Planet Nine – 3 Years on
There is no doubt that the scientific advocacy for a Planet X body has been significantly strengthened by the work of Brown and Batygin, who published their first paper about ‘Planet Nine’ three years ago (1). Mike Brown is a renowned astronomer in academic circles, whose speciality is hunting down distant Kuiper Belt Objects and dwarf planets in the outer solar system.
Following on from the work of Trujillo and Sheppard (2), he and his Caltech colleague Konstantin Batygin studied odd similarities in the orbits of distant scattered disk objects (SDOs) which lie beyond the regular Kuiper belt. Certain orbital properties of these eschewed objects seemed to be gathered into place within a common clustering, and the astrophysicists determined that something massive located well beyond them must have been responsible for shepherding these objects into such a serendipitous arrangement. They advocated a renewed search for Planet X, which had been confined to the doldrums of astronomy for decades, and re-branded the object ‘Planet Nine’ (3).
Planet Nine is thought to be a super-Earth object, upwards of 10 Earth masses. Searches for exoplanets have determined that such planets are common enough elsewhere, but, so far as we know, absent from our own shores within this solar system. Planet Nine is likely twenty times further away from us than Pluto, maybe more, and how such an object could have ended up so far away from the rest of the planets has vexed scientists. Of course, it remains hypothetical, because, despite the observational strength of modern day astronomy, Planet Nine has not been located. Its position is unknown (beyond ruling out certain sections of the sky), as its existence can only be inferred from the clustering data, but not determined directly from it.
Despite its 3-year long ‘no-show’, Brown and Batygin stand by their initial paper, and have published a follow-up paper this month to continue to argue their case (4). It primarily responds to the arguments raised by scientists working for the Outer Solar System Origins Survey (OSSOS) who found similar objects which did not seem to belong to the P9 cluster (5), and who went on to argue that the evidence for Planet Nine should be dismissed due to inherent observational bias in the data (6). At the time, Batygin quickly refuted that criticism, finding more patterns in the outer solar system snow, although I wondered whether the OSSOS data may be opening up another issue entirely about Planet Nine’s argued-for position (7). Anyway, Brown and Batygin’s new paper presents their subsequent work about the issue of observational bias, and offers a robust analysis leading to following conclusion:
“From this now more complete understanding of the biases, we calculate that the probability that these distant KBOs would be clustered as strongly as observed in both longitude of perihelion and in orbital pole position is only 0.2%. While explanations other than Planet Nine may someday be found, the statistical significance of this clustering is now difficult to discount.” (4)
Having rebutted their critics on one front, the Caltech team face another problem this month, this time in the form of an alternative explanation for the clustering anomalies proposed by researchers from the University of Cambridge. This new hypothesis involves the possible existence of a very significant disk of objects beyond the Kuiper belt, with a combined mass of 10 Earth masses, or perhaps less.
This massive ring of material would be eccentrically inclined to the invariant plane of the planets. The astrophysicists’ calculations and simulations show that such a massive eccentric disk might have the gravitational pull to create the observed clustering of extreme SDOs (8), but the mass required represents a couple of orders of magnitude of mass greater than the known Kuiper belt. Arguing that studies of other young star systems show extended debris disks, the authors seem quietly confident about the potential existence of such a massive extended disk:
““If you remove planet nine from the model and instead allow for lots of small objects scattered across a wide area, collective attractions between those objects could just as easily account for the eccentric orbits we see in some TNOs,” said [Antranik] Sefilian, who is a Gates Cambridge Scholar and a member of Darwin College.” (9)
Brown points out that it’s unusual in science for a new hypothesis – in this case the proposed existence of Planet Nine to explain the observed clustering of SDOs – to not face a barrage of counter-hypotheses. For some reason, all of the attention up until this point has been focussed upon the statistical credibility of the cluster properties. Brown acknowledges the new Cambridge paper is the first stab at an alternative explanation for the extended SDO cluster (11). In fact, a similar explanation has already been offered within academic circles, by a group based in Colorado led by Ann Marie Madigan last summer. The Colorado group argued that a significant amassed collection of distant asteroids could explain the observed anomalies (10). Mike Brown explains the difference between these two papers: “…although the hypotheses sound similar, they are really totally unrelated. The one from last summer doesn’t actually explain…what we see. This one, at least, does.” (12)
Despite actually offering what appears to be a mathematically credible explanation, Brown is sceptical of the new Cambridge paper on a couple of fronts: (1) The required mass of the disk (as above), and (2) its provenance (11). How could such a warped extended disk have been shaped in the first place? This raises another vexed question about how such a weird disk came to be, which flies in the face of the Cambridge authors’ claim to have provided a simpler explanation than Planet Nine.
It is known that the invariant plane of the planets is warped away from the solar equatorial plane by about 6-7 degrees. Planet Nine, on the other hand, is likely to be inclined by about 30 degrees, and may itself represent an explanation for this warping, should its mass be significant enough to have shaped the rest of the solar system in this way. One of the several strands of evidence pointing towards the existence of a Planet Nine/X body is the ~6 degree tilt of the invariant plane of the planets away from the Sun’s own equatorial plane. In other words, like the Earth, the Sun’s axis is tilted away from the plane of the planets.
All things being equal, the Sun and the planets should have formed out of a common rotating disk of primordial matter – the coalescing pre-solar nebula. It’s understandable that many of the planets engaged in a bit of to-and-fro during the early period of planet-forming, and so ended up a little skewed. But the Sun is the dominant player, and it should take a considerable gravitational influence to draw the planets away from its own equatorial plane. Yet, the Sun is seemingly a lonely star. So, that pesky 6 degree tilt has to be explained by something. Maybe a passing star pulling at the planets at some point in the past; or maybe the Sun had an early companion (within its birthing dense core) which affected the system’s alignment; or maybe another significant planet strongly inclined to the ecliptic, influencing the others over time (13).
