Building a habitable planet

There is a lovely podcast on Geology Bites which covers most of my planetary science degree in the space of half an hour, and I enjoyed it so much it was due a little blog post. To hear the whole thing in detail, here is the full link to it.

Over 6000 exoplanets (planets outside of our solar system) have now been found, and the new direction of research has been to look for habitable life in them.

What makes planets habitable? How could they be formed? How would we detect life if it was there? Astrophysicist Anat Shahar joins the Geology Bites podcast and focuses on the second of these questions, particularly, how might a rocky planet acquire water, one of the conditions we believe to be necessary for life?

Artist’s impression of a range of exoplanets that have been detected so far. Image credit: NASA/JPL

What characterises a rocky planet to have life?

Currently, the only planet known to host life is Earth, so given our sample size of 1, we don't have much to go on in terms of diverse lifeforms. Astrophysicists use Earth to see what characteristics are needed for life. The requirements thought to be needed are as follows: firstly, liquid water to transport nutrients and act as a solvent for chemical reactions, secondly, an atmosphere and magnetic field to shield life from harmful radiation, and finally, a scheme of recycling for nutrients. It is believed a planet would need all of these to sustain life in the long term.

Water has useful properties as a solvent, as it can bring nutrients into cells and allow for waste to be excreted. While there have been places found on Earth where life exists without oxygen or sunlight, this has not been the case for water (an example of this is hydrothermal vents on the bottom of the sea). In 4.5 billion years of Earth's existence, water is the only chemical that has ever been found to act as the planet's solvent, so its fair to say that water does a pretty good job if no better alternative were used in this timescale.

Magnetic fields on Earth protect life on Earth from potentially harmful ionising particles, such as energetic cosmic rays or solar wind, acting as a shield. Other protections could be to have a very thick atmosphere such as the atmosphere on Venus. Alternatively, subsurface life could be shielded from excessive solar radiation by being underneath a crust. Mars has evidence of a former magnetic field, and current missions are underway to search for life that may have existed beneath the surface.

Missions are underway to determine whether Mars may have had subsurface life. In 2024, NASA’s Perseverance Mars rover explored a rock nicknamed "Cheyava Falls," which has features that may bear on the question of whether the Red Planet was long ago home to microscopic life. Image credit: NASA/JPL-Caltech/USGS

Nutrients have to be available for life to use, so there must be some form of replenishing mechanism for the nutrients to be recycled, otherwise they would run out and the ecosystem would die. Surface catalysts are particularly important to ensure that reactions are able to happen. Plate tectonics play a role in moderating the atmosphere and climate on geological timescales, as well as recycling nutrients by subduction, volcanism and mountain building. Feedback mechanisms from plate tectonics help keep temperatures and atmospheres stable over long timescales. Volcanism can occur without plate tectonics however, bringing materials buried in the planet interior to the surface. However, it would be harder to replicate the plate tectonic impact on atmosphere through volcanism alone. 

The Habitable Zone and water on planets

The Goldilocks zone, otherwise known as the Habitable Zone, is the region around a star where an orbiting planet would have a surface temperature such that liquid water can be sustained on the surface. Too far away, and there would be insufficient heat from the host star to provide planetary temperatures above 0 Celcius, so any water would be frozen as ice. Too close to the host star, and any water on the hot planet's surface would have boiled away. But how does the water get to a planet in the first place?

This is an active area of research. On Earth, a potential theory for how it acquired water is that the planet was born in its planetary disc with water molecules on it already. Another theory suggests that water could be transported in from comets in the outer solar system. Through studying exoplanets, it is hoped answers to this question can be found. Since over 6000 exoplanets have been found already, this large and growing number means population statistics can be performed on a large scale. Many of these were found to be rocky, with a hydrogen rich atmosphere. When such planets form, in their early stages they will be very hot, molten and like a magma ocean, surrounded with a hydrogen rich atmosphere. What chemistry occurs here with both the atmosphere and the planet? This area has been taken up by theoretical astrophysicists, and it was found that theoretically water should be able to form under these conditions. Experimentally however, this is challenging to show in a lab. Scientists have attempted to recreate these conditions through using a silicate, magma ocean like sample, squeezed between two diamonds, in order to create high pressure on this material sample. This is because large rocky exoplanets have high atmospheric and surface pressure. This silicate is then surrounded with a hydrogen rich gas. A laser is then used to heat the sample to a high temperature, to simulate the conditions of stellar radiation on such a planet. The result has been found to be water in two forms: firstly as a liquid due to the reduction of the iron oxide inside the silicate which reduces to iron metal and forms water, and secondly through the dissolving of hydrogen from the atmosphere in the silicate magma ocean.

