The discovery of 2012 VP113, a sizable object roughly twice Pluto's distance from the Sun, has dynamicists wondering whether a super-Earth-size perturber lies undiscovered even farther out.This week's issue of Nature features an interesting announcement by Chadwick Trujillo (Gemini Observatory) and Scott Sheppard (Carnegie Institution for Science). These two observers have found an object orbiting the Sun, designated 2012 VP113, that they first spotted 17 months ago with the 4-m Blanco Telescope on Cerro Tololo in Chile and then followed up last year with the 6.5-m Magellan-Baade reflector at nearby Las Campanas Observatory.
With an estimated diameter of roughly 280 miles (450 km), this 23rd-magnitude blip hardly ranks as one of the giants of the outermost solar system. More interesting is 2012 VP113's distant, highly elongated orbit, which brings it 80 astronomical units from the Sun at its closest and a whopping 472 a.u. away at its farthest. It takes 4,600 years to loop around the Sun. Another such object, 90377 Sedna, is likewise distantly adrift. Both lie well outside the Kuiper Belt, which extends outward only to about 50 a.u.
Sedna caused quite a stir after its discovery in 2003, because it resides in a kind of orbital "no man's land". Its perihelion (closest point to the Sun) is 76 a.u. — too far away to have been flung out there by a close pass with an outer planet. Dynamicists have speculated that a star passed very close to our solar system in primordial times, drawing objects out of the Kuiper Belt and into Sedna-like orbits before moving on. It's easy to think that 2012 VP113 had the same fate.However, what's got dynamicists buzzing about this new find is not so much its distance, but instead what it has in common with Sedna's orbit. Both have perihelia near the plane of the ecliptic. So do several other far-out objects, 12 in all, whose distance from the Sun averages at least 150 a.u.
This isn't due to some kind of observational bias, note Trujillo and Sheppard, and it's statistically unlikely to be mere coincidence. Importantly, this kind of orbital alignment means there was no close-passing star at the dawn of solar-system history, because the orbits' orientations would have become randomized in the eons since by gravitational nudges from the outer planets.
Instead, the observers suggest, this might be the handiwork of a super-Earth-size planet roughly 250 a.u. from the Sun, in what's considered the inner Oort Cloud of comets. This rogue world would have enough mass to perturb objects like 2012 VP113 and Sedna inward.
Notably, Trujillo and Sheppard aren't saying this "Planet X" is the real deal. But their computer simulations suggest that it would indeed push objects into these otherwise unexplained orbits. "This is at the suggestive stage," Trujillo cautions. "There are many possible configurations of perturber(s) that could cause the effect."
"If you're asking me whether they've found a planet, the answer is no," comment dynamicist Hal Levison (Southwest Research Institute). "But I'm uncertain about what it means" because the observational data look sound to him.
Levison explains that big primordial objects, Earth-size or bigger, could easily have been thrown into the Oort Cloud after encountering the growing cores of what became to outer planets. For example, some have speculated that Uranus got knocked over into its sideways spin after colliding with an Earth-mass object. However, he cautions, the chance of ending up in the Inner Oort Cloud is small, only about 2%. So lots of objects must have been tossed around to get one to "stick".
Interestingly, way back in 1991, Alan Stern (then at the University of Colorado) postulated that the Kuiper Belt and Oort cloud should contain roughly 1,000 objects the size of Pluto, numerous Mars-sized objects, and potentially a few biggies potentially with masses of up to several times Earth's.
So, can this modern-day "Planet X" be found? Not directly, it seems. Something that far out and having a dark surface (which is most likely) would be too faint to sweep up by any deep telescopic survey now under way.
What about NASA's Wide-Field Infrared Survey Explorer (WISE)? It's especially good at detecting the heat given off by dark objects. But 250 a.u. is too far away to get much warmth from the Sun. A recent analysis of WISE observations rules out anything larger than Jupiter or Saturn in extremely distant solar orbits. Kevin Luhman (Penn State University), who published his assessment last January, thinks WISE would not spot anything as small as Uranus or Neptune closer in.
"This is too faint for WISE," notes project scientist Ned Wright (University of California, Los Angeles). "Earth has a small internal heat output, about 10,000 times less than Jupiter, so a super-Earth would probably heat itself to about 40 K, too cold for WISE to detect."
"We just have circumstantial evidence, so we need to find more of these Sedna-type objects," explains Sheppard. "We do have several more candidates in our ongoing outer-solar-system survey."
Meanwhile, Levison is eager to start digging deeper into the intriguing orbital alignments of 2012 VP113, Sedna, and their kin — either to uncover alternate perturbation schemes or to strengthen the theoretical case for a distant planet-mass instigator.
For more information, you can read Carnegie's press release or, better still, Sheppard's website devoted to the Inner Oort Cloud. And check out the special interactive app by Ron Baalke (Jet Propulsion Laboratory) for displaying the orbits of Sedna and 2012 VP113 together.
C. Trujillo and S. Sheppard, "A Sedna-like body with a perihelion of 80 astronomical units" Nature, March 27, 2014.