. Size-wise, as Science Magazine describes in a pithy analogy, “If Pluto were as big as a basketball, Sedna would be a softball and the new world a mere golf ball.” Science Daily says this is probably one of thousands of bodies out there that await discovery.
What’s weird about VP113 is that, like Sedna, it has a highly elliptical orbit, so 83 AU is its nearest point, or perihelion, far beyond Neptune’s orbit. That means, “Whereas Pluto orbits the sun every 248 years, the new world requires 4340 years and Sedna 12,600 years to do the same.” Since planets are assumed to form from a circular dust disk, theoreticians have a problem. They postulate that Sedna, VP113 and similar bodies were jolted out of their original orbits by gravitational encounters with other bodies. The smaller the body, the easier this could happen; that’s why Chariklo’s rings are a problem.
Analysis of the orbits of these two minor planets suggests there is something tugging on them. (Note: orbital anomalies led Adams and LeVerrier to predict and discover Neptune in the 19th century.) That’s why USA Today, New Scientist and Live Science are teasing readers about a possible super-Earth-size “Planet X” farther out there. (New Scientist has the clearest diagram of the orbits.) The discoverers toyed with this idea in their models and found a possible fit if the super-Earth were at 250 AU, but admitted it was not a unique solution: “This configuration is not unique and there are many possibilities for such an unseen perturber.” Veteran planet hunterr Mike Brown of Caltech commented, “It is possible that some undiscovered large object out there is doing this, but there are likely many other explanations, too, most of them sadly more mundane.” An Earth-sized planet at that distance would be undetectable by modern methods, the paper said.
Commenting on the paper in Nature, Megan Schwamb says “The discovery of a second resident in a region of the Solar System called the inner Oort cloud prompts fresh thinking about this no-man’s-land between the giant planets and the reservoir of comets of long orbital period.” If “A decade after its discovery, Sedna still remains one of the strangest objects in the Solar System,” now it has a contender. She is dumbstruck by these objects:
To all intents and purposes, in the current architecture of the Solar System, Sedna and 2012 VP113 should not be there. These objects are in a no-man’s-land between the giant planets and the Oort cloud where nothing in the known configuration of the modern-day Solar System could have emplaced them. Effectively frozen in place and untouched as the Solar System evolved to its present state, their orbits preserve the dynamical signature of whatever event scattered these bodies to such distances and detached them from the giant-planet region.
Maybe passing stars did it, she considers. “There is no direct way to identify whether the Sun was ever in a stellar nursery or to find its siblings,” though, she admits. “But indirectly, the mark from those early stellar encounters, like fingerprints at a crime scene, could provide evidence for the sculpted distribution of orbits in the inner Oort cloud.” As for that inner Oort Cloud, its existence is cloudy: both minor planets would be far to the interior of its assumed position. Does it even exist? “Our knowledge of the inner Oort cloud is, in many ways, in the same state as the study of the Kuiper belt was in the 1990s, when the first Kuiper-belt objects were discovered — 62 years after Pluto’s detection.” She assumes these objects will shed light on it. But they are not even part of it, so how can they?
These stories contribute to a pile of indictments of solar system astronomers that show: (1) every body they observed with spacecraft or improved instruments did not match predictions, and (2) everything else was a surprise. They are experts at math and observation, but when it comes to prediction, psychics might be a better bet. Megan’s confession is the take-home lesson: “Sedna and 2012 VP113 should not be there.” That line sounds familiar. If you watch the DVDs by Spike Psarris (CreationAstronomy.com), he quotes similar statements – by leading astronomers – in reputable journals – about various planets and stars. They shouldn’t be there. But they are. Maybe the scientists are the ones who shouldn’t be there.