How do we measure the age of stars and planets?

Astronomers used the Hubble Telescope to take the Hubble Extreme Deep Field in 2014. (Image source: NASA, ESA)

This article was originally published on The Conversation. This article is published on Space.

Adam Burgasser is a professor of astronomy and astrophysics at the University of California, San Diego, and an observational astrophysicist who studies the coolest stars, brown dwarfs, and exoplanets.

Measuring the age of planets can help scientists understand when they formed and how they evolved - and, in the case of planets, whether life had time to evolve on them.

However, the age of objects in outer space is very difficult to measure.

Sun-like stars have maintained the same luminosity, surface temperature and radius over billions of years. The characteristics of a planet - such as temperature - are generally determined by the properties of the star, not by the planet's age or evolution.

So determining the age of a star or planet is like trying to guess the age of a person who looks exactly the same from birth until retirement.

Guessing the age of a star

Fortunately, stars slowly change their brightness and color over time, and using very precise instruments, astronomers can compare photometric data with predictions from mathematical models to predict what will happen to a star as it ages, and thus estimate its age.

Stars not only shine, they also rotate. Over time, their rotation speed will slow down, just like a spinning wheel will slowly slow down after encountering friction. By comparing the rotation speeds of stars of different ages, astronomers can establish a mathematical relationship that describes the rotation speed and age. This method is called gyrochronology.

Researchers estimate that the sun is 4.58 billion years old. (Image source: NASA)

The rotation of a star also generates a strong magnetic field and magnetic activity, such as stellar flares - powerful bursts of energy and light on the surface of a star. A continuously decaying magnetic field can also help estimate the age of the star.

Another more complex method of estimation is called asteroseismology, which measures the oscillations within a star. Astronomers study the shock waves generated on the surface of a star as it passes through the star's interior. Young stars have different oscillation patterns than old stars. In this way, astronomers estimate the age of the sun to be about 4.58 billion years.

The principle of asteroseismology (Source: Wikipedia)

Piecing together the planet's age

Radioactive elements are key to determining the age of planets in our solar system. These special atoms decay slowly over long periods of time. Naturally occurring in rocks, radioactive elements have helped scientists determine the age of many items, from rocks to bones and jars.

Using this method, scientists can determine that the oldest known meteorite is about 4.57 billion years old, which is almost the same age as the sun. The oldest rocks found on Earth are younger, at around 4.4 billion years old. Similarly, analysis of lunar soil brought back by the Apollo missions shows that the oldest rocks on the moon date back to 4.6 billion years ago.

Craters on the moon (Photo credit: Tomekbudujedomek/Moment via Getty Images)

While studying radioactive elements is a powerful way to determine the age of a planet, it usually requires having a bit of rock on hand. Astronomers can usually only study a photo of a planet. Astronomers usually determine the age of rocky planets by counting craters; older surfaces have more craters than younger ones. However, erosion caused by water, wind, cosmic rays and lava from volcanic eruptions can easily erase older impact craters.

These methods don't work for gas giants like Jupiter, because their solid cores are tightly wrapped by their outer atmospheres. But astronomers can still determine their ages by counting the number of impact craters on the moons of gas giants or studying the distribution of specific meteorites that were scattered by them. This is consistent with the method of measuring the radioactive elements content or counting impact craters of rocky planets.

Due to the limitations of existing technology, we cannot currently directly measure the age of exoplanets.

An exoplanet discovered through direct imaging can be seen as just a dot in a telescope (Source: space.com)

How accurate are these estimates?

Our own solar system provides the best check on accuracy, because astronomers can compare the amounts of radioactive elements in samples from Earth, the Moon or meteorites with the age of the Sun derived from asteroseismology, and the results agree very well.

Stars in clusters like the Pleiades and Omega Centauri are thought to have formed at about the same time, so the ages of individual stars in these clusters should be estimated to be about the same. In the atmospheres of some stars, astronomers can detect radioactive elements like uranium, a heavy element found in rocks and soil. These radioactive elements have been used to estimate ages by other methods. Astronomers think that planets are about the same age as the stars they orbit, so improving methods for estimating stellar ages can also help study the ages of planets. By studying some subtle clues, we can come up with a rough range of ages for exoplanets orbiting main-sequence stars.

BY:Adam Burgasser

FY: Chen Li

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