YIR V: ‘Planet Nine’ – Pushing the boundaries of the solar system

by Chris Gebhardt

In 2016, our understanding of our cosmic home might have taken its most significant leap forward since the discovery of Uranus shattered our then perception of a six-planet system and expanded the solar system for the first time.  This year, in January came the announcement of a mathematical and orbital-characteristic inferred “discovery” – more a prediction based on evidence – of a massive, super-Earth, ice giant in a 200 x 1,200 AU orbit that, if confirmed, would represent the ninth planet in our solar system.

‘Planet Nine’ – a hypothetical find beyond the Scattered Disc:

First proposed in 2014 by Chad Trujillo and Scott S. Sheppard, who inferred the existence of a yet-unknown massive body from strikingly similar oddities in the orbits of two Trans-Neptunian Objects (Sedna and 2012 VP113), Planet Nine was thrust firmly into the public and planetary spotlight on 20 January 2016.

On that day, a second team of researchers – Michael Brown and Konstantin Batygin – announced a much more detailed explanation of Planet Nine, including its effect on six observed Trans-Neptune Objects (TNOs) and its proposed mass and orbital characteristics – all of which Brown and Batygin argue would explain the highly improbable configuration of a group of TNOs in the Scattered Disc.

The Scattered Disc is a group of icy objects in the outer reaches of the solar system.  By distance, portions of the Scattered Disc overlap with the Kuiper Belt; however, its limits extend farther above and below the ecliptic plane and well beyond those of Kuiper Belt.

In all, the new research and mathematical equations suggest that Planet Nine is 10 times more massive than Earth, has a diameter of two to four times that of Earth’s, and is located in a highly elliptical orbit with a period of 10,000 to 20,000 years and a semi-major axis (radius of the orbit at the orbit’s two most distant points) of 700 AU – 23 times farther from the Sun than Neptune.

Moreover, Planet Nine’s orbit is inferred to hold an aphelion of 1,200 AU and a perihelion of 200 AU with an inclination of 30° to the ecliptic plane – roughly in line with the other known TNOs with large inclinations to those observed elsewhere in the solar system.

At aphelion, Planet Nine would be in the general location of the Orion and Taurus constellations, while perihelion would place the planet in near Serpens, Ophiuchus, and Libra.

This mathematical and other-body observationally inferred discovery of a planet is not new to our exploration of the solar system.

Neptune’s visual discovery on 23 September 1846 was preceded by a mathematical prediction of Neptune’s existence and location based on observed perturbations in Uranus’ orbit.  

So precise, in fact, were Urbain Le Verrier’s calculations of Neptune that the planet was discovered within 1° of where Le Verrier predicted it would be on the very first night that observations to find the planet were undertaken.

If it’s there, where did it come from?

Importantly, based on current observations of the extreme outer solar system, the existence of such a massive planet like Planet Nine is possible.

A 2009 survey by NASA’s Wide-field Infrared Survey Explorer (WISE) concluded that a Neptune-sized object could exist at a distance greater than 700 AU; a 2014 study subsequently ruled out the existence of a Jupiter-mass planet to a distance of 26,000 AU – two discoveries that would not rule out Planet Nine’s exitance at its proposed mass, diameter, and distance.

If Planet Nine does indeed exist, there are currently four ideas regarding its creation and how it got to its present-day orbit.

The first, and most likely, explanation is that Planet Nine formed with Jupiter and Saturn and was ejected to its distant, eccentric orbit following close encounters with these two gas giants during the nebular epoch of the early solar system.

Once ejected, Planet Nine’s eccentricity, which was initially much greater, was reduced and its perihelion raised either by a process called dynamical friction (loss of momentum and kinetic energy of moving bodies through gravitational interactions with surrounding matter in space) from the gaseous remnants of the solar nebula that the Sun and solar system formed in, or by gravitational interactions with the other stars that formed in the same birth cluster as the Sun.

This would also serve to explain the mass and size of Planet Nine – as this type of ejection would have greatly halted Planet Nine’s development and left it at roughly the same mass as Uranus and Neptune.

