What Is Meant By “Habitable Zone” And How Do We Define It?

Our Solar System’s HZ:

Ongoing studies of our planet and its neighbors have revealed some very interesting similarities between Earth and other planets of the Inner Solar System. For instance, Mercury, Venus, and Mars are similar in composition, being terrestrial (i.e., rocky) planets like Earth.

Equally interesting are the telltale indications that Venus and Mars also occupy our Sun’s HZ and were actually quite similar to Earth during their early history. To break it down, Earth and the other planets of the Solar System are believed to have formed roughly 4.56 billion years ago from a protoplanetary disk.

What Is Meant By "Habitable Zone" And How Do We Define It?
Illustration of a star surrounded by a protoplanetary disk. Credit: NASA/JPL-Caltech

Over the next hundreds of millions of years, these planets began to cool, and their internal structures began to differentiate between the core, mantle, and crust. Primordial atmospheres began to form as well, largely composed of either volcanic gases and/or elements left over from the solar nebula that formed our Sun.

In either case, one of the key components in this mix was water. Not only did the formation of our planets involve a large amount of water, but water also began to concentrate on the surfaces of Venus, Earth, and Mars at around the same time (ca. 3.8 billion years ago).

Around this time, Venus is believed to have had oceans on its surface. This is based in part on data gathered by the Galileo spacecraft as it conducted its flyby of Venus in 1990, which revealed that Venus’ highland regions are likely composed of felsic rocks (the formation of which require water).

This ocean would have likely vaporized during Venus’ early history as a result of rising temperatures. This is theorized to have contributed to the runaway greenhouse effect that caused Venus’ atmosphere to thicken and become the incredibly dense and hot one it is today.

What Is Meant By "Habitable Zone" And How Do We Define It?
Comparison between the habitable zones of the TRAPPIST-1 system and the Solar System. Credit: NASA/JPL-Caltech

On Mars, the situation was quite different. Being at the outer edge of our Sun’s HZ, it does not receive enough energy to maintain liquid water on its surface, and its atmosphere has remained one that is primarily composed of carbon dioxide (96%, with argon and nitrogen making up most of the remaining 4%).

Earth, meanwhile, occupies a space between the two extremes. Because of this, it is able to maintain liquid water on its surface, as well as the all-important hydrological cycle – where water evaporates to because vapor in the atmosphere, then condenses to form clouds and return to the surface in the form of precipitation.

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