The invention of the telescope allowed man to view the millions of stars and planets spread throughout the universe, sparking the question ‘are we alone?’ Mathematical probability suggests that we are not, supposed UFO sightings suggest we are not (although they are often evidence of dubious credibility) and the search for other life forms continues.
As technology has developed, sophisticated telescopes and space probes have let us view space thousands of light years away. Almost 2,000 exoplanets have been discovered in the last 50 years. These are planets that orbit a star like Earth does.
Currently, it is believed that a star and its planets form from a collapsing cloud of dust and gas in interstellar space. This shared origin means that the star and planet have the same basic chemical genesis. To begin with, gravity pulls the shrapnel closer together. As the cloud’s centre gets more and more compressed, it also gets hotter, creating a fiery, dense core. Inherent atmospheric motion causes other particles in the collapsing cloud to churn and rotate in the same direction. This creates thin circumstellar disks that are the birthplace of planets.
In the atmosphere closer to the hot star, gas is consumed and cleared out, leading to the formation of small, rocky terrestrial planets such as Earth. Planets further from the star have a higher composition of gas with cold, icy surfaces. To host life as we know it, a world must have liquid water and be orbiting a star in order to obtain energy from its radiation.
The ‘Goldilocks zone’ is the area around a star that is neither too hot nor too cold for water to be liquid. This is where we find that Earth’s composition is very different from most other planets in the Goldilocks zone.
Most strikingly, the planets in other hospitable zones are lower in heavy metals such as iron than Earth is. Planets rich in heavy metals orbiting metal-rich stars tend to be larger and enveloped in huge gaseous atmospheres unlike Earth. Vardan Adibekyan of the Institute of Astrophysics and Space Sciences in Portugal suggests that this is because Earth is much younger. Heavy metal elements form when a very large star, much larger than our own, undergoes a supernova that scatters particles into interstellar space.
For them to combust in this way, stars must either accumulate too much matter or run out of nuclear fuel at the end of their life time.
So Earth was formed at a time when more stars were too old and had exploded, releasing heavy metals that came together in the cloud that formed our solar system.
Studying the composition of exoplanets in other stars’ habitable zones is essential to our understanding of how life can form. It reveals how these differences in heavier metal abundance affect the suitability of an exoplanet for life and, in the long term, for our chances of colonising said planet.