The Habitable Zone

Earth as the “Goldilocks Planet”

The Earth is unique. As far as we know, it is the only planet in our solar system that has given rise to carbon-based life. Life on Earth, however, could not have taken hold without the serendipitous good fortune of being in just the right place, having just the right size and composition, and meeting a number of other seemingly unlikely requirements.

While we do not yet know whether life has arisen on other planets in the universe, there is growing evidence that life is at least a rare occurrence, and that intelligent life may indeed be exceptionally rare. The Earth is the “goldilocks” planet: not too cold, not too hot, but just right to support life. Here we take a closer look at some of the conditions that allow life to survive, and thrive, on our home planet.

Get in the [Habitable] Zone

In the story Goldilocks and the Three Bears, Goldilocks must try two soups – one too hot and the other too cold – before she finds the one that’s just right: warm, but not too hot to eat. Our solar system, as it turns out, is similar in that some planets are far too hot, and others far too cold, to sustain life as we know it. And our solar system has far more “cold soup” than “hot soup”; only two planets, Mercury and Venus are too hot to support life, the rest are too cold. While here on Earth we may complain when there’s a cold spell or a heat wave, these temperature “extremes” are nothing compared to what you find on other planets. Sure, 0 °C may seem cold and 40 °C may seem hot, but consider the average temperature found on Venus (460 °C) or Mars(-63 °C). Earth, however, has a very comfortable average temperature of 14 °C or (57.2 °F).

habitable zone
In our solar system, the Earth is just the right distance from the Sun to have liquid water on the surface and support the development of life. Source: NASA.

So why is the Earth’s global temperature so moderate? The primary reason is that Earth is in just the right place in the solar system. It orbits the Sun within a relatively narrow band known as the habitable zone. The habitable zone is a range of distances from a star in which a planet will receive enough heat to maintain liquid water on its surface (assuming water is present). This zone changes depending on the size and temperature of the star (or stars) at the center of the solar system. For our own star, the Sun, and our own solar system, the habitable zone extends from just beyond the orbit of Venus to just before the orbit of Mars. Earth, no surprise, is nearly at the dead-center of our solar system’s habitable zone.

Of Planet Sizes and Atmospheres

Photo of Earth and the Moon
Earth is the right size to maintain a moderately thick atmosphere with the right composition of gases to support a variety of life. The Moon, however, is too small to maintain a similar atmosphere. Source: NASA.

While the Earth is just the right distance from the Sun, it also happens to be just the right size. Size matters when it comes to planets due to the fact that gravity is tied to mass. The more mass an object has, the more gravity it has. Gravity is important because it not only holds you and I to the surface of the planet, along with all the objects we see around us, but it also keeps the various gasses that make up the atmosphere from simply slipping out into space. If the planet is too small it may not have enough gravity to hold onto heavier gases like nitrogen and carbon dioxide. Mercury, for example, there is only a trace atmosphere composed of light elements like oxygen, hydrogen and helium. Not only is Mercury small, however, it is also very hot, which increases the escape velocity of molecules in the atmosphere, allowing them to diffuse out into space at a faster rate than on a cooler planet like Mars.

Our atmosphere has the right composition and the right thickness to comfortably support life. If the atmosphere was much thicker than it is, we’d be crushed by the weight of the gas molecules all around us. Such would be the case on Venus, which has an atmospheric pressure 92 times that of Earth. Another important feature of our atmosphere is the presence of a an ozone layer, which absorbs harmful UV radiation from the Sun, protecting life on the surface. The health of the ozone layer became the planet’s top environmental concern 30 years ago when it was discovered that man-made chemicals, most notably chlorofluorocarbons (CFCs), were interacting with, and depleting, the ozone layer.

Earth’s Shield: the Magnetic Field

Earth's magnetic field
The Earth’s magnetic field protects the planet from deadly solar radiation.

The ozone layer is not the only shield the Earth has against deadly solar radiation. The planet has an enormous magnetic field that extends tens of thousands of kilometers out into space. It is believed to be generated by the the movement of the planet’s liquid metal outer core around the solid metal inner core. The magnetic field deflects much of the solar wind – the stream of charged particles – that the Sun regularly sends the Earth. Despite the fact that the Earth orbits some 93 million miles away from the Sun, the amount of solar radiation that reaches the Earth on a daily basis is staggering. Without the magnetic field to protect the planet, the solar radiation would likely kill all life living on the surface of the planet. Every once in a while, bursts of solar radiation make it through the magnetic field near the northern and southern poles. When the charged particles interact with the upper atmosphere (the thermosphere),  light energy is released, creating wispy bands of color in the night sky called auroras.

Once in a Blue Moon

In addition to being the right distance from the Sun, the right size, and having a most useful magnetic field, the Earth has been blessed with a rare planetary partner: the Moon. While most other planets have multiple moons – heck, Jupiter has 63 of them – the Earth’s Moon is exceptionally large relative to the size of the Earth. The Moon is quarter the diameter and 1/80th the mass of Earth, making it the fourth largest moon in the solar system, and the largest Moon relative to the size of its host planet. Why is the Moon so important? The Moon not only produces the tides, making highly diverse tidal zones possible, it also helps the Earth maintain a stable rotation about its axis. Without the gravitation force of the Moon, the Earth would begin to wobble much more with a constantly-shifting axis of rotation, resulting in abrupt and severe climate changes around the globe. If this wasn’t bad enough, consider all the animals that rely upon the moonlight to locate and capture prey or find their way to breeding areas or other important locations. Entire ecosystems would be disrupted, potentially leading to mass-extinctions. Although as a species we could probably survive without the Moon, it is uncertain whether our civilization could.

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