Goldilocks Zone: The Search for Habitable Planets

Scientists have been in pursuit of finding habitable planets for years. The Goldilocks zone is a theoretical region around a star where the temperature conditions for the existence of liquid water are suitable. This term was coined by the Chinese astrophysicist Su-Shu Huang in 1959.

Liquid water is a fundamental factor for life based on carbon as we know it. This region is defined as the area where life can be potentially sustained due to the existence of liquid water.

Planets existing in the Goldilocks zone’s outer line remain frozen just like Mars. Planets existing in the inner line of the Goldilocks zone remain in evaporating oceans just like Venus. Planets in the middle of the zone include suitable conditions for liquid water to exist just like Earth.

However, as research has shown, the presence of water on a planet does not definitively constitute evidence for life. This term has evolved over the years, revealing that there are other different determinants of life.

Etymology of the Name Goldilocks

The Habitable zone is sometimes referred to as a goldilocks zone. The name “Goldilocks” derives from the English fairy tale “Goldilocks and the Three Bears.” The story was first published anonymously in 1837.

In Goldilocks and the Three Bears, Goldilocks finds three alternatives. They are porridge, chairs, and beds. Two of them are too much (too hard or too soft), but one is just right. The concept of Goldilocks has been adapted to research on habitable planets based on this story

• Too Close and Too Hot: Planets that are very close to their stars will have higher temperatures. High temperatures make it impossible for them to contain liquid water. Venus and mercury can be given as examples of this.
• Too Distant and Too Cold: Planets that are very far from their stars will be colder. Low temperatures will cause any water to freeze. This could make it impossible to find liquid water, as observed on Mars.
• Just Right: The planets in the middle are in the right zone for liquid water to exist, which will have the temperature of that Goldilocks Zone. The world can be given as an example of this.

Location of the Goldilocks Zone

The location of the Goldilocks zone around a star depends on the type of star, and its size. The Goldilocks zone around a star is determined through a series of calculations that use our Sun’s physical properties as a constant value for comparison. Then the star’s absolute luminosity, which is related to the star’s absolute magnitude, is calculated.

Image-1. Habitable Zones of Different Stars. (Image Credit: NASA/Kepler Mission/Dana Berry)

The types of stars are divided into F, G, K, and M, respectively. F-type stars become brighter and hotter, while M-type stars become smaller and colder. The Sun is a G-type star. It will be possible for bigger and hotter stars to support life in more distant regions.

The Goldilocks zone is defined by an inner and outer boundary. A runaway greenhouse effect similar to that observed on Venus is observed at the inner boundary.

At this distance, the water reservoir boils, and hydrogen and oxygen decompose. Hydrogen is lost in space, while oxygen combines with rocks to form oxidized compounds.

The outer limit of the habitable zone is determined by the distance at which the greenhouse effect is insufficient to sustain liquid water and the CO2 in the atmosphere begins to condense.

Planets in the Goldilocks Zone

Mercury in our solar system is too hot to support life, while Venus and Mars are located at the boundaries of the Goldilocks zone. The Goldilocks zone also includes the ones known as Kepler-22b, Kepler-62e, Kepler-62f, Kepler-69c, Kepler-61b, and Kepler-47c.

Venus, Earth, and Mars all orbit within the sun’s habitable zone. It is thought that there were once liquid water and oceans on Venus. It is also thought that a runaway greenhouse effect has destroyed the oceans located on Venus.

Venus’ runaway greenhouse atmosphere makes the planet even hotter than Mercury, which is much closer to the sun. That atmosphere eliminates the possibility of life on the planet’s surface.

Image-2. North Polar Ice Cap of Mars. (Image Credit: NASA/JPL/Arizona State University, R. Luk)

Even though the research is ongoing as to whether there is liquid water on the surface of Mars, it is proven that glaciers exists. With average temperatures around -65°C (-85°F), Mars is a cold planet. At these low temperatures, water is more likely to exist as ice.

Polar caps of Mars are made up of water ice and dry ice which is carbon dioxide in frozen state. The northern polar cap, Planum Boreum, consists mainly of water ice. The southern polar cap called Planum Australe also has significant quantities of water ice but covered by a layer of dry ice.