Moreover, for the inferred orbit to be achieved under this model, Planet Nine would have to have been ejected between 3 to 10 million years after the formation of the solar system.

Importantly, this ejection of Planet Nine 3-10 million years after solar system formation would not detract from the Nice model of solar system development – which indicates that a transfer of energy from the outer disk of planetesimals left over from the initial formation of the solar system disturbed the fragile quadruple resonance of Jupiter, Saturn, Neptune, and Uranus.

This destabilization of the quadruple resonance resulted in an increasing eccentricity of the inner ice giant (seventh planet – in this case, Neptune) that subsequently pulled all four gas and ice giants outward from the Sun.

The further breakdown of the secular resonance between the inner ice giant and outer ice giant (Uranus) led to gravitational interactions between the two, which eventually flung Neptune out of its orbit and sent it hurtling through the outer disk.

As Neptune swept through the outer disk, it disturbed the planetesimals – forming the Kuiper Belt while sending some planetesimals even farther out, creating the Scattered Disc seen today.

At the same time, Neptune flung a large number of planetesimals toward the sun.  Some of these were grabbed by Jupiter (the gas giant’s trojans seen in its L4 and L5 Lagrange Points) while others continued into the solar system, accounting for the Late Heavy Bombardment between 4.1 and 3.8 billion years ago.

Thus, given the timing of the events believed to have led to the orbital realignment of the gas and ice giants and the Late Heavy Bombardment, Planet Nine would have been ejected 490 million to 697 million years prior – giving enough time for the four remaining gas and ice giants to harmonize and then destabilize.

Thus, Planet Nine would not be responsible for the Late Heavy Bombardment.

The second possibility of Planet Nine’s genesis is that it’s not native to our solar system.  Under this explanation, Planet Nine would have formed in the far reaches of its original parent system and in the same birth cluster as the Sun.

As the young stars and systems interacted, the Sun passed close enough to Planet Nine to “grab it” and “steal” it from its parent.

However, given the current orbit Planet Nine is understood to be in, this explanation is only 1-2% probable.

Conversely, Nine could have formed with the rest of the solar system in an extremely distant, circular orbit that was then perturbed to its current eccentricity when the solar system passed close to another star in its birth cluster.

This type of formation would radically reshape our understanding of the early solar system as the Sun would either 1) have to have possessed a massive disk of material far larger and more expansive than previously thought possible or 2) there would have to have been a major outward drift of solid material escaping the accreting disk that formed the eight planets.  This outward drifting material would then have had to form a ring in which Planet Nine would then have accreted.

Under this model, with such a distant and eccentric orbit, the odds that Planet Nine would not have been stolen by another star are only 10%, meaning the planet would have to have beaten extraordinary odds to remain with the solar system through its birth and dissolution of the Sun’s birth cluster.

Finally, the fourth possibility is that Planet Nine originated in the inner solar system and was ejected to its current location as Jupiter migrated inward from its formation orbit in the early development era.

This is highly unlikely though not improbable as the chance of such an object being ejected by Jupiter at just the right the velocity to wind up in a stable orbit beyond the Scattered Disc and not be thrown completely out of the solar system is only 2%.

Regardless, Planet Nine will greatly aid our understanding of early solar system development and could help answer the question of why our solar system has no planets with masses between Earth and Neptune (which is 17 times the mass of Earth) – an occurrence that makes our solar system rather unique among observed planetary systems to date.

How are we trying to find it?

While all of this conjecture of how Planet Nine formed and where it came from – as well as the effects it’s having on the extreme TNOs observed – is valid, nothing can be answered completely until the planet is actually found visually or by concrete methods of indirect detection.

To this end, given the extreme distance of Planet Nine, scientists are employing both indirect and direct methods of detection in a concerted effort to narrow the possibilities of where Planet Nine is in its orbit, where its gravitational effects might be seen, and what those gravitational effects could help us explain about the observed solar system.

