An introduction to exoplanets
An introduction to exoplanets

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3.1  Stars, planets and chemical elements

Astronomers know that planets form along with their parent star from vast clouds of gas in space, so the age of the Earth is the same as the age of the Sun: 4.5 billion years (4.5 × 109 years). The Sun itself is middle-aged; it will spend another 4.5 billion years as a main sequence star, converting hydrogen into helium. Then, in the final stages of its life, the Sun, like most stars, will use further nuclear reactions to create heavier elements that didn’t even exist when the Universe was young. The whole Universe, including the Milky Way, is about 14 billion years old. This means there was almost time for another star like the Sun to complete its entire main sequence life before the Sun formed. More massive stars live their lives much more rapidly (on astronomical timescales!), the most massive taking as little as a few million years to leave the main sequence.

At the end of a star’s life it expels material into space, returning it to the Galaxy’s reservoir of gas, including the new elements it has created. This material then goes on to form subsequent generations of stars, as shown in Figure 8. This cycle underpins the formation of rocky planets and of life itself. Only material which has been processed by now-extinct stars contains the chemical elements needed to form rocky planets: chemicals such as iron, oxygen, silicon and magnesium. Similarly, only material which has been processed by now-extinct stars contains the chemical elements which are needed for life: carbon, nitrogen and oxygen.

Described image
Figure _unit9.3.1 Figure 8  The lifecycle of stars and the cycle of chemical enrichment of the material in the Galaxy

Until recently, astronomers didn’t know if there were many rocky planets in our Galaxy which are older than Earth. It could have taken billions of years for the Galaxy to accumulate enough iron, oxygen, silicon and magnesium in its gas for rocky planets to form.

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