Many regions on Mars contain permafrost, a mixture of ice and soil. The presence of permafrost suggests that water ice is stable in these areas just below the surface.

Image-3. Habitable Zones relative to the size of a star. (Image Credit:Wikipedia)

In our solar system, the Earth is clearly in the Goldilocks zone with much of its surface covered by liquid water. Venus is too hot, Mars is too cold, and Earth is just right.

Major Constituents of Terrestrial Planetary Atmospheres

Water vapor (H₂O) and carbon dioxide (CO₂) are regarded as greenhouse gases. The greenhouse effect is a natural process causing a much warmer surface of planets.

• The atmosphere of Earth consists of 78% nitrogen (N₂), 21% oxygen (O₂), 0.93% argon (Ar), 0.04% carbon dioxide (CO₂), and a trace amount of (H₂O).
• Venus has a thick and dense atmosphere. The atmosphere of Venus consists of 96.5% carbon dioxide (CO₂), and 3.5% nitrogen (N₂). In addition, it contains trace amounts of argon and water vapor (H₂O). Due to the existence of large amounts of carbon dioxide (CO₂), Venus is exposed to the greenhouse effect. This effect raises the surface temperature of Venus to around 867°F (464°C).
• Mars has a thin atmosphere. The pressure in Mars’ atmosphere is less than Earth’s. The atmosphere of Mars consists of 95.3% carbon dioxide (CO₂), 2.7% nitrogen (N₂), 1.6% argon (Ar), and 0.13% oxygen (O₂). The main greenhouse gas is CO₂. However, low atmospheric density causes a weak greenhouse effect.

Surface Temperatures of Planets

Distance from the its star is main factor for planetary surface temperatures. Planets tend to get colder the farther a planet is from the Sun. Apart from the distance from its stars, various factors such as its surface albedo, and the greenhouse effect have an impact on the average temperature of planets.

Albedo refers to how much sunlight a planet reflects. Planets with high albedo are expected to have cooler temperatures. Earth with its white clouds can reflect more sunlight, causing cooler temperatures.

Planets with low albedo are expected to be hotter. Mercury with its dark rock surface absorb more sunlight, causing hotter surface.

Calculating a rough estimate of the planet’s effective temperature (Te) can provide information about the actual surface temperature (Ts) and the existence of liquid water. The effective temperature (Te) can be calculated with this formula:

• Te=(S(1-A)/σ)^0.25

Where:

• S represents the amount of solar energy received by the planet per unit area.
• A is the planet’s albedo. This value is in between 0 and 1.
• σ is the Stefan Boltzmann constant.

In order to find the actual surface temperature (T s), the greenhouse effect must be considered.

• Δ T represents the warming effect of greenhouse gases.
• Ts=Te+ΔT

Image-4. Habitable Zones. (Image Credit: NASA/Kepler Mission/Dana Berry)

This formula essentially calculate the balance between incoming solar radiation and incoming solar radiation. Assuming the planets acts like a perfect blackbody, the estimate temperature of planets are:

• Venus: 867°F (464°C)
• Earth: 59°F (15°C)
• Mars: -85°F (-65°C)

Photosynthetic Habitable Zone

A photosynthetic habitable zone (PHZ) is a modification of the traditional concept of habitable zones (HZ). This new idea was named by Cassandra Hall in 2023.

This idea considers not only the presence of liquid water but also the availability of proper light for oxygenic photosynthesis. This approach recognizes that liquid water alone is not sufficient for complex life. Besides liquid water, the availability of appropriate light for photosynthesis is also crucial.

This process ranges from 400 to 700nm in wavelength and uses sunlight’s energy; it has been responsible for producing oxygen on earth enabling complex life forms.

PAR (Photosynthetically Active Radiation) refers to the range of light wavelengths (400 to 700nm). Photosynthetic organisms use this range of light for energy.

Oxygenic photosynthesis is the process by which plants, algae, and cyanobacteria convert light energy, water, and carbon dioxide into oxygen and glucose. Oxygen production is a significant biosignature of life

Based on datas from Cassandra Hall’s research, some planets such as Kepler-452b, Kepler-1638b, Kepler-1544b, Kepler-62e and Kepler-62f are believed to be in the photosynthetic habitable zone.