Did a Galactic-Scale Collision Lead to Us?

Why do we have a planet that we can walk around on, and generally mess up? As most of us know, the atoms we use, apart from hydrogen, will have originated in a nova or supernova, and some of the planet possibly even from collisions of neutron stars. These powerful events send clouds of dust into gas clouds, but then what? We call it dust, but the particle size is mainly like smoke. Telescopes like the Hubble space telescope have photographed huge clouds of gas and dust in space. These can be quite large, thus the Orion molecular cloud complex is hundreds of light years across. These giant clouds can sit there and do very little, or then start forming stars. The question then is, what starts it? The hydrogen and helium, which are by far the predominant components, with hydrogen masses about ten thousand times as much as anything else except helium, are always colliding with each other, and with dust molecules, but they always bounce back because there is no way to lose their kinetic energy. The gas has been around for 13.6 billion years, so why does it collapse suddenly?

To make things slightly more complicated, the cloud does not collapse on itself. Rather, sections collapse to form stars. The section that formed our solar system would probably have been a few thousand astronomical units across (an astronomical unit, AU, is the distance between Earth and the Sun), and this is a trivial fraction of such giant clouds. So what happens is sections collapse, leaving the cloud with “holes”, a little like a Swiss cheese.

For us, about 4.6 billion years ago such a piece of a gigantic gas cloud started to collapse upon itself, which eventually led to the formation of the solar system, and us. Perhaps we should thank whatever caused that collapse. A common explanation is that a nearby supernova sent a shockwave through the gas, and that may well be correct for a specific situation, but there is another source of disruption: galactic collisions. We have observed these elsewhere, and invariably such collisions lead to a good generation of stars. Major galaxies do not collide that often because they are so far away from each other. As an example, in about five billion years, Andromeda will collide with the Milky Way. That may well initiate a lot of formation of new stars as long as there is plenty of gas and dust clouds left.

However, there are some galactic collisions that are a bit more frequent. There is something called the Sagittarius Dwarf Spheroidal Galaxy which is approximately a tenth the diameter of the Milky Way. It comprises four main globular clusters and is spiralling around our galaxy on a polar orbit about 50,000 light years from the galactic core and passes through the plane of the Milky Way periodically. It apparently did this about five to six billion years ago, then about two billion years ago, and one billion years ago. Coupled with that, a team of astronomers have argued that star formation in the Milky Way peaked at around 5.7, 1.9 and 1 billion years ago. The argument appears to be that such star formation arose about the same time that the dwarf galaxy passed through the Milky Way. In this context, some of our nearest stars fit ths hypothesis. Thus Tau Ceti, EZ Aquarii,  and Alpha Centauri A and B are about 5.8 billion years old, Procyon is about 1.7 billion years old, while Epsilon Eridani is about 900 million years old.

However, if we look at other local stars, we find Earth, Lacaille 9352 and Proxima Centauri are about 4.5 billion years old, Epsilon Indi is about 1.3 years old, Alpha Ophiuchi A is about 750 million years old, Sirius is about 230 million years old, and Wolf 359 is between 100 – 300 million years old. Of course, a galaxy passing through another galaxy will consume a lot of time, so it is not clear what to make of this. There is always a temptation to correlate and assume causation, and that is unsound. On the other hand, the more massive Milky Way may have stripped some gas from the smaller galaxy, and a wave of gas and dust on a different orbit could have long term effects.

In case you think the stars in a galaxy are on well-behaved orbits around the centre, that is wrong. Because the galaxy formed from the collision and absorption of smaller galaxies the motion is actually quite chaotic, but because stars are so far apart by and large they ignore each other. Thus Kapteyn’s Star orbits the galactic centre and is quite close to our Sun, except it is going in the opposite direction. We “meet again” on the other side of the galaxy in about 120 million years. So to summarize, we still don’t know what caused this solar system to form but we should be thankful that we got what we did. Our system happens to be just about right for our life to form, but as you will see, when it comes out, from the second edition of my ebook “Planetary Formation and Biogenesis” there are a lot of things that could have gone wrong. Let’s not help more things to go wrong.