h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Today (9th Feb 2005) astronomers have witnessed the first example of a star being ejected from the Milky Way. Its mistake, apparently, if that's what it was, was to travel so near to a Black Hole that it underwent the vast acceleration of the 'slung-shot effect' and was catapulted out into deepest nothing, travelling, it is said a several million kilometers per hour and bound for who knows where - or when?

This is fascinating news and the immediate question it raises for those of us occupying a planet attached to a star that could, conceivably, suffer the same fate, is "What happened to its planets?" - assuming it had some. It is very difficult to imagine that such a radical outcome could arise without any planetary structure being ripped apart and planets, if left attached to the star at all following such acceleration, being disposed in totally new orbits and relationships to their mother star on which, conceivably, life forms and eve3n civilizations would have been relying for millions of years.

Do we have expert readers in this forum who would be able to spell out the effects of this trauma for us? It would be fascinating to have some insight into this process and the possible outcome.

riccardo1

My guess is that the proximity to the black hole caused more havoc to the orbits of the planets than the sling shot effect. Remember orbits are a balance between gravity and inertia. You put a new large gravitational effect into the formula and it changes everything.

As far as we know, the Sun is a lone star continuing a lone journey through the Milky Way. It's not paired up with any other star, and it's currently far enough away from other stars not to be bothered too much by them.

But then again, it has been travelling around the Milky Way for 4.5 billion years, so it's plausible enough to assert that it did come very close to major gravitational sources at certain times in its history, and that its course might have been distorted by the interactions. However, the interactions, if they did happen, did not cause the Earth and all the planets to shoot away from the Sun, as we wouldn't be here if this were the case.

So, if we were shot out of the Galaxy, then we might well survive the encounter without noticing that much was happening at all.

Many thanks for these thoughts.
I am sure that among the risks the Earth faces from natural and other causes (if 'man-made' shouldn't also be considered as 'natural', ultimately), this one of possibly being slung out of the Milky Way is one of the remoter ones - but it does offer interesting speculations for us to consider - doomsday or not.

riccardo1

Mind you - for a star to be shot out of a galaxy assumes that it has been accellerated to such an extent that it exceeds the galaxy's escape velocity. I imagine that this speed is quite huge, so the event causing the acceleration might well be a killer.

Good point. The velocity required could indeed be enormous. But how about considering how, theoretically, such a velocity could be achieved at a gradual but very protracted accelerative rate that would not necessarily be a killer.

However, whether relatively gradual velocity increase accords with the physics of a star's being accelerated by the forces of a black hole is a different matter.

And it was gravitational forces from black hole that were identified as the likely cause of the star's escaping our galaxy (this by the scientists reporting the first evidence of such an event).

Furthermore there is an issue of the relative impact of the gravitational forces from the black hole on any planets circulating round the star ........ It is difficult to envisage the planets not being seriously disrupted in their orbits during such a process - however gradual - and this does not seem conducive to survival of any life forms on the planets!

Does this mean that the more one considers such an event the less likely seems any prospect of survival of our species were the ejected star to have been the Sun dragging the Earth with it into space? I think it probably does. Shame! Could have been a great adventure - even though an adventure pursued over aeons!

Just a concluding more optimistic speculation: I envisage that during the star's journey through the vastness of space there should be plenty of time for life to evolve afresh as long as
a) the star was still 'young' enough
b) there were suitable conditions pertaining on any surviving planets.
But the life evolving, I imagine, may or may not lead to something like mankind - an intriguing thought!

riccardo1

According to one of the laws of gravitation, the gravitational force felt by the planets because of the star is far greater than that felt because of the black hole, because of the relatively short distance between the planets and the star compared to that between the planets and the black hole. The gravitational force increases with the mass, but it reduces by a great factor with the increase in the distance.
So then, the planets might not have felt much of gravity of the black hole and still remained in their orbits around their star and headed for exile into the intergalactic space with their star.
I'd love to be on one such planets, just to see the night sky changing every night and gradually to be able to see the galactic disk, where I once lived, apear in the night sky.

shashikanth