Adam Hibberd
I have woken up at some ungodly hour and what of all things should I be obsessing about? You guessed it – that weirdness of weirdnesses, to use a word employed many times by H. P. Lovecraft himself, the truly eldritch interstellar object known as ‘Oumuamua.
I occasionaly grasp for a mental image of this almost-lone extrasolar visitor to our abode, but I dare not fill in the unknown aspects of this celestial body to give me a full picture, though my imagination demands it, my scientific integrity simply does NOT allow it.
All we have is its strange set of observed characteristics, one of which being its tumbling motion. That got me to thinking, what exactly sets off a chaotic tumbling state in a body such as ‘Oumuamua? Many scientists believe the likely explanation is that ‘Oumuamua was struck by an object – possibly in its planetary system of origin, indeed many asteroids in our Solar System experience the same phenomenon and with the same explanation.
But why should ‘Oumuamua have left its natal planetary system in the first place? Possibly the collision was a cause of this, but I think that would be unlikely, more likely gravitational resonances or an encounter with a massive planet in its host planetary system was the cause, after all Jupiter is known to have done exactly this to comets in our own system.
But there is a BIG problem here and I shall endeavour to explain the logic below.
What happens when Jupiter ejects a body is that the body’s so-called hyperbolic excess (its speed reached at a great distance, in other words entering into interstellar space) is very small WITH RESPECT TO OUR OWN SOLAR SYSTEM.
Look at it this way, let’s say you throw a ball gently out of a moving train. It is clearly the case that, although the ball’s velocity relative to the train is small, relative to the ground that ball has a velocity which is almost precisely the velocity of the moving train, so anyone observing that ball on the ground would get a good idea of the velocity of the train by measuring the velocity of the ball.
This should also be the case for ‘Oumuamua, by measuring its velocity in interstellar space, we should be able to trace its velocity to its natal system. But therein lies a big mystery.
What we find is that ‘Oumuamua’s velocity in interstellar space was virtually zero – technically it was very close to the Local Standard of Rest (LSR), which is the mean velocity of all the stars in our vicinity as they rotate around our Galaxy’s centre. Going back to our train analogy, that means the train – or planetary system – it came from had almost zero velocity w.r.t. the LSR. What is the likelihood of that happening? My mind now is cast back to my time as a pupil at Stoke Park Comprehensive school where I impressed my chemistry teacher Dr Brooks with the following deduction.
We were studying what is known as the ‘Maxwell Distribution‘, this is what you get when you plot on the horizontal x-axis the speed of the molecules of a particular volume of gas and on the vertical axis the number of molecules in this volume which have this speed.
What one finds is that there is a peak speed, that is there is a MOST likely speed for a molecule and as the possible speeds increase, the number of molecules with these speeds reduce – in fact the graph decays and approaches zero to the right.
But what happens to the left of this peak in the curve – that is as the speed of the molecules decrease to zero?
What we find is that there is a similar decrease on the left side of the peak until the curve actually touches the horizontal axis, AT THE ORIGIN. Another way of looking at this is that the number of molecules with zero speed is actually zero. This is in line with the observation that the only way a gas could have ANY molecules with no speed is at minimum energy or ABSOLUTE ZERO, which is impossible, right?
Now let’s apply this to ‘Oumuamua’s system of origin which, as we have seem, had almost zero speed w.r.t. the LSR, and make the anaolgy of the speed of stars in the galaxy with the speed of molecules in a gas. A zero speed w.r.t the LSR for a star should actually be exceedingly unlikely, in precisely the way it is for molecules in a gas.
It seems the more you think about ‘Oumuamua the stranger – and more eldritch – it gets.