What Are The Factors That Make A Planet Habitable?

The main factors that make a planet habitable are: orbiting inside its star’s habitable zone (the “Goldilocks zone” where liquid water can exist), a stable, long-lived host star, the right mass and size to hold an atmosphere and a magnetic field, a roughly circular orbit, a moderate rotation rate, and the right mix of chemicals (especially liquid water) on the surface.

Life has been on its way to dominating planet Earth for the past 3.7 billion years. What started as microbes and single-celled organisms have evolved into giant multi-cellular plants and animals capable of doing many complex things. It was a long journey to get here, and many factors had to fall in Earth’s favor to make life possible.

For starters, there are a few general requirements for a planet to be habitable.

What Are The Factors That Make A Planet Habitable?

• It has to be a comfortable distance away from a star (Habitable Zone)
• The stars around it have to be ‘stable’.
• It should not have a very low or very high mass.
• It must rotate on its axis and revolve.
• It should have a molten core.
• It should hold an atmosphere.
• It should contain liquid water and other compounds that are required for life.

However, that’s not all. There is a wide variety of reasons (including those listed above) that could make life possible on any planet. These reasons could range from cosmological factors to the star system in which it resides and its nature.

This article will describe how the star system and the conditions on the planet make it conducive to life, followed by an exploration of the cosmological factors involved.

Factors Of The Star System That Determine Habitability Of A Planet

First, we will consider how the star system could affect the habitability of its planets. The necessary conditions for planetary habitability in the star system are:

1. It should be within a certain distance from the star to achieve a temperature where water could exist in the liquid state, while also ensuring that compounds like proteins and carbohydrates (in the case of carbon-based life forms) do not break down,
2. The planet should be in a stable orbit around the star for a long time.
3. The orbit of the planet should be circular, or very close to it. This ensures that the conditions remain somewhat the same during the planet’s entire revolution around the star.
4. It should not be too close to a giant planet, which would cause a continuous shower of asteroids to be directed toward it or might disturb the planet’s orbit. In fact, a gas giant that is far away would prevent large asteroids from hitting the habitable one and destroying any life on it. Jupiter performs a similar function in our solar system.
5. The star system should not be close to cosmic explosions, like supernovae, gamma-ray bursts, etc.

The first three points reference the concept of a ‘habitable zone’ or ‘Goldilocks zone.’ This is a zone that exists in each planetary system that depends on the star’s mass and luminosity.

A planet that stably orbits its star inside the habitable zone will have a temperature range that can support life and ensure that water can exist in a liquid state.

The importance of water has often been emphasized when it comes to life on other planets. Water is an essential ingredient for transporting nutrients and chemicals between cells. It is also capable of dissolving many substances, more than any other liquid. For this reason, it has been called the universal solvent.

A planet within an appropriate distance from the star would also receive the right amount of energy needed to run the chemical reactions for life and for cells to function. The light energy from a star is the primary source of energy for the planets and any life form that may develop on it. Too much or too little light energy from the star will harm life and make the planet uninhabitable.

So, for planets to support life, they must be within this habitable zone throughout their lifetime. If it’s too close to the star, not only will there be extreme temperatures (both hot and cold), but the planet will be locked tidally with its star, such that only one side will ever face the star.

If the planet is too far away, the temperature will be too low for any processes necessary to sustain life. The low temperature will also result in the water mostly existing in a solid state, which is unsuitable for life.

Factors Related To The Planet That Determine Its Habitability

In the case of a planet, the following factors play a role in its ability to host life:

1. Mass and size
2. Atmosphere
3. Constituent chemicals and water
4. Rotation rate

The planet’s mass and radius affect habitability in many ways. It determines the extent of production of a magnetic field, which is necessary to protect the planet from charged matter coming from the star.

The mass also determines the gravitational force. This plays a role in atmospheric height and retentivity. A planet with too much mass will end up as a gas giant. If it is too low in mass, its gravity won’t be strong enough to hold onto a substantial atmosphere at all, which is just as detrimental to life.