Photomicrograph of the diamond anvil used in Shahar’s high-pressure experiments. Between the diamond tips is a gasket that holds a silicate magma ocean-like sample prepared in the lab and the hydrogen. For scale, the diamond's tip facing the sample is about 300 microns across. Credit: Geology Bites

Both mechanisms show that water can be created as a natural consequence of planet formation, and that external sources of water are not necessarily needed to bring water to a planet, as this can be created in magma ocean planets with hydrogen rich atmospheres anyway. To reach these pressures and temperatures, planets need to be at least the size of 1/3 and Earth mass, and bigger planets have higher pressures. As they have large masses, bigger planets have stronger gravity, and hence any retained atmosphere is also thicker as it is harder for the gas to escape than it would be on a less massive planet with weaker gravity (note that in astrophysics, massive doesn't necessarily mean huge, it means something with a lot of mass, although this will likely be pretty big anyway). Thicker atmospheres prevent heat from escaping, so the planet remains molten for longer, allowing more time for water to be created in the mechanism that was shown in the lab. Hence, large rocky planets have billions of years to carry out these reactions and create large quantities of water. 

Light elements such as hydrogen can quickly escape from an atmosphere, either being radiated from the top of the atmosphere by the planet's host star, or the atmosphere is radiated from the bottom through the molten magma heat from the planet itself. The bigger the planet and thicker the atmosphere, the longer it would take to escape, allowing more time for water to be made. This could have been the process that formed water on Earth too. 

Potentially, one could conclude from these experiments that water production could be relatively common in rocky planets, as a natural consequence of planetary formation. However, it should be noted this doesn't necessarily result in oceans, as some could go into the planet interior, become water vapour or even enter the planet's metallic core as hydrogen. Exactly how hydrogen and oxygen separate into different planetary layers is still an ongoing research area, where astronomers are now looking to see, if, supposing water production is common, where does it all go? If it goes into the mantle magma ocean, does it stay here? If it crystallises (as on Earth), does the water get pushed out as an ocean, or stay trapped in the mantle and have to be released through volcanic activity? 

Interestingly, on Earth there could be as much if not more water in the mantle as there is in the oceans. In exoplanets, it is currently believed that the majority of hydrogen makes its way into planetary cores, as iron and hydrogen bond together well, after which some may stay in the mantle, some my go to the surface and some into the atmosphere. 

A snapshot of Earth, featuring the Hubble telescope in orbit. Image credit: ESA/Hubble

Intelligent life on other planets in the galaxy

The Drake equation expresses the number of active communicating civilisations (intelligent life) in the Milky Way in terms of a number of factors. This calculation is carried out by making a number of estimations. These are approximately as follows: how many stars have planets around them in the Milky Way, of which a fraction have habitable conditions, of which a fraction have some form of life, of which a fraction of those are intelligent life, of which a fraction of civilisation releases signals for some length of time. Many of these were completely unknown in 1961 when Frank Drake came up with this equation to approximate the number of active, communicative extraterrestrial civilisations in the Milky Way galaxy. We now know that the fraction of stars with planets around them is quite high, and as water is a key aspect in supporting life, and recent research has shown that water production could be a common occurrence in planet formation, we are making our way through whittling down the various unknowns in the equation. The later terms of the equation are quite challenging, not least because they hinge on the definition of what counts as intelligent life in the first place.

