Celebrating Sol’s 4,568 millionth birthday

Wednesday, December 19th, 2007

I’m glad I held on to the very end of this year before proclaiming what the coolest news of 2007 was, because it’s just arrived, and I would have hated to prematurely give the crown to something else. Scientists at UC Davis have pinned down the age of the solar system to 4,568 million years, with a margin of error of about one million either way. This is by far the most precise estimate of the age of the solar system I’ve ever seen. In my astronomy classes (from 2003-2007) we were taught that the solar system was 4.5 billion years old. Now we have two more significant figures! I can’t really explain why, but knowing that figure with such precision is meaningful to me. I’ve already committed it to memory and I shan’t forget it. It’s easy to remember; each digit is monotonically increasing, with the first three being sequential (yeah, that’s how I handle larger numbers).

In the brief period of time between when I saw the headline and when I read the rest of the article, all sorts of thoughts raced through my mind. The most prominent was, “How did they do it?” My first guess was radioisotope dating of material from asteroids (as there’s no rock native to Earth that dates back that far). Uranium-238 has a really long half-life (4.468 billion years), so that was my guess as to what isotope they used to do the dating. The other possibility I came up with was that they used some form of advanced computer modeling of the dynamics of the solar system, but I would be really surprised if anyone could get such accuracy from a simulation.

Well, it turns out my first guess was correct. The scientists at UC Davis performed a radioisotope analysis on samples from asteroids that date back to the beginning of the solar system. My guess of uranium-238 was incorrect though; they actually used manganese-53, which decays to chromium-53 with a half-life of 3.74 million years. That’s a very short half-life relative to the age of the solar system, so the final amount of manganese-53 left in these asteroid samples is incredibly minuscule compared to what it once was. But thankfully, there was still enough left with which to measure to determine the age. As for the type of radioisotope analysis performed, they specifically used radioisochron dating, which compares relative quantities of the unstable isotope (manganese-53) with its resultant decay product (chromium-53) to determine age.

Of course, I was so excited about this news that I immediately told everyone about it, including all of my friends who happened to be online at the time, a friend who happened to call me, and my family. There were two common misconceptions that I’d like to address just in case anyone else who reads this joyous news happens to wonder about them.

How do we know it’s from the solar system? The vast, vast majority of the stuff in our solar system was all born here, and at the same time to boot. The chance of some substantive object being propelled with enough velocity to leave another star system and then get captured by ours is incredibly small. Pick anything at random in this solar system (Sol itself, the planets, moons, asteroids, comets, dust, etc.) and the odds are incredibly good that it’s from around here. But to deal with the off-chance that you end up with something not from around here, you perform the same test on a variety of different asteroids. The chance that they all happen to be from outside the solar system is negligible.

How do we know it’s from the beginning of the solar system? This is a very valid question to ask, at least of materials found on Earth. Earth is a very lively place, with all sorts of geological and chemical processes going on that continually break down older rocks. There isn’t anything on Earth that has been left untouched since the formation of the solar system (if I remember correctly, the oldest rock we’ve found dates back to a paltry 3.8 billion years). So that’s why you need to look at asteroids. Asteroids are too small to have tectonic activity capable of metamorphosing rocks, and out in space, they certainly aren’t hot enough to cause the other kind of metamorphic activity that would change the structure of the rock. So they’re unchanged since the formation of the solar system simply because there’s no mechanism that could change them.

And keep in mind, everything in the solar system formed at the same time in a very rapid process (hey, less than a million years is definitely rapid compared to 4.568 billion years). So all you have to do is test a variety of different asteroids and note that the oldest of the resultant ages all tend to cluster around one number. We wouldn’t expect to find any chunk of asteroid older than the age of the solar system, and indeed, we haven’t. You know it’s good science when the experimental results are repeatable, and these are definitely repeatable.

So look for scientists to continually narrow down the age of the solar system even further in the coming decades as they get more data and more accurate test equipment, but never again will we experience a jump from imprecise estimate to quantified value — with margin of error! — in this number so meaningful to us, the age of the solar system. Remember this day.