Three New Objects Extend Hunt for Planet X

The two scientists, Scott Sheppard and Chad Trujillo, who first recognised the clustering of objects thought to reveal the presence of ‘Planet Nine’ (1), have announced the discovery of three new objects.  All three are highly distant objects (2).  Two of them are extended scattered disk objects beyond the traditional Kuiper Belt, and fit reasonably well into the afore-mentioned cluster.  The third, perhaps even more amazingly, is an object whose elongated orbit reaches way out into the distant Oort Cloud of comets, but which also never comes closer than the planet Neptune.   So, this is the first outer Oort cloud object with a perihelion beyond Neptune, designated 2014 FE72.

Here’s how the announcement of these three new objects has been described in a press release from the Carnegie Institution for Science (3), where Scott Sheppard works:

The new objects they have submitted to the Minor Planet Center for designation include 2014 SR349, which adds to the class of the rare extreme trans-Neptunian objects. It exhibits similar orbital characteristics to the previously known extreme bodies whose positions and movements led Sheppard and Trujillo to initially propose the influence of Planet X.

“Another new extreme object they found, 2013 FT28, has some characteristics similar to the other extreme objects but also some differences. The orbit of an object is defined by six parameters. The clustering of several of these parameters is the main argument for a ninth planet to exist in the outer solar system. 2013 FT28 shows similar clustering in some of these parameters (its semi-major axis, eccentricity, inclination, and argument of perihelion angle, for angle enthusiasts out there) but one of these parameters, an angle called the longitude of perihelion, is different from that of the other extreme objects, which makes that particular clustering trend less strong.

“Another discovery, 2014 FE72, is the first distant Oort Cloud object found with an orbit entirely beyond Neptune. It has an orbit that takes the object so far away from the Sun (some 3000 times farther than Earth) that it is likely being influenced by forces of gravity from beyond our Solar System such as other stars and the galactic tide. It is the first object observed at such a large distance.” (4)

3newobjects

In their paper, the astronomers argue that the orbital properties of these objects provide further evidence for the existence of an unknown massive planet on an eccentric orbit.  The more objects fitting this pattern, the more unlikely it is that this is some kind of statistical ‘blip’ which can be explained away as coincidence.  They maintain that the configuration of this growing cluster of objects is ‘anti-aligned’ to the perturbing orbit of the Planet X objects shaping their collective anomalous trajectories.  The parameters of this as-yet undiscovered Planet X object (or Planet Nine, as named by Mike Brown and Konstantin Batygin in their influential paper published in January 2016 (5)) are becoming steadily more constrained with each new piece of evidence that comes to light.

 

An inner Oort Cloud object connected with Planet X

The discovery of 2014 FE72 is particularly remarkable.  It currently lies about 61 AU away, and achieved perihelion back in 1965 when it was just 36AU away – still quite a long way beyond Neptune, in the Kuiper Belt.  But its elongated orbit takes it out to about 3,800AU, within the inner Oort Cloud of comets, in a circuit that lasts a remarkable 86,000 years (6).  So, 2014 FE72 effectively transits between the inner Oort Cloud and the Kuiper Belt without ever entering the planetary zone of the solar system.  This places it into a quite different category of object than standard long period comets, and extends the range of the ‘Sednoids’ considerably.

This is obviously an amazing discovery, particularly given this object’s current distance.  But it doesn’t seem to fit readily into the pattern of Sednoids that make up the Planet Nine cluster.  I say this because this cluster is envisaged to be a collection of Kuiper Belt Objects which have been drawn up into extreme orbits by Planet Nine’s gravitational influence.  Yet, 2014 FE72’s huge orbit takes it well beyond the range of even Sedna: four times as far, if the figure are to be believed (and Sheppard and Trujillo are cautious about this orbit, given the short observation arc and extreme eccentricity that they’re working with (6)).  As they describe it (2), this object appears to be an inner Oort Cloud object.  Indeed, their press release indicates how this object’s aphelion distance makes it subject to external forces, like the galactic tide:

“Another discovery, 2014 FE72, is the first distant Oort Cloud object found with an orbit entirely beyond Neptune.  It has an orbit that takes the object so far away from the Sun (some 3000 times farther than Earth) that it is likely being influenced by forces of gravity from beyond our Solar System such as other stars and the galactic tide.” 32)

So then why include it in an academic paper whose title points towards Planet X?  The implication seems to be that the undiscovered planet has drawn this inner Oort Cloud object down into the solar system.  This, potentially, puts this object into a similar category as Niku and Drac, whose retrograde motions may imply a similar origin (7).  I think this emerging pattern may also indicate that Planet X (Planet Nine in Dr Brown’s parlance) is perturbing objects from both ends of its orbit: some up from the Kuiper Belt, some down from the inner Oort Cloud.  This, in turn, implies that its reach into the outer solar system is considerably longer than that envisaged by Brown and Batygin: either in terms of a greater orbital length, or greater mass – or both.