The Drake Equation, a mathematical formula for the probability of finding life or advanced civilisations in the Universe, as revised by two University of Rochester researchers in 2016. Credit: University of Rochester

Detecting life on exoplanets and biosignatures

Assuming there is some form of extraterrestrial life, how would this be detected? Potentially, we could constrain the search by looking for planets with liquid water, a magnetic field to protect from damaging radiation and a nutrient recycling mechanism. However, exoplanets are very far away, and the information we can find on them for now is quite limited. For example, looking for a magnetic field on a planet, this can be detected indirectly by observing interactions between stellar and planetary magnetic fields in the same system. Potentially this could even be measured directly, as the magnetosphere of the planet would interact with the star in such a way that 'sunspots' could be formed on the star's surface. The Habitable Worlds Observatory, an upcoming NASA mission which will search for habitable planets, may investigate this further. Looking for recycling mechanisms would be harder, and may require looking for signatures in a planet's atmosphere. These are called biosignatures, chemicals in an atmosphere which point towards there being life on the planet. An ideal biosignature is a chemical that can only be made by living organisms (biotically), with strong spectral features, that disappears quickly unless replenished. Unfortunately, it is quite difficult to rule out a chemical as definitely not being creatable by non-living mechanisms (abiotically), so even if we think we have found a biosignature, it could just be that is has been made abiotically, we just haven't found a way to do it yet, so false positives are hard to rule out. Again, this is an active area of research. For recent updates on an exoplanet where as of last year there was the 'strongest evidence yet' of life found elsewhere, see my previous post from April 2025 here.

Water vapour has already been detected in exoplanet atmospheres with a method called transit spectroscopy. How this works is that a planet briefly passes in front of its host star as it orbits, blocking out a small fraction of the star's light. If we know the star's radius, we can calculate the planet's size and distance from the star using the fraction of light blocked out, which is rather neat. Even more excitingly we can probe the chemical fingerprint of a planet's atmosphere using telescopes such as the James Webb telescope, to find out an atmosphere's chemical composition.

Phosphine is one potential biosignature candidate, and excitingly, this may have been detected in the atmosphere of the planet Venus. However, there is some debate about this, with spirited debates between academics on the validity of these results. Furthermore, in recent times there has been some research which suggests that some geological processes may be able to form phosphine abiotically, so its detection isn't a guarantee that life is around, as potentially there are other mechanisms to create phosphine.

In addition to water, there are other chemical requirements thought to be necessary for life, or at the very least, life as it is on Earth. Known as the CHNOPS elements, these are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Before the planet is at its hydrogen rich atmosphere and magma ocean stage, all the elements are formed in the protoplanetary disc before the planet comes together. All the CHNOPS elements condense into little grains and form their way into planets, so when the planet is at its magma ocean stage, these elements can also interact with the hydrogen rich gas. How this works exactly, is very much still shrouded in mystery. Would these elements dissolve into the magma ocean, as hydrogen was found to do in the lab experiments mentioned earlier? Or would they instead stay in the atmosphere? Or form another condensable phase? The new goal is to investigate how carbon and nitrogen move throughout the different planetary layers in the atmosphere, metallic core and mantle at these high temperatures and pressures, which Shahar and other astronomers are attempting to recreate in labs across the world. Again, another area of active research. 

So there we have it, as far as we know extrapolating from life on Earth, the requirements we believe to be necessary are liquid water to transport nutrients and act as a solvent for chemical reactions, an atmosphere and magnetic field to shield life from harmful radiation, and a scheme of recycling for nutrients. Given the exciting developments through working with geophysicists which have revealed that water may be a common consequence of planet formation, it seems now is a particularly exciting time to be joining in the search for around other planets. Interdisciplinary research is becoming increasingly important in planetary science, and I certainly had a taste of that last year, with lectures zooming in to bits of astro-biology/chemistry/physics and even my first exposure to geology. Obviously I am still wedded to astrophysics, yet I can't deny it is pretty cool being able to dip in to other sciences at times, like sticking your head through different doors to find a whole new room of science you'd never known about before. That seems an accurate metaphor for life sometimes, a series of random rooms to stick one's head in, find out some more, and move on to the next one, ideally gathering more pearls of wisdom along the way. :))