Meet Planet Nine
Researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun.By comparison, Jupiter is about 317 times the mass of Earth, Saturn is about 95 times the mass of Earth, Neptune is 17 times the mass of Earth, and Uranus is about 15 times the mass of Earth, which is the next most heavy planet. The Sun, in contrast, has a mass about 333,000 times the mass of the Earth. So, Planet Nine should have a mass on the same order of magnitude as Neptune and Uranus, but should be much smaller than Saturn and Jupiter.
The researchers, Konstantin Batygin and Mike Brown, discovered the planet's existence through mathematical modeling and computer simulations but have not yet observed the object directly.
If it is a moderate sized gas giant would expect it to have a mean radius of about 15,000-16,000 miles or so, gravity that might be quite comparable to Earth gravity. But, if it is a rocky planet (like the core of Jupiter or Saturn beneath their clouds and oceans) it could be roughly half the radius and have a surface gravity closer to four times the gravitational pull on the surface of the Earth.
It would have a surface temperature of less than 70 degrees Kelvin (colder than liquid nitrogen), unless it is generating its own heat through some kind of nuclear process insufficiently powerful to cause it to become a star (something similar makes the magma at the center of the Earth a liquid). This is a temperature low enough that many "conventional" superconductors would start to display their superconducting properties.
The authors suspect, for reasons set forth in the final section of their paper, that Planet Nine "represents a primordial giant planet core that was ejected during the nebular epoch of the solar system's evolution." In other words, it is probably about four billion years old and has been with us from the very early days of the solar system.
A Once In A Lifetime Discovery
To give you an idea of how far out Planet Nine must be, the Planet Neptune takes about 165 Earth years to rotate around the sun. Planet Nine would take roughly a hundred times as long to do so.
"This would be a real ninth planet," says [Mike] Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy. "There have only been two true planets discovered since ancient times, and this would be a third. It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting."The last such discovery was 150 years ago.
Pluto used to be called the Ninth Planet, but is so small that it has been demoted to dwarf planet status. Dwarf planet sized objects (and some objects that are smaller) that are very far from the sun all called Kuinper Belt Objects, such as Sedna.
Brown has discovered more dwarf planets than anyone who has ever lived and now may land the discovery of the only remaining true planet left in the solar system.
Is It Real? Why Should It Exist?
The linked Science Direct article (really a Cal Tech press release) is quite rich in explaining how the discovery came about and what evidence supports their conclusion, yet very readable. I came away from it convinced that they are almost certainly correct despite not having directly observed it yet.
The discussion in the initial section of the paper demonstrates how deep the recent literature is on explaining the various phenomena that Batygin and Brown have finally appeared to crack with their Planet Nine hypothesis.
The physics of the solar system can be calculated very precisely because they involve just a single force (gravity) operating with extremely little friction, according to classical mechanics, in a weak gravity regime where general relativity makes only slight corrections to the very simple Newtonian GMm/r^2 force rule.
The general relativity perturbations whose magnitude can be calculated with great precision in principle. But, the general relativity effect is very small, because Planet Nine, unlike Mercury whose perihelion is tweaked slightly from the Newtonian expectation by general relativity due to its proximity to the strong gravitational field of the Sun, and the direction of the general relativity effect can be calculated much more easily than the full exact general relativity calculation with multiple bodies. So, one can add a one directional error bar for systemic differences between general relativity and Newtonian gravity that is quite small to the Newtonian prediction. Indeed, the correction is probably dwarfed by other experimental uncertainties in the astronomy observations of the solar system objects used an inputs in the model of the solar system used to make the prediction.
So, basically, one is left making predictions using a computer model constructed using only high school physics and calculus and a wealth of available data points on all of the known masses in the solar system, that is still phenomenally accurate to the limits of the precision of state of the art telescope measurements of solar system objects. (The initial analysis of the multi-body gravitational dynamics that flow from Newtonian gravity is done not using this "dumb" N-body simulation method, but with a more advanced mathematical physics concept known as a Hamiltonian which is an equation that adds up the potential and kinetic energies of all of the objects in a system which must stay constant due to the conservation of energy that has been known in its current form for solar system dynamics since at least 1950. But, it all flows from applying high school physics and calculus to this complex multi-body situation.)
Convincingly, a lot of seemingly unrelated properties of Kuinper Belt Objects, including some that were not the basis of the original formulation of the model are all consistent with a Planet Nine hypothesis.
