How old is the Solar System? We know now, from a variety of astronomical, geological and solar evidence that we’re looking at about 4.5 billion years. But in the 19th century, there was a huge problem. Here’s why.
On one hand, you had evolutionary biology and evolutionary biologists.
Charles Darwin had discovered not only that organisms evolve, but had determined the mechanism of natural selection. His voyage on the HMS Beagle had allow him to make the observations necessary to confirm the validity of his theory and its mechanism.
A little bit about geology and the age of the Earth was known, and Darwin used that to his advantage. He reasoned that, in order for organisms to evolve to the point they are today based on the observed rate of change, the Earth needed to be at least many hundreds of millions of years old. This reasoning was sound, the biology was sound, and the geology was sound.
“At least hundreds of millions of years” even fits in very well with our modern view of the Earth and the Solar System, which shows about 5-600 million years from the Cambrian explosion to the present day. So, as long as the Earth is at least that old, there’s enough time — at least in principle — for evolution to take us from simple organisms to the present day.
But, there was a problem. How could the Earth be hundreds of millions of years old, and how could evolution take place on it over that time, unless the Sun was that old, too?
Well, enter Lord Kelvin, a.k.a. William Thomson. One of the most respected physicists of the day, Thomson decided to calculate how old the Sun was. How did he do it? Well, he knew the following three things:
- the mass of the Sun,
- how gravity and gravitational energy worked, and
- the rate of energy emitted by the Sun.
Based on how big the Sun was, he figured out the rate it must be contracting to emit the amount of energy it was giving off, and found that the upper limit for the age of the Sun was 20-40 million years.
But both Darwin and Thomson couldn’t be right! You can’t have many hundreds of millions of years of evolution if the Sun is only a few tens of millions of years old! Thomson knew about gravitational energy, electromagnetic energy, and chemical energy, and calculated that, even if the Sun made use of all three, gravity gave the oldest age, and the age was far too short. He challenged Darwin and his followers to resolve the problem, and of course they could not.
But the Sun really is much older than that. What’s its secret?
Yes, Ivy Mike, it’s nuclear fusion! More powerful than gravity, electromagnetism or chemical energy, nuclear fusion converts hydrogen nuclei into heavier elements, emitting tremendous amounts of energy in the process. This is what drives most stars, allowing them to burn as brightly as they do for billions and billions of years.
But Thomson was entirely right in his calculations. It turns out that there are stars that don’t undergo nuclear fusion, that simply gravitationally collapse and emit light as a result. These stars are called white dwarfs, and they also shine for billions of years, but are much, much dimmer than our Sun or any other fusion-driven star we’ve ever found. Take a look at Sirius A (the bright, fusion-driven star) and B (the tiny, dim white dwarf) below.
And that secret — that of the atomic nucleus — is the solution to this battle of the beards. Now, the only question that remains is whose beard is better? I leave that debate to you.