Monday, 24 November 2014

Circumstellar Habitat Zones

The circumstellar habitat zone is the region around a star - in our case, the sun - within which a planet must circulate in order to be able to support liquid water on its surface. This zone is more commonly referred to as the Goldilocks zone (GZ). Dependent upon the size of the star and the amount of energy it emits, the position of the GZ will vary. The more energy a star emits, the further away the GZ will be positioned; and vice versa. There are many other contributing factors to this GZ equation, but for the simplicity of this article and to avoid detraction from its main point, the above stated facts are entirely sufficient.   

The earth is 4.54 billion years old; it has a diameter of 7,891 miles; and circles the sun once a year. The average diameter of the earth's orbit around the sun is 186,411,358 miles. This means that usually - due to the earth's orbit of the sun not being entirely circular - that the earth is a distance of 93,000,000 miles from the sun. This distance is also known as an Astronomical unit (AU). Due to earth's positioning within the solar system's GZ, it is able to support liquid water on its surface, and maintain life. Therefore, this poses a rather large question: How many planets in the universe could potentially harness life? How many planets - just like earth - could be sitting within the GZ of their local star, hosting liquid water allowing life to thrive? 

Take a seat.

Let's start small. First of all there are planets and stars. Planets and stars are situated within a solar system - effectively a star and its orbiting planets -, just like earth is positioned alongside its fellow planets circulating around the sun. Next, solar systems are located within a galaxy - just as our solar system is positioned within the Milky Way. And last but not least, galaxies are contained within the universe - which as far as we know at this current point in time, there is only one. Our galaxy - the Milky Way - is estimated to contain 200 - 400 billion stars, and be 100,000 light years across. Light travels at roughly 300,000,000 meters per second - or 186,411 miles per second. In order to make these figures more manageable it may be easier to envisage that light travels around the earth roughly 8 times per second. Therefore, if light can travel around the earth 8 times per second, relentlessly, for every second of existence, it would be reasonable to conclude that if it takes 100,000 years to span the Milky Way, then the Milky Way is very large indeed. Although it is by no means the largest of the universe's galaxies.

The observable universe is 28,000 megaparsecs across (1 megaparsec is the equivalent of 3.26 million light years). However, the actual size of the entire universe is unknown. Therefore, for the sake of the remainder of this article we will assume that the universe is the size of the current observable universe - 28,000 megaparsecs. 

There are an estimated 200 billion galaxies - varying in sizes - within the known universe. One of which is the Milky Way, within which there are an estimated 300 billion stars. Therefore, if on average each galaxy contained the same number of stars as the Milky Way - 300 billion -, then the number of stars within the universe would be a cool 60,000,000,000,000,000,000,000 (60 sextillion). On top of this, it is estimated that there are 1,000,000,000,000,000,000,000,000 (1 septillion) planets within the universe, of which 40,000,000,000 (40 billion) are thought to be circulating a living star. However, of the 40 billion planets circulating living stars, only 11 billion of those planets are believed to be positioned within the GZ. If this is the case, then there are 11 billion planets within the known universe that could potentially be harnessing life. This is a massively significant number, as with statistics like this, the search for extraterrestrial life seems to be a genuinely worthwhile endeavor. Furthermore, if the entire universe were to be twice the size of the observable universe - which it would not be far fetched to hypothesise -, then there would be roughly 22 billion planets capable of hosting life, and so on. These numbers are phenomenal, and breathtaking. However, no matter how amazing they seem to be, they should be taken within a rather large pinch of salt.


The problem is, this whole topic of planet numbers, star numbers and the chances of extra-terrestrial life within the universe is a gargantuan can of worms. This article has provided an incredibly simplified overview of the entire issue. The complexities, theories, statistics and proposed equations that surround this matter could easily fill hundreds if not thousands of volumes. There is simply too much to write within one single article. If you wish to know more, then my personal recommendation for a mind-bending and suitable starting point would be the Fermi paradox.


I feel at this point as the article draws to a close, it would be quite apt to quote Arthur C. Clarke, for he sums up the entire situation perfectly, "Two possibilities exist: either we are alone in the universe or we are not. Both are equally terrifying".

An unimaginably simplified model of the known universe. 

Earth positioned exactly 1 AU from the sun, sitting within this solar system's GZ.

A very basic depiction of earth's orbit around the sun. 

The milky way is 100,000 light years across, and is the galaxy we call home.

- Until the next Butterfly...