Studies of protoplanetary disks in young star systems is revealing similar warps elsewhere. The latest case concerns a very young single protostar system known as L1527. This system is so young that there is an implication that the warping may be occurring in the primordial cloud itself (14). The disk in question is effectively in two parts, where the warping issue affects the inner disc out to some 40-60 AU from the star (15). In the perceived absence of a companion object causing this effect, it is thought that the gravitational effect of the cloud itself is causing the warp in the protoplanetary disk.
But here’s the thing: Just because there isn’t a self-evident, luminous companion object near to L1527, doesn’t mean that there isn’t a darker companion lurking around somewhere nearby, tugging at the disk. It has been suggested that all stars begin life within dense cores, containing at least two protostars (16). In turn, this has implications about the potential for failed stars being ubiquitous companion objects (17). So, maybe L1527 does have an unseen sub-stellar companion affecting the structure of its inner disk.
Another item of interest to add is news about another misaligned disk, this time around a young binary star system (19). In this case, the disk orbits at right angles to the orbit of the two stars which make up the binary HD 98800, meaning that the disk is in a perpendicular polar misalignment (19). Furthermore, the authors state that despite the extreme misalignment, the disk itself has physical properties similar to those around single stars, including, therefore, potential planet forming conditions.
So it is clear that such warped arrangements are by no means confined to the solar system, can be pretty extreme, and can appear very early on in the lifetime of a star system. What’s less clear is why they arise in the case of sible star system, seemingly minding their own business. Some kind of distant, dark companion object pulling at the rest of the system seems a reasonable enough explanation – one that was already present, or co-forming, within the stellar birth cluster. Perhaps that might be a body the size of Planet Nine (a proposed super-Earth), perhaps something bigger still.
So, happy third birthday, Planet Nine! You may still be a mere twinkling in a Californian astronomer’s eye, but you’ve already evoked a modern renaissance in the history of Planet X.
Written by Andy Lloyd, 24th January 2019
1) K. Batygin & M. Brown “Evidence for a Distant Giant Planet in the Solar System” The Astronomical Journal, 20 January 2016, 151(2)
2) Chad Trujillo & Scott Sheppard “A Sedna-like body with a perihelion of 80 astronomical units” Nature, 27 March 2014, 507: 471-474, http://www.nature.com/nature/journal/v507/n7493/full/nature13156.html
3) Andy Lloyd “Massive Planet X Now Urgently Sought by Top Planet-Hunters” 20-23 January 2016, http://www.andylloyd.org/darkstarblog34.htm
4) Michael Brown and Konstantin Batygin “Orbital Clustering in the Distant Solar System” The Astronomical Journal, 22 January 2019, 157(2)
5) Cory Shankman et al. “OSSOS VI. Striking Biases in the detection of large semimajor axis Trans-Neptunian Objects”, 19th June 2017, The Astronomical Journal, 14 July 2017, 154(2)
6) Josh Sokol “New haul of distant worlds casts doubt on Planet Nine”, 21st June 2017 http://www.sciencemag.org/news/2017/06/new-haul-distant-worlds-casts-doubt-planet-nine
7) Andy Lloyd “Planet Nine: Are They Digging in the Wrong Place?” 3 July 2017 http://andy-lloyd.com/planet-nine-digging-wrong-place/
8) Antranik Sefilian and Jihad Touma. ‘Shepherding in a self-gravitating disk of trans-Neptunian objects.’ The Astronomical Journal 21 January 2019, 157(2) https://iopscience.iop.org/article/10.3847/1538-3881/aaf0fc/pdf
9) Sarah Collins “Mystery orbits in outermost reaches of solar system not caused by ‘Planet Nine’, say researchers” 21 January 2019 https://www.cam.ac.uk/research/news/mystery-orbits-in-outermost-reaches-of-solar-system-not-caused-by-planet-nine-say-researchers
10) Daniel Strain “Collective gravity, not Planet Nine, may explain the orbits of ‘detached objects'” 4 June 2018 https://www.colorado.edu/today/2018/06/04/collective-gravity
11) Mike Brown “Is Planet Nine just a ring of icy bodies?” 22 January 2019 http://www.findplanetnine.com/
12) @plutokiller replying to @darkstarandy, 24/1/19
13) Andy Lloyd “Does Planet Nine Solve the Riddle of the Sun’s Obliquity?” 30th July 2016 http://andy-lloyd.com/planet-nine-solve-riddle-suns-obliquity/
14) RIKEN Press Release “Early protostar already has a warped disk” 1 January 2019, http://www.riken.jp/en/pr/press/2019/20190101_1/
15) Nami Sakai et al. “A warped disk around an infant protostar” Nature, 31 December 2018, https://www.nature.com/articles/s41586-018-0819-2
16) Sarah Sadavoy & Steven Stahler “Embedded Binaries and Their Dense Cores” MNRAS, 21 August 2017, 469(4): pp3881–3900
17) Andy Lloyd “The Sun was Born with a Companion” 15 June 2017 http://www.andylloyd.org/darkstarblog51.htm
18) Peter Thorley “Double star system flips planet-forming disk into pole position” 14 January 2019 https://warwick.ac.uk/newsandevents/pressreleases/double_star_system/
19) Grant Kennedy et al. “A circumbinary protoplanetary disc in a polar configuration” 15 january 2019 Nature Astronomy Letters, https://www.nature.com/articles/s41550-018-0667-x.epdf with thanks to Lee