Floating above your head are about 300 million exoplanets – planets outside our solar system – that could be habitable.
But as much as it’s easy to think all they need is oxygen and water for life to flourish, scientists have discovered that isn’t quite the case.
For life to even have a tiny chance of forming, two other substances need to be there when a planet’s core is formed, a new study has found.
Six chemical elements have long been thought to be essential for all Life As We Know It: hydrogen, carbon, oxygen, phosphorus and nitrogen.
Researchers from ETH Zurich wrote in a paper, published in Nature Astronomy, that scientists need to focus more on the last two.
Phosphorus is a key ingredient in human DNA and is one of the rarest ingredients for life in the cosmos. Nitrogen, meanwhile, is needed to create the proteins required for cells to function.
This is why how life came to be is very hard to say. Which came first, the protein of living cells, or the DNA that makes them?
Craig Walton, the study lead at the Center for Origin and Prevalence of Life, isn’t sure of the exact answer. But he knows roughly when they have to be present to give life its best shot at forming.
He said: ‘During the formation of a planet’s core, there needs to be exactly the right amount of oxygen present so that phosphorus and nitrogen can remain on the surface of the planet.’
Exactly that happened when Earth was formed some 4.6 billion years ago – in other words, we won the chemical lottery.
At the time, a wobbly blob of cosmic dust and gas collapsed under its own weight. Most was squeezed to create a star – the sun – and the rest began spinning around it.
As all this material whirled around, some began to stick and clump together, forming planets. Heavier metals formed, like iron, sank to create the core, while lighter materials were flattened into the mantle and crust.
But if there’s too little oxygen, phosphorus will get stuck to the heavy metals and get dragged into the core, giving it no chance to help life.
On the flip side, too much oxygen means the mantle gets crammed with so much phosphorus that there’s not enough room for nitrogen. So, the gas just floats into space.
The Earth was just the right distance from the sun to make it in the chemical equivalent of the Goldilocks zone, where all six ingredients to life can exist at once.
Mars, the researchers found, was too far out of this zone for enough phosphorus or nitrogen to sustain life.
What is the Goldilocks zone?
Traditionally, the Goldilocks zone refers to the narrow band around a star where a planet is just warm enough for the surface to retain liquid water.
Too close to the sun and the liquid evaporates, like on Mercury. Too far away and you get an icy wasteland.
Scientists use this version of the Goldilocks zone to figure out where habitable, Earth-sized planets could be, a measure called eta-Earth.
But the new study suggests we could make the search for alien life easier by looking for planets in a chemical Goldilocks zone instead.
The researchers said: ‘The amount of oxygen available during the formation of a planet can mean that many planets are chemically unsuitable for life from the very beginning, even if there is water present and they otherwise appear to have the right conditions for life.’
Given that planets are made up of the same material as their host star, it means astronomers can easily narrow down their search for alien life.
Walston said: ‘This makes searching for life on other planets a lot more specific.
‘We should look for solar systems with stars that resemble our own sun.’
According to NASA, there are at least 100 billion stars in the Milky Way, of which about four billion are like our sun.
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