New Simulations Point to Oort Cloud Disturbance in Gemini

The shard-like asteroid from deep space which shot through the solar system last years, known as ‘Oumuamua, set many an astronomer’s heart racing.  The peculiar body was determined to be the first confirmed interstellar asteroid to have been observed (1).  It’s possible, though, that other comets which pursue so-called hyperbolic orbits (moving fast enough to escape the solar system) also have an interstellar origin, rather than having originated from the Oort Cloud.  A team of Spanish astrophysicists, who have more than a passing interest in the topic of Planet X, have performed powerful computer simulations to build up a picture of the trajectories and spatial origins of various hyperbolic comets (2).  The objects they chose to consider have inbound velocities greater than 1km/s

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)

This new work could stoke up this contentious argument still further, to the dismay of astronomers who might have hoped this whole subject had been put to bed long ago.  There is also the potential for more red dwarf flybys emerging from the cosmic woodwork over time, too.  There has been a reliance upon the Hipparcosastrometric catalogue to make systematic searches for such objects in our galactic neighbourhood.  The problem with this is that this catalogue seems to contain a relatively small number of M dwarfs compared to their known galactic population.  Increased use of another astrometric catalogue, known as Gaia, should pull up more of these neighbouring, dim red dwarfs which Hipparcos has missed (3).  Perhaps the use of Gaia will bring out more besides, like ultracool brown dwarfs in our immediate neighbourhood?

Written by Andy Lloyd,  9th March 2018

References:

1)  Andy Lloyd “‘Oumuamua’s Many Cousins” 21st January 2018, http://andy-lloyd.com/oumuamuas-many-cousins/

2)  C. de la Fuente Marcos, R. de la Fuente Marcos & S. J. Aarseth “Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies.” 2nd February 2018,https://www.researchgate.net/publication/322899125_Where_the_Solar_system_meets_the_solar_neighbourhood_patterns_in_the_distribution_of_radiants_of_observed_hyperbolic_minor_bodies

3)  Eric Mamajek et al “The Closest Known Flyby of a Star to the Solar System” The Astrophysical Journal Letters, 2015, 800(1): L17

4)  Anil Ananthaswamy “Stellar intruder’s daring fly-by of the solar system” 19th February 2015 https://www.newscientist.com/article/dn27004-stellar-intruders-daring-fly-by-of-the-solar-system/

5)   Nick Strobel “Kepler’s Laws of Planetary Motion” 6th April 2010 http://www.astronomynotes.com/history/s7.htm

6)  C. Bailer-Jones “Close encounters of the stellar kind” Astronomy & Astrophysics, 19th February 2015, 575: A35

7)  J. Matese, P. Whitman & D. Whitmire “Cometary evidence of a massive body in the outer Oort cloud” Icarus, 1999, 141, pp354-336

8)  J. Murray “Arguments for the presence of a distant large undiscovered Solar system planet”, Mon. Not. R. Astron. Soc., 1999, 309, pp31-34

9)  M. Davis, P. Hut & R. Muller “Extinction of species by periodic comet showers”. Nature, 1984, 308 (5961): pp715–717

10)  Correspondence from Carlos de la Fuente Marcos, 8th March 2018

 

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