Following adjustment for the Sun's own movement through space towards the Solar Apex, interstellar visitors would likely have a more or less random distribution to their radiants (the position in the sky from which they came, rather like meteor showers striking the Earth's atmosphere).  The Spanish team carried out statistical analysis on the emerging sky maps of these radiants, and looked for patterns or clusters of these origin points.  Statistically significant patterns did indeed emerge from the data.  A particularly large source was located in the zodiacal constellation Gemini.  Such a clustering might indicate a number of possibilities, which the astrophysicists explore in their paper. 

One possibility is a close flyby of a star in the past which could have disrupted the outer edges of the distant Oort Cloud, sending comets in-bound towards the Sun.  Looking at the tracking of candidate flybys in the (by Cosmic standards) relatively recent past, Carlos de la Fuente Marcos, Raul de la Fuente Marcos & S. J. Aarseth argue that there is a possible correlation between this cluster of hyperbolic orbit radiants in Gemini, and a close flyby of a neighbouring binary red dwarf system known as Scholz's star some 70,000 years ago (2).  At a current distance of about 20 light years, Scholz's star may be a close neighbour to the Sun relatively speaking, but even so it took a while for it to be discovered. This was probably because of a combination of factors:  Its proximity to the Galactic plane, its relative dimness, and its slow relative movement across the sky (3).  Its distance was less than a light year 70,000 years ago, and its rapid movement away from us in the intervening time helps to explain why it was difficult to detect as a neighbouring binary star:  Its retreating motion is mostly along our line of sight, making it difficult to differentiate from background stars.

Other patterns that emerged from an analysis of the radiants of hyperbolic comets may have the same, or other explanations:

"It is difficult to attribute to mere chance the near coincidence in terms of timing and position in the sky between the most recent known stellar fly-by and the statistically significant overdensity [picked up in the study]. It is unclear whether other clusterings present may have the same origin or be the result of other, not-yet-documented, stellar fly-bys or perhaps interactions with one or more unseen perturbers orbiting the Sun well beyond Neptune." (2)

Note that there is another possibility that they consider:  An embedded planetary object among the comets.  Perhaps that is also a possibility for the Gemini clustering too?  I say this because a quick review of the stellar flyby hypothesis for Scholz's star picks up the fact that comets jilted into a Sun-crossing orbit by the visiting red dwarf would take a very, very long time to get here:

"The visitor would have perturbed comets in the outer Oort cloud, but those comets’ orbits won’t take them near the sun for hundreds of thousands, perhaps even millions of years." (3)

Seventy thousand years seems like a long time, but it may not be enough to witness the arrival of perturbed long-period comets from the very outer realms of the solar system.  That said, we are talking about hyperbolic trajectories here, rather than the usual, calmer parabolic ones.  So, perhaps in the case of these rapidly moving objects, there has been time enough? 

My back-of-an-envelope calculation shows that if 1I/'Oumuamua travelled at the same rough velocity as it did when it was observed over the last 70.000 years, that it could have covered about 6 light years in that time.  This is, of course, further than the nearest star.  But that would assume that 1I/'Oumuamua was travelling in a straight line, like a cosmic bullet, for all that time.  Even if it was on that kind of cosmic trajectory, and had come under the influence of the Sun, then there would be an additional sling-shot effect at play at the point when we astronomers observed it at perihelion, which would have speeded its pace up considerably.  Further, if this was a long-period comet which had been given a gravitational nudge by a passing star, then its motion would not be a simple straight line velocity through space either.  According to Kepler's 3rd law, comets speed up when closer to the Sun, nearing perihelion, and then slow down when moving back toward their most distant, aphelion position (4).  So, a perturbed comet from the outer Oort cloud would take a lot longer to get here than my back-of-an-envelope calculation suggests.  A stellar flyby would not cause a perturbed long-period comet to come shooting straight at us like a bullet from a gun.  Instead, it would take a more leisurely route around the celestial houses.

If it was not Scholz's star that caused this particular disturbance in Gemini, then older stellar flybys could have set these cosmic wheels in motion.  There is something of a back-catalogue of stars which likely invaded the Sun's personal space at some point over the last few million years (5).