Indirect detection:

One of the first instruments employed in the search for Planet Nine was the Cassini spacecraft in orbit of Saturn.

Since Cassini arrived at Saturn in 2004, the craft has taken very specific measurements of Saturn’s orbit of the Sun.  An examination of those measurements for unexplained perturbations – deviations – yielded results that, while neither proving nor disproving the existence of Planet Nine, helped scientists eliminate large portions of Planet Nine’s proposed orbit in which the planet could not be.

Simply put, Planet Nine’s gravity, if it were in certain portions of its orbit between 2004 and 2016 (the years bounding the Cassini data investigation), would have noticeable effects on Saturn’s position.

Since all of Saturn’s orbital parameters during this time were exactly as predicted, Planet Nine’s gravity could not have been affecting the massive ringed planet – thus demonstrating that Planet Nine could not be in certain locations of its proposed orbit.

This led to an understanding the Planet Nine could not be located in portions of its orbit bounded by a true anomaly (the angular parameter that defines the position of a body moving along a Keplerian orbit that is the angle between the direction of periapsis and the current position of the body as seen from the point around which it orbits) of -130° to -110° or -65° to 85°.

Further analysis of data from Cassini determined that Planet Nine was likely located at a true anomaly of 107.8° to 128.8° – which would place it at a distance of approximately 630 AU.

Moreover, another way to validate Planet Nine’s existence and further refine where in its orbit it could be is by finding additional extreme TNOs whose orbits could only be explained by a massive perturber (in this case, Planet Nine).

Scheduled for completion in 2023, the Large Synoptic Survey Telescope – which will be capable of mapping the entire sky in a just a few nights – will greatly aid our ability to detect these distant, small, extreme TNOs.

Under Brown and Batygin’s mathematical deductions of Planet Nine, its orbit, and its mass, the pair also find that if Planet Nine does exist, so too will a large population of extreme TNOs with semi-major axes greater than 250 AU and overall orbits with low eccentricities that are aligned with Planet Nine.

Moreover, further analysis released in May indicated that if Planet Nine is indeed in its predicted orbit, it would cause a tenfold increase to the number of extreme TNOs in high inclination and high-perihelion with moderate semi-major axis orbits – something which, if enough extreme TNOs in these kinds of orbits are found, would greatly lend evidence for Planet Nine’s existence.

But orbital studies of Saturn along with the search for more extreme TNOs aren’t the only indirect methods being used.

In fact, one of the other methods relates to an investigation into the mysterious spin-orbit misalignment of the solar system.

As observed, the Sun’s axis of rotation is tilted 6° from the orbital plane of the gas and ice giants.

Why this is remains a mystery; however, if Planet Nine – in its proposed orbit with its proposed mass – is factored in, the spin-orbit misalignment can be explained by the gravitational perturbations from Planet Nine on the Sun and the outer giant planets over billions of years.

While this does not directly or indirectly prove Planet Nine’s existence, it does lend more evidence toward its presence in the far reaches of the solar system – though other explanations for the spin-orbit misalignment are certainly possible.

Direct detection:

Given all of the analysis and predictions, Planet Nine could be spotted with existing visual and infrared telescopes.

Indeed, if the planet were located near its perihelion, existing surveys of the sky would contain the photographic proof of Planet Nine’s existence.

However, such searches turned up nothing, and the fact that Saturn’s orbit has had no observed perturbations since 2004 leads to a fairly conclusive indication that Planet Nine is near its aphelion – meaning it would be at its farthest and dimmest (in the visible spectrum).

Unfortunately, this would also place the planet in a section of the sky that is extremely complicated and star-rich – as Planet Nine’s aphelion crosses the plane of Milky Way, where light pollution from the galactic core makes visual detection methods difficult… though not impossible.

Nonetheless, an object of Planet Nine’s size and mass would have a temperature of roughly 47 K (-226° C ; -375° F).

At this temperature, it would be visible in the infrared spectrum and its radiation signature could be detected from Earth-based telescopes and easily detected by the James Webb Space Telescope – which is still just under two years out from launch.

