by Prasad Ganti

The Goldilocks Zone

While there are billions of galaxies and each galaxy harboring billions of stars, the probability of finding life elsewhere in the universe is significant. There may not be life resembling humans, in our neighborhood. Like in our solar system. Life seems to evolve with the right combination of different factors. Referred to as the goldilocks zones (scientifically called the Circumstellar Habitable Zone). Let us consider what those factors are.

The position of Earth in our solar system is supposed to be unique for life formation. It is not too close to the Sun nor too far away from it. At the right distance, it is neither too cold nor too hot. Where there is a solid surface of rocks, where there is a breathable environment, where the range of temperature swings is mild. Where water exists in liquid form. Where there are no violent phenomena like volcanoes on Venus, or the giant storm on Jupiter. Yes, there are storms and volcanoes on the earth, but they are comparatively very mild.

Mercury is hot, Venus is a runaway greenhouse, Mars is rocky but cold, asteroid belt is some junk leftover from the solar system formation, Jupiter onwards are gaseous giants which have very little solid cores. Earth’s distance from Sun is falls within the goldilocks zone. For other stars, the goldilocks zone varies. Depending on the star’s temperature which determines the radiation, star’s size which determines gravitational fields etc.

Our earth does have protection from Sun’s solar winds, which generate very energetic charged particles. The earth’s magnetic field, resulting from the molten metallic core of iron and nickel in the innermost layer, protects us from all the harmful radiation which could have prevented life from forming in the first place, or demolishing it once formed. But the earth is not immune from all the disasters which could befall in our solar system. There are extraneous objects like the meteors and asteroids which swing by and threaten a collision. The most famous asteroid collision wiped out the dinosaurs sixty five million years ago.

Earth also has the unique feature called plate tectonics. Which means that the crust is made of several thin plates, like a jigsaw puzzle. Due to the hot and molten core in the interior, these plates float and move and collide with one another. Resulting in volcanoes, earthquakes etc. They also recycle metals to the top. Diamonds, which result from the cooking of carbon in the earth’s interior under extreme heat and pressure, would not come to the surface without the plate tectonics. Because of plate tectonics, the continents have moved around and will move in the future as well. Such changes are very small compared to human lifespans.

Another factor is that the presence of moon has been a stabilizing factor in the earth’s orbit around the sun. The earth and the moon are tidally locked with each other. After life formed on earth, for the first two billion years, it was very simple. Just single celled organisms. It is said that accidentally, one form of life called archaeon gulped another form, a bacteria. This led to creation of complex cells called eukaryotes, where the genetic material DNA is contained in a separate compartment called the nucleus. And then to multicellular organisms, and the evolution of sex about a billion years ago. And then the explosion of species, called the cambrian explosion, about half a billion years ago.

There could be microbial life on one of Jupiter’s moons called europa, which is supposed to be harboring an ocean of liquid water beneath its surface. Another candidate is titan, saturn’s moon, which is supposed to be having lakes of organic chemicals like methane. The presence of organic chemicals is supposed to be conducive to evolution of life. Organic chemicals have been found in clouds in interstellar regions in our galaxy, and confirmed using radio telescopes. After all, in a simple explanation, life is just an amalgamation of complex organic chemicals. DNA is an organic chemical, which comprise genes, