Eight ingredients for life in space

For generations, scientists have been scouring our galaxy for evidence of life on other planets. They are searching for a specific set of circumstances and chemicals to come together in the right place, at the right time.

Dr Anne Jungblut, a specialist in life living in extreme conditions, and Dr Paul Kenrick, a specialist on the early evolution of life, explain what they are looking for.

1. Water

Almost all the processes that make up life on Earth can be broken down into chemical reactions - and most of those reactions require a liquid to break down substances so they can move and interact freely.

Liquid water is an essential requirement for life on Earth because it functions as a solvent. It is capable of dissolving substances and enabling key chemical reactions in animal, plant and microbial cells.

Its chemical and physical properties allow it to dissolve more substances than most other liquids. Other characteristics that make it a good habitat for life are its heat conduction, surface tension, high boiling and melting points, and its ability to let light penetrate it.

Anne said, 'As water plays such an essential role in life on Earth, the presence of water has been vital in the search of other habitable planets and moons'.

2. Carbon

Many complex molecules are needed to perform the thousands of functions sustaining complex life. Carbon is the simple building block that organisms need to form organic compounds such as proteins, carbohydrates and fats.

Carbon's molecular structure allows its atoms to form long chains, with each link leaving two potential bonds free to join with other atoms. It bonds particularly easily with oxygen, hydrogen and nitrogen.

The free bonds can even join with other carbon atoms to form complex 3D molecular structures, such as rings and branching trees. 

Carbon molecules are also strong and stable, so they are perfect to build a body with.

Anne said, 'Carbon is one of the most abundant chemical elements on Earth and a major part of all living organisms. Therefore one working hypothesis is that life on other planets might also be carbon-based.'

3. Nitrogen

Carbon is a fundamental component of organic compounds, but it can't do it alone. The complex proteins required for life are built up from smaller compounds called amino acids - simple organic compounds that contain nitrogen.

Nitrogen is also needed to make DNA and RNA, the carriers of the genetic code for life on Earth.

Many bacteria can convert nitrogen from the atmosphere into a form that is used in living cells.

Anne said, 'Plants cannot use atmospheric nitrogen. They rely on ammonium and nitrate created by bacteria in the soil and water, and animals obtain it through their food.

'Finding biochemically usable nitrogen could be a big clue for biological activity on another planet.'

4. Phosphorus

Phosphorus is a key component of adenosine triphosphate (ATP), an organic substance that acts as life's molecular unit of currency.

ATP transports chemical energy around the body's cells, powering nearly every cellular process that requires energy.

Phosphorus is a vital element in cell membranes, the layer surrounding the inside of cells that controls the movement of substances in and out.

And like nitrogen, phosphorus is necessary to create DNA and RNA.

Anne said, 'The phosphate group acts like glue in DNA, so the bodies of living organisms would not work without it'.

5. Sulphur

Sulphur is part of most biochemical processes on Earth, and most enzymes cannot function without it. It is also a component of many vitamins and hormones.

In the absence of light and oxygen, it is possible to use sulphur as an energy source. Bacteria that live under severe environmental conditions are called extremophiles and have been found to gain their energy for growth from sulphur and hydrogen alone.

Anne said, 'Some micro-organisms are able to grow under extreme conditions such as permanently frozen lakes, deep-sea hydrothermal vents, high radioactive radiation and hypersalinity.

'They expand our understanding of the capability of some life forms to resist extreme stress. This helps us to understand how habitable other planets might be.'  

6. Luck

Having all the right chemicals on the same planet seems fortunate. And Earth – a tiny planet in the middle of an enormous universe – is lucky to have enough of the right chemicals to support a vast abundance of life.

Dr Paul Kenrick, a Museum palaeobotanist, said, 'Over time, major catastrophes such as impact by asteroids and massive volcanic eruptions have wiped out many species.

'However, the gaps created afforded opportunities for the survivors to flourish. These accidents along the road mean chance has a huge role in shaping our destinies.'

7. Time

The development of complex life takes billions of years, and there’s no shortcut in the journey from single-celled organisms to complex life.

Earth is 4.5 billion years old, but in its earliest stages it was far too hot to support life. The oldest fossil evidence of life comes from rocks that are 3.4 billion years old. It took a long time to evolve plants and animals from single-celled organisms.

It is possible that life exists on other planets - but it is likely such life would have a lot of evolutionary catching up to do.

Paul, explaining why it took complex life so long to form, said: 'The key building block of life is the cell, with its complex genetic and biochemical systems. Animals and plants are all made of cells, so cells had to evolve first. 

'To make tissues and organs, cells need to multiply, specialise in function, and co-operate. The evolution of these basic building blocks and their integration took time. 

'Larger organisms require even more specialised and integrated cellular systems. The fossil record tells us that this took billions of years.' 

8. Location

Earth falls into the Goldilocks zone, meaning it is just the right distance from the Sun: not too hot or too cold to have liquid water on the surface.

Astronomers are searching for planets that are a similar distance from their own host stars.

Life needs an energy source to power growth - either the right amount of light from a star or chemically generated energy. Life also needs protection from certain wavelengths of solar radiation. Exposure to ultraviolet B damages DNA, but this wavelength is mostly absorbed by the ozone layer.

Paul said, 'In the search for life in the solar system, one strategy is to follow the water. Liquid water may exist below the dry surface of Mars and the frozen surface of Jupiter's moon Europa. The search for life has broadened to consider worlds far from the Sun.'