And infrared detection is perhaps the best method of directly observing Planet Nine because of its distance. Near aphelion, Planet Nine is predicted to have an absolute magnitude of 22, making it 600 times fainter than Pluto.

Regardless, visual detection is still possible, and Brown & Batygin and Trujillo & Sheppard are each cooperatively using the Subaru telescope at the Mauna Kea Observatory on Hawaii in what is expected to be up to a five year search for Planet Nine.

Likewise, the Dark Energy Survey (optical and near-infrared) using the Victor M. Blanco Telescope in Chile is also searching for Planet Nine at the portion of its orbit in which data from Cassini indicates the planet may be.

Furthermore, a review of archival visual data from the Catalina Sky Survey (to apparent magnitude 19) and Pan-STARRS (to magnitude 21.5) was undertaken, but the data did not identify the planet.

Data from the WISE mission was also reviewed but returned no detections of the planet.

Other data points, sky surveys, and data are being reviewed as well – and new software to search this already collected information is being developed – to see if Planet Nine could have already been imaged without recognition of what it is.

Missing a planet in the visual record would certainly not be a first.  

Uranus was somewhat routinely observed before its official discovery in 1781.

This was mainly due to the fact that, while faint, it is the farthest planet visible to the naked eye and its long orbit produces a very slow motion across the sky.

Neptune, too, was observed before its discovery.  Galileo himself observed and plotted Neptune on 28 December 1612 and 27 January 1613, but because the planet had just begun its retrograde trek across the sky on the first day Galileo saw it, he mistook it for a fixed star.

Likewise, James Challis – working from a mathematical prediction of Neptune’s existence from John Couch Adams, a less accurate prediction than Le Verrier’s – observed Neptune on 4 and 13 August 1846 but did not recognize it for what it was.

So too was Pluto observed for its discovery.

The big question: is it a planet or a dwarf planet?

The simple answer is that it is extremely probable – with high certainty – that Planet Nine, if it exists, is indeed a planet under all three definitional requirements set by the International Astronomical Union (IAU).

According to Mike Brown, Planet Nine would contain a mass high enough to clear its immediate orbit over the course of 4.6 billion years of development and would thus dominate its immediate neighborhood.

Thus, it would 1) be a celestial object in orbit of the Sun, 2) have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes hydrostatic equilibrium (a nearly round shape), and 3) has cleared the neighborhood around its orbit.

With its current, TNO-observed inferred parameters, the only way Planet Nine would fall into the grey area between planet and dwarf planet is if it is the smallest of its inferred mass – still ten times that of Earth – and also located in the very farthest possible orbit for such a mass.

Under these circumstances, it is possible – though still not likely – that a debate about whether Planet Nine would then meet the critical third requirement for planethood could occur.

While the third criterion is the most controversial and argued portion of the planet definition (and the one that doomed Pluto while at the same time elevating Ceres, Eris, Haumea, and Makemake), it has since been widely interpreted to mean “gravitationally dominating its area”.

Under this understanding, Planet Nine would certainly qualify as a planet as it is already observed to dominate its area and the known objects around it.

However, should Planet Nine be located, and if its actual mass and/or orbit creates uncertainty toward the third criteria, the IAU would have to make a binding decision regard Planet Nine’s status.

*Click here for Part 1 of NASASpaceflight.com’s 2016 Year In Review*

*Click here for Part 2 of NASASpaceflight.com’s 2016 Year In Review*

*Click here for Part 3 of NASASpaceflight.com’s 2016 Year In Review*

*Click here for Part 4 of NASASpaceflight.com’s 2016 Year In Review*

(Images: Caltech; R. Hurt (IPAC); Mike Brown; Konstantin Batygin; NASA; JPL; Astronomy Journal; R. Gomes et al.; L. Calçada; ESO; M. Kornmesser; Museum Victoria; CNRS, Paris and Cote d’Azur observatories; A. Cuadra; National Astronomical Observatory of Japan; Esther Linder, Christoph Mordasini, University of Bern)

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