But why would it be detrimental to life? What role does the atmosphere play in harboring life on a planet?

Apart from determining the climatic and weather patterns of the planet, the atmosphere traps heat and helps it maintain a stable temperature range. It blocks the harmful radiation coming from its star and outer space. It also provides the gaseous compounds, like nitrogen and carbon dioxide, which are necessary for life-supporting processes.

The next factor is the constituent chemicals and water on the planet. Although the planets and moons from the same star system would have a similar chemical makeup, certain types of compounds are necessary for the development of life. If it’s a carbon-based life form, chemicals needed to make proteins and carbohydrates should be present.

Apart from the availability of compounds needed to create nutrients, there should also be a system that circulates the water to be accessible to various life forms. On Earth, this occurs in numerous ways, like the water cycle, carbon cycle, etc., and during events like volcanic eruptions.

The rotation rate of the planet is also fundamental for its habitability. It is the primary driver of air circulation in the atmosphere via the Coriolis effect. The Coriolis effect is a fictitious force that arises due to Earth’s rotation. It causes a shift in the motion of air when seen from the ground.

The Coriolis effect and the circulation of atmospheric air are responsible for the circulation of heat and motion and the distribution of clouds. Any changes in the planet’s rotation rates could change its climatic conditions, which would significantly affect the creation and evolution of life.

In the case of planets like Venus, where the rotation rate is slow (a day there lasts around 116 Earth days), the Coriolis effect becomes very weak. Such a scenario would result in clouds becoming stationary and very thick, thus blocking any light from reaching the surface.

There are planets like Earth that rotate much more rapidly. This heightened Coriolis effect results in better cloud motion in the form of bands and allows for the better passage of sunlight.

Cosmological Factors And The Anthropic Principle

We finally come to the third set of factors, which are the cosmological factors. Here, the main principle is the role of physical constants (like the speed of light, Planck’s constant, etc.) and the strength of the fundamental forces. The fundamental forces are the gravitational force,  electromagnetic force, and weak and strong nuclear force.

To give some idea about how these quantities could play a role in the creation of life, let’s consider the gravitational force first. If gravity became just slightly weaker, it would not be possible for thermonuclear fusion to occur inside stars, so they would die out. If it is higher, the stars would use up their fuel much faster, giving no time for life to evolve. In either case, life could not be present.

Let’s look at the electromagnetic force. If it became a little weaker, electrons would not get bound to the atom’s nucleus, and molecules would not form. If slightly stronger, electrons would remain within the atom, and reactions would not happen. In both cases, the creation of life could not take place.

This is true for the other two fundamental forces and the physical constants. If any of the four fundamental forces became slightly stronger or weaker, the formation of life would be impossible anywhere in the Universe.

In a sense, it looks like our Universe has a special quality attached to it. After all, the physical constants and the fundamental forces seem finely tuned to allow for intelligent life. It has often led to creationism ideas, namely that our Universe is all based on a divine plan. However, this is unscientific, as it does not give rise to any predictions that can be tested.

In this scenario, physicists have established two principles to explain this fine-tuning. The principles are called the Weak and Strong Anthropic principles.

The Weak Anthropic principle states that our present Universe is in a state that allows for the existence of intelligent life (or observers). It is pretty straightforward and implies that the properties of the Universe are sufficient for observers to be created and to evolve. Properties of the Universe include quantities like its age and the values of the fundamental constants.

However, the Strong Anthropic principle states that for intelligent life to occur, having these properties is a necessity.

The two anthropic principles seem to imply that life in our Universe exists because the properties of the Universe permit it. In any other Universe, one with different kinds of properties and different values of physical constants, intelligent life may not necessarily exist.

What Makes Earth Habitable?

So far, we have talked about habitability in general terms. But the one planet we know for certain is habitable is our own, so it helps to see how Earth manages to tick every box at the same time.

