Life cannot begin on a planet unless certain chemical elements can be found in sufficient quantities. The two most vital are phosphorus and nitrogen. Phosphorus helps construct DNA and RNA, which store and transmit genetic information, and it also plays a key role in how cells manage energy. Nitrogen is a significant component of protein, which is important for constructing cells and helping them function. Without enough phosphorus and nitrogen, life cannot emerge from non-living matter.
The recent research, led by Craig Walton, a postdoc at ETH Zurich’s Center for the Origin and Prevalence of Life, and ETH Zurich Professor Maria Schönbachler, shows that these elements are already available in the suitable amounts when the core of a planet forms. “During the formation of a planet’s core, there needs to be just the right amount of oxygen present so that phosphorus and nitrogen can remain on the planet’s surface,” explains Walton, lead writer of the study. On Earth, this appears to have happened about 4.6 billion years ago, giving our planet an unusually fortunate chemical place to begin. The result could affect how scientists search for all times beyond Earth.
How the essential composition of the planet affects habitability.
Planets begin as bodies of molten rock. As they’re formed, their contents are separated by weight. Heavy metals corresponding to iron sink inward and form the core, while lighter materials remain at the highest and eventually turn out to be the mantle and later the crust.
Oxygen levels are critical during this phase. If oxygen is just too low when the core is formed, phosphorus binds with heavy metals corresponding to iron and is pulled downward. Once this happens, it isn’t any longer available in parts of the planet where life can develop. If there is just too much oxygen, phosphorus stays within the mantle, but nitrogen is more more likely to escape and be lost to the atmosphere.
Chemical Goldilocks Zone
Using extensive modeling, Walton and his co-authors found that each phosphorus and nitrogen are abundant within the mantle only under a really narrow range of moderate oxygen conditions. They describe it as a chemical Goldilocks zone.
“Our models clearly show that Earth is exactly within that range. If we had a little more or a little less oxygen during early formation, there wouldn’t be enough phosphorus or nitrogen for life to develop,” says Walton.
The team also found that other planets, including Mars, formed under oxygen conditions outside of this Goldilocks zone. On Mars, this meant that there was more phosphorus within the mantle than on Earth, but less nitrogen, making conditions difficult for all times as we realize it.
A brand new option to search for all times beyond Earth
The findings could change how scientists take into consideration habitat. Until now, a lot of the attention has been focused on whether a planet has water. Walton and Schönbächler argue that this is just not enough.
A planet can have water and still be chemically unsuitable for all times in the primary place. If oxygen levels were incorrect in the course of the formation of the core, the planet may not have kept phosphorus and nitrogen in places where life could use them.
Why are stars just like the Sun most vital?
Astronomers can infer these chemical conditions by studying other solar systems with large telescopes. The oxygen available during planet formation is determined by the chemical composition of the host star. Because planets are mostly product of the identical material as their star, the star’s composition helps shape the chemistry of the whole planetary system.
This implies that solar systems with very different chemistry than ours could also be poor candidates within the search for all times. “This makes the search for life on other planets much more specific. We should look for solar systems with stars that are similar to our own Sun,” says Walton.