For example, it turns out that the trick to making these models work with a Ninth Planet that influences the dynamics of a lot of Kuinper Belt objects is for a key property of the Ninth Planet orbit and of the affected Kuinper Belt object fall quite exactly into relatively small integer ratios of each other such as 2:1, 3:1, 5:3, 7:4, 9:4, 11:4, 13:4, 23:6, 27:17, 29:17, and 33:19, because if they lack a common denominator, the objects orbits will never collide even though their orbits overlap (tiny friction effects due to things like tidal effects on the shape of the objects eventually probably would collider, but over time periods too long relative to the age of the solar system for this to have actually happened). Remarkably, this turns out to be the case.
Similar objects have been observed around other stars, but not to date, around our own.
Why Hasn't It Been Directly Observed?
It appears that a big barrier to observation is that its location has been pinned down only to a particular, very long orbital path around the Sun (which is also quite wide due to margins of errors in the astronomy measurements and calculation uncertainties), rather than to a specific location. Also its great distance from the Sun means that it is not illuminated strongly (and like all planets does not make its own light) and due to its distance and not super huge diameter (compared, for example, to large gas giants and stars) should be only a tiny, almost point-like object in the night sky.
Depending on the albedo (i.e. reflectivity) of Planet Nine's surface, it may not even be visible via a telescope at all except when it obscures some other known object like a star by passing between it and the Earth. Gas giants and planets with atmospheres like Venus reflect 40-65% of the sunlight that hits them, and gas giants are also larger (because the density of gases and liquids is lower than a rocky core), but rocky objects without much in the way of atmospheres like Mercury, the Moon and Mars reflect only 10%-15% of the light that hits them and also have about half the radius of a gas giant of comparable mass. So, a rocky Planet Nine would reflect only about 6% or so of the light of a gas giant Planet Nine comparable to Uranus or Neptune, making it significantly harder to detect directly. Since a planet like this is basically unprecedented in the solar system, there is no really strong reason to favor a rocky Planet Nine hypothesis over a gas giant Planet Nine hypothesis which would be very similar to Uranus and Neptune but significantly colder.
Has Planet Nine Been Seen Already? Probably Not.
Maju notes at his blog a pre-print purporting to have possibly observed a new planet which may or may not be related. The Vlemmings, et al. paper that he notes states that "we find that, if it is gravitationally bound, Gna is currently located at 12−25 AU distance and has a size of ∼220−880 km. Alternatively it is a much larger, planet-sized, object, gravitationally unbound, and located within ∼4000 AU, or beyond (out to ∼0.3~pc) if it is strongly variable." The Liseau preprint to which he links also appears to be describing the same object using the same data.
Neptune is about 30 AU from the Sun, so according to the press release, Planet Nine's orbit should be about 600 AU from the Sun. This isn't a good fit for the object described by Vlemmings, although without a closer read of the preprints it is hard to see what assumptions were made in order to rule out the possibility definitively. If the Vlemmings paper is observing a new planet, at any rate, it is probably not observing Planet Nine.
The paper open access paper and its abstract as as follows:
Recent analyses have shown that distant orbits within the scattered disk population of the Kuiper Belt exhibit an unexpected clustering in their respective arguments of perihelion. While several hypotheses have been put forward to explain this alignment, to date, a theoretical model that can successfully account for the observations remains elusive.
In this work we show that the orbits of distant Kuiper Belt objects (KBOs) cluster not only in argument of perihelion, but also in physical space. We demonstrate that the perihelion positions and orbital planes of the objects are tightly confined and that such a clustering has only a probability of 0.007% to be due to chance, thus requiring a dynamical origin.
We find that the observed orbital alignment can be maintained by a distant eccentric planet with mass greater than approximately10 m⊕ whose orbit lies in approximately the same plane as those of the distant KBOs, but whose perihelion is 180° away from the perihelia of the minor bodies.
In addition to accounting for the observed orbital alignment, the existence of such a planet naturally explains the presence of high-perihelion Sedna-like objects, as well as the known collection of high semimajor axis objects with inclinations between 60° and 150° whose origin was previously unclear.
Continued analysis of both distant and highly inclined outer solar system objects provides the opportunity for testing our hypothesis as well as further constraining the orbital elements and mass of the distant planet.Konstantin Batygin and Michael E. Brown, "Evidence for a Distant Giant Planet in the Solar System." Astronomical Journal (January 20, 2016); DOI: 10.3847/0004-6256/151/2/22.
Hat tip to Maju for altering me to the discovery in the comments on another post.
Michael Brown's website, or his blog, which is in the sidebar, make no mention of the discovery (perhaps due to a publication embargo, or perhaps because he simply no longer maintains either of them).