How do we know the age of celestial bodies?

Importance of knowing the age of things

Humans, throughout their short individual journey - period of time existing in the universe - and relatively longer if one considers a collective period since the existence of the race itself, have had an avid fascination with the past and finding out the why of things. Although, this could apply for areas of study such as history, or paleontology, knowing the age of things allows us to obtain new pieces of a puzzle about the history and origin of the universe as we know it, therefore, also the way it works. However, to achieve that we need benchmarks and indicators that allow us to obtain that important knowledge. Some of them could be characteristics such as radiation, color, among others.

Age of a planet: Earth

Determining the age of a planet can be considered a somewhat complicated task, in the first instance because in some cases, planets can renew or change their outer layers, as in the case of Earth. Earth has volcanoes that periodically expel magma, which becomes a new rocky cover on its surface. The movement of the tectonic plates generates new mountains and, in the case of the oceanic floor, it is constantly renewed: it forms at mid-ocean ridges and sinks beneath the continents at subduction zones. Nonetheless, it is possible to know the age of planet Earth thanks to the ingenious uranium-lead dating method and zircon. This wonderful mineral is extremely useful because it contains zirconium and small traces of uranium. Uranium atoms are similar enough to zirconium atoms to sometimes take their place in the crystal structure. However, uranium atoms are unstable and end up decaying into lead atoms. Knowing that lead could not have got there on its own, we can conclude that the more lead you find in a zircon crystal, the older it is. One of the oldest "pieces" we know of on Earth is a zircon. From uranium-lead measurements, we can conclude that this planet is at least 4.4 billion years old. The accepted age of the Earth, about 4.54 billion years, comes from the lead composition of meteorites. Similar tests with other radioactive elements can be used to date other bodies, such as meteorites and lunar rocks.

Age of a star

As for the age of a star, other methods are used to determine it. In 2015, a study presented at a meeting of the American Astronomical Society in Seattle described a new method of calculating age from rotation speed. Specifically, it applies to "cool" stars smaller than or about the size of the Sun, the most common kind in our galaxy. The method starts from the premise that stars spin fast when they are young but, like a spinning top, slow down over time. The observations were made with data from the Kepler space telescope. However, a 2016 study showed that this clock becomes less reliable for stars more than halfway through their main-sequence lifetimes: magnetic braking weakens and they spin faster than expected. The Sun is close to this transition.

On ages and the Universe

Now that we know how the ages of certain astronomical objects can be measured, how is it possible to measure the age of the Universe? The answer lies in the expansion of the Universe. Approximately 100 years ago it was believed that the universe had no beginning or end, therefore, knowing its age was a question no one asked, or at least most people did not. It was Lemaître who formulated ideas about how wrong it was to think that the universe was eternal, and presented them to Einstein in 1927, but Einstein told him that, although his mathematics was impressive, his physics was abominable. However, Lemaître's ideas were confirmed thanks to the American Edwin Hubble, who realized that the Universe was expanding through the discovery of galaxies outside the Milky Way.

He noticed another indicator: galaxies moved away faster the farther they were. However, Hubble's calculations led him to an incorrect age of 2 billion years, younger than the Earth itself: he underestimated the distances to the galaxies and, at the time, the expansion was also assumed to be slowing down. Before reaching the corrected age of the Universe, it is necessary to talk about supernovae, explosions of stars so violent that they are sometimes compared to atomic bombs; they emit light so intense that it lets us see - for a short time - the deepest parts of the Universe. Years later, a group of scientists, who were studying supernovae, came to the conclusion in 1998 that the expansion of the universe was accelerating, and that for this reason the Universe was older than a steadily slowing expansion would imply. They received the Nobel Prize in Physics in 2011. The age obtained after redoing the calculations was a little under 14 billion years, and measurements of the cosmic microwave background by the Planck satellite pin it down to about 13.8 billion years. Even so, different ways of measuring the expansion rate do not fully agree, a discrepancy known as the "Hubble tension".