Or perhaps we are returning to similar arguments put forward by the likes of John Matese and Daniel Whitmire (re Tyche) (6), John Murray (7), and even Richard Muller and company (regarding our old friend 'Nemesis') (8).  Dr de la Fuente Marcos put to it me this way:

"As explained in our work, a significant fraction of the objects with inbound velocities above 1 km/s have radiants observed projected towards the area we mention in Gemini. We argue that this may be consistent with the most recent known stellar encounter, but we do concede that other explanations are possible, including the presence of yet-to-be found perturbers (bound to the Sun)." (9)

Nonetheless, the 'L/T transition state' is a variable zone where the properties of 'inbetweener' planetary-mass brown dwarfs remain very difficult to predict.  Multiple factors are at play, including complicated chemistry within the clouds and weather variations across the brown dwarf's atmosphere.  So when does this patchiness begin?  At what temperature; at what age; at what colour, even?

"“L dwarfs” are hot brown dwarfs, which range in effective temperature from ~1400 K to just over 2000 K. They have significant cloud decks—made of iron and rock!—which blanket the surface in a dusty layer. Brown dwarfs cool as they age, and eventually become “T dwarfs.” These T dwarfs are colder brown dwarfs (under ~1200 K). They seem to have clear atmospheres, without a significant cloud layer.

"Objects transition very quickly from being dusty L dwarfs to clear T dwarfs. While brown dwarfs at 1400 K are covered with thick clouds, brown dwarfs that are just 100-200 K cooler are cloud-free. But we don’t know how exactly this happens. Do the clouds just sink down below the visible atmosphere? Do they break up and become patchy?" (4)

Dr Brown seemed optimistic that his 'Planet Nine' would be discovered around this time - to solve its 'existential difficulties' as he put it recently (2).  But Planet X has proven an elusive object for nigh on 150 years of searching, and it seems that it is not about to give its location away just yet - assuming, of course, that it does actually exist in the first place.  In the meantime, ground-based telescopes located in the southern hemisphere could quite easily continue the search over the coming summer.  That's if anyone else is actually still looking for it, that is:  Although much of the sky has been effectively ruled out as a potential location for Planet Nine, the areas where it may yet lurk remain vast.  There's no guarantee that Mike Brown's favoured spot, which has not changed despite various published papers further constraining Planet Nine's position, is correct.  Looking for a needle in a haystack involves first locating the right haystack...

There is more bad news on the way for Planet Nine, this time in the form of results from a search for the object in the WISE and NEOWISE databases.  The search focuses upon a particular infra-red wavelength, and covers up to 90% of the sky. 

"In the present work, we extend our 3.4µm (W1) search to cover more than three quarters of the sky and incorporate four years of WISE observations spanning a seven year time period. This represents the deepest and widest-area WISE search for Planet Nine to date." (3)

Having dismissed a number of possible matches, the astronomers conducting the search concluded that, within those bounds, Planet Nine is simply not there.  However, this bleak conclusion is the polar opposite of Brown's statistical analysis of the chances of there being an additional sizeable planet out there.  He continues to stand by the theoretical work which underpins the existence of Planet Nine, as described in this month's issue of Scientific American:

"In the months following the announcement, many of Planet Nine’s most fervent seekers (Brown chief among them) predicted it would probably be found by the end of the following winter—that is, by now. In January of this year Brown was still bullish: Based on “statistically rigorous calculations” incorporating all the available data, there is only a one-in-10,000 chance the planet is not out there, waiting to be found. In other words, Brown’s best guess is Planet Nine has a 99.99 percent probability of being real." (4)

I wrote to Mike Brown this week, just to give him the opportunity to say a few words for the new (quite scientific) Dark Star book. As of yet, no reply.  This is not uncommon.  Most of the astronomers I write to (generally to ask them about aspects of their published work) do not reply.  This is frustrating for me, but they're busy people and, well, I'm not exactly a big-time journalist or editor.  Also, there's the whole Nibiru thing.  Nonetheless, I would welcome the chance at some point to flesh out the reason why I think this object defies efforts to detect it.  If nothing else, it would be worth putting these unusual ideas to the test.