 

Aligned?

There’s a second issue which I’d like to address, which is made obvious by the diagram published alongside the press release.  One of the new objects, 2013 FT28, appears to be aligned with Planet X, not anti-aligned.  The authors allow that this object does not fit into the cluster as well as the other Sednoids, largely due to its quite different longitude of perihelion.  They also argue for the anti-alignment in their abstract (2), which places Planet X in the opposite side of the sky from the bulk of cluster.  Yet, it seems curious that 2013 FT28 seems to buck this trend.

 

An Exoplanet?

Journalist Mark Kaufman interviewed Scott Sheppard in the wake of this announcement, allowing us to gain some insight into the astronomer’s opinions about Planet Nine (which his team is actively hunting).  Sheppard estimates the probability that Planet Nine exists at about 80%.  He likens the current search to the run-up to the discoveries of other outer solar system planets – Uranus and Neptune – and the dwarf planet Pluto (8).  In terms of the origin of this proposed Planet X body, Sheppard’s working hypothesis is that it’s a rogue intruder from the planetary system of another star in the Sun’s initial birth cluster.

“If this turned out to be the case, then we’d know that there were other suns being formed nearby our sun,” Sheppard said. “It would have to be a very dense solar environment, and that would also tell us a lot about the formation of our solar system… Actually, it’s quite possible that the planet has already been in some way imaged.  That happened with Uranus, Neptune and Pluto — they were observed but not understood before they were actually detected.  Who knows, proof of Planet X {or Planet 9} may already exist in some observatory archive.” (8)

This reflects ideas about the origin of the purported Planet Nine first published by Alexander Mustill et al back in April (9).  They argue that stars are capable of stealing planets from each other in these early, slow moving star clusters, and that effectively Planet Nine initially began life as the offspring of one of the Sun’s sisters:

“A number of lines of evidence suggest that the Sun formed in a sizeable cluster of a few thousand stars , and previous dynamical studies have shown that orbiting bodies at large radii can be transferred between stars in the slow close encounters typical in open clusters.  It is indeed possible for the Sun to have captured such a planet from another star in a close encounter in its birth cluster.  Our study is complementary to the recent work of Li & Adams (10), who also identify capture in a cluster as a possible source for Planet 9.  Whereas these authors consider the capture of planets initially on circular or moderately eccentric orbits, we focus on a scenario in which the Sun captures a highly eccentric planet with a semimajor axis of several hundred au but a pericentre of ~10 au.” (9)

exoplanet

Both of these sets of workers find the chances of such a successful capture occurring to be 2% or less (much less, depending upon the constraints applied to Planet Nine orbital characteristics).  This seems a pretty bleak possibility, until one considers the considerably remoter probabilities attached to competing theories (i.e. how a sizeable planet could have formed out there way beyond Neptune, and the preliminary protoplanetary disk).

So, the capture of an eccentric rogue planet originating from outside the solar system.  My goodness, where have we heard that before?

 

Written by Andy Lloyd,  3rd September 2016

References:

1)    Chad Trujillo & Scott Sheppard “A Sedna-like body with a perihelion of 80 astronomical units” 27th March 2014, Nature, 507, 471-474, http://www.nature.com/nature/journal/v507/n7493/full/nature13156.html

2)  Scott Sheppard & Chad Trujillo “New Extreme Trans-Neptunian Objects: Towards a Super-Earth in the Outer Solar System” 31st August 2016 http://arxiv.org/abs/1608.08772

3)  Carnegie Science “Hunt for ninth planet reveals new extremely distant solar system objects” 29th August 2016, https://carnegiescience.edu/news/hunt-ninth-planet-reveals-new-extremely-distant-solar-system-objects

4)  Carnegie Institution for Science “Hunt for ninth planet reveals new extremely distant solar system objects” 29th August 2016 https://www.sciencedaily.com/releases/2016/08/160829093242.htm with thanks to Lee

5)   K. Batygin & M. Brown “Evidence for a Distant Giant Planet in the Solar System” 20th January 2016, The Astronomical Journal, Volume 151, Number 2, http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22

6)  ‘2014 FE72’ https://en.wikipedia.org/wiki/2014_FE72

7)  Andy Lloyd “Going the Wrong Way Round” 14th August 2016 http://www.andylloyd.org/darkstarblog41.htm

8)  Marc Kaufman “The Case Strengthens For “Planet 9”” 1st September 2016 http://www.manyworlds.space/index.php/2016/09/01/the-case-strengthens-for-planet-9/

9) Alexander Mustill “Is there an exoplanet in the Solar system?” 26th April 2016 MNRAS Letters Volume 460, Issue 1Pp. L109-L113, http://mnrasl.oxfordjournals.org/content/460/1/L109

10)  Gongjie Li & Fred Adams “Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine” 26th February 2016 http://arxiv.org/abs/1602.08496

Images Credit:  Robin Dienel, and NASA

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