Earth orbits about 150 million km from the Sun, which is exactly one astronomical unit, placing it comfortably inside the Sun’s habitable zone. At that distance, the surface stays warm enough for water to remain liquid, and today water covers roughly 71% of the planet. Its atmosphere is around 78% nitrogen, 21% oxygen and 1% other gases, which traps heat, blocks harmful radiation and supplies the compounds that living things depend on.

Deeper down, Earth’s rapid rotation and molten nickel-iron core generate a global magnetic field that deflects the solar wind and shields both the atmosphere and the surface from damaging radiation. And unusually for a rocky planet, Earth has a large Moon. Calculations by Jacques Laskar and colleagues showed that the Moon keeps Earth’s axial tilt remarkably steady, wobbling only within a couple of degrees over tens of thousands of years. Without it, our tilt could swing chaotically between 0° and about 85°, triggering violent shifts in climate. Moonless Mars, by comparison, already sees its tilt vary between roughly 0° and 60°.

None of these features is enough on its own. What makes Earth special is that it happens to have all of them together, and has kept them stable for billions of years.

Plate Tectonics: Earth’s Natural Thermostat

There is one more factor we have not touched on, and many scientists consider it crucial for keeping a planet habitable over billions of years: geological activity. On Earth, plate tectonics drives the carbonate-silicate cycle, a slow feedback loop that behaves like a planetary thermostat.

Here is how it works. When the climate warms, rainfall and the chemical weathering of silicate rocks both speed up. Weathering pulls carbon dioxide (CO₂) out of the atmosphere and locks it away in carbonate minerals, which cools the planet back down. When the climate cools, weathering slows, but volcanoes keep adding CO₂ to the air, so the planet gradually warms again. This negative feedback has helped hold Earth’s surface temperature within a life-friendly range even though the Sun has grown steadily brighter over its lifetime.

Plate tectonics is what keeps the whole cycle turning. Subduction drags carbon-rich rock down into the mantle, volcanoes at ridges and arcs return CO₂ to the atmosphere, and mountain-building constantly exposes fresh rock for weathering. A rocky planet without this recycling could lose its thermostat and tip into a runaway greenhouse or freeze solid, which is one reason geological activity now features heavily in discussions of exoplanet habitability.

How Do Scientists Search For Habitable Exoplanets?

We can list the conditions for habitability, but how do astronomers actually check whether a distant world meets them? The first step is finding rocky planets that sit inside their star’s habitable zone. The harder step is reading their atmospheres.

The main tool is transmission spectroscopy. When a planet passes in front of its star, a sliver of starlight filters through the planet’s atmosphere on its way to us, and the gases there absorb specific wavelengths, leaving telltale fingerprints in the spectrum. The James Webb Space Telescope used exactly this method to detect carbon dioxide in the atmosphere of the hot gas giant WASP-39 b.

Astronomers are especially interested in ‘biosignature’ gases that might hint at life, such as oxygen (O₂), methane (CH₄) and nitrous oxide (N₂O). One intriguing clue is chemical disequilibrium: spotting gases like oxygen and methane together, since they react with one another and should not last long unless something keeps replenishing them. The catch is that these signals are hard to read. Atmospheres of small rocky planets produce faint signals, some of these gases can build up without life at all (oxygen can accumulate from evaporating oceans, for instance), and different teams can interpret the same Webb data in different ways. A convincing sign of life will probably have to wait for future telescopes.

Conclusion

To conclude, we can see that many factors determine the feasibility of life on a planet. From the kind of Universe and the star system to the planet itself, we have seen how even a slight change in the conditions could hinder the development of life.

These factors include the values of the physical constants, the strength of the fundamental forces, the type of star, the size of the planet, the distance between them, etc. Things like the nature of the atmosphere present on the planet, its constituent minerals and compounds, and the amount of energy it receives from its parent star will also play a role.

The nature of the Universe determines if the creation of life is possible within the entirety of that Universe. In the case of star systems, so long as it is within the habitable zone, multiple planets could host life, given that all other factors are similarly in their favor!