In the most concise terms, the Habitable Zone is the orbital distance at which a planet, orbiting a star, receives enough heat to maintain water in its liquid form on the surface, avoiding both freezing and evaporation.
But, is this distance the same for all stars in the Universe? Not exactly…
The primary factor determining this distance is, naturally, the amount of energy a star emits into its surroundings. As one would expect, our home, planet Earth, is situated within the habitable zone of our star, the Sun. This enables us to have liquid seas, water vapor in the air, and polar ice, showcasing water’s presence in all three forms.
Additional Information: A star with 1.5 times the mass of the Sun doesn’t produce merely 1.5 times the energy but approximately seven times as much. A star with four times the Sun’s mass generates 180 times more energy. If it’s 10 times more massive, it means it emits 20,000 times more radiant power. Conversely, a star with 80% of the Sun’s mass produces only 35% of its energy, while a star with a quarter of the Sun’s mass emits a mere 1% of the Sun’s energy.
The habitable zone around the Sun, as a rough estimate, starts approximately 110-120 million kilometers away from the Sun and extends to a distance of 230-250 million kilometers. This distance may slightly vary depending on the planet’s mass and atmospheric density within this region.
Venus, much closer to the Sun in our solar system, falls slightly outside the inner limit of the Habitable Zone. On the other hand, Mars, farther away from the Sun than Earth, lies precisely on the outer boundary of this habitable zone. Thus, a planet with a slightly larger mass, like Venus or Earth, replacing Mars, could host liquid water, similar to our own planet.
Today, astronomers are earnestly searching for Earth-like planets within the habitable zone in the Universe, akin to our Solar System. However, it is believed that conditions suitable for life might exist on only a few of the roughly 400 Earth-like planets discovered around other stars using various methods.
One of the most notable examples is Gliese 581c (whose existence remains contentious). This planet orbits a red dwarf star located 20 light-years away from Earth. With a diameter 1.5 times that of Earth and a mass 5-6 times greater than our Earth’s, it orbits a much cooler star than our Sun. Consequently, the habitable zone of this star lies much closer to it. As a result, Gliese 581c completes one revolution around its star much faster, approximately 13 Earth Days according to our estimations.
The habitable orbital distance for each star, where liquid water can exist on a planet’s surface, is unique to that star. Covering these distances for each star type in this article would extend its length. Nevertheless, you can find these distances defined individually for each star in our “nearby stars” and “brightest stars” article series on our website.
The definition of the “habitable zone” we explained earlier is based on a planet’s ability to support liquid water on its surface, thus applying to planets that meet Earth’s conditions. Nevertheless, life might develop under vastly different circumstances.
For instance, a planet situated so far from its star that it’s completely frozen could still harbor life. As seen with the moons Europa and Enceladus in our Solar System, liquid water oceans might exist beneath the ice shells of planets, providing an environment for life to thrive. These organisms can harness the volcanic heat emanating from the planet’s core to compensate for the lack of energy from the Sun.
For life to emerge, a solvent liquid is essential for molecules to combine. Water is the most suitable liquid for this purpose. Nevertheless, life forms not dependent on water are conceivable. Liquid hydrocarbons, as on Saturn’s ultra-cold moon Titan, could serve as a suitable medium for life’s development. While the possibility remains uncertain, planets where liquid methane mimics water on Earth (forming seas, evaporating, raining, and freezing) might support methane-dependent life.
In summary, the “habitable zone” as we currently understand it in the universe pertains to life forms dependent on water. However, there is potential for non-water-based life forms to exist. Although we have no evidence of such creatures in the universe, if they do exist and are engaged in astronomy and the quest for life beyond their home planet, they would consider regions with methane in liquid form, far from scorching hot orbits close to their stars, as potential life zones.
Nonetheless, liquid is a prerequisite for life, and for substances to remain liquid, they must steer clear of the extremely hot orbits near stars. Consequently, it remains a hypothetical notion that any form of life, regardless of its nature, could develop and advance on a planet orbiting so close to its star that it’s blazing hot.