Is this why we haven't found aliens yet? Scientists claim looking for planets with water is POINTLESS - and say we should focus on worlds with phosphorus and nitrogen instead

Scientists hunting for life beyond Earth have long thought that water is one of the key indicators. 

But a group of experts now says that looking for life on water–rich planets could be a waste of time.

Instead, they claim we should be turning our telescopes to worlds that are filled with phosphorus and nitrogen.

Life as we know it simply cannot form without these two elements - even if there's abundant water. 

Phosphorus is required to make DNA and RNA, which store and transmit genetic information in all lifeforms.

Nitrogen, meanwhile, is an essential component of proteins, which are the basic building blocks of cells.

This means that life can only form on worlds within the 'chemical Goldilocks zone', where there is just the right amount of nitrogen and phosphorus in the rocky mantle.

Lead author Dr Craig Walton, of ETH Zurich, told the Daily Mail: 'You could feasibly have a planet that looks great with oceans and even dry land, but there is no life and never will be because the other elements you need are simply all but absent.'

While no life as we understand it can survive without liquid water and oxygen, looking for planets that have these ingredients might be misleading.

This is because a planet's 'oxygen balance' at the moment of its formation determines how much phosphorus and nitrogen remain available for life.

When planets cool out of molten rock, a sorting process happens in which the heavy elements like iron sink towards the core, while lighter ones float to the surface to form the mantle and crust.

If there is too much oxygen present, phosphorus gets locked in the mantle while nitrogen is forced out into the atmosphere and eventually lost into space.

On the other hand, if there is too little oxygen, phosphorus binds with other heavy elements and is dragged down to the core, where it can't be used to kickstart life.

Dr Walton says: 'Having too much or too little oxygen in the planet as a whole – not in the atmosphere per se – makes the planet unsuitable for life because it traps key nutrients for life in the core.

'A different oxygen balance means you have nothing to work with left at the surface when the planet cools and you form rocks.'

Using numerical modelling, the researchers found that there is a very narrow band where there is just enough oxygen for both phosphorus and nitrogen to be abundant in the mantle.

By an incredibly lucky chance, Earth happens to sit right inside the chemical Goldilocks zone, ensuring that it had the right balance for other planets to form.

However, this likely means that habitable worlds are far rarer than astronomers thought.

Dr Walton suggests that there might be just one to 10 per cent as many habitable planets as previously suggested.

That might have serious consequences for how scientists search for life, and how we think about the future of human space exploration.

While we currently see an abundance of oxygen as a sign of habitability, the researchers' calculations suggest this may signal that the world cannot support life.

'It would be very disappointing to travel all the way to such a planet to colonise it and find there is no phosphorus for growing food,' says Dr Walton.

'We'd better try to check the formation conditions of the planet first, much like ensuring your dinner was cooked properly before you go ahead and eat it.'

Closer to home, the researchers' findings also suggest that Mars sits just outside of the chemical Goldilocks zone.

Dr Walton says: 'Mars is fairly similar to Earth, and its formation conditions mean there is more phosphorus, not less. This means growing food there might be relatively easy.'

However, other chemicals are also a lot more abundant at the surface, poisoning the soil with harsh salts.

Additionally, the planet has significantly lower levels of nitrogen near the surface, making it especially unsuited to life.

'It is not that different, but it is not currently habitable, Elon Musk will have to come up with a clever way to change the composition to grow food there.'

In the future, the researchers suggest that searches for alien life should pay much more attention to the chemical composition of exoplanets.

This is very difficult to measure directly, but astronomers can get a clue by looking at the type of star the planet orbits.

Since planets primarily form out of the same material as their host star, their chemical compositions are closely related.

Solar systems that have similar stars to our own are, therefore, a much more promising place to look for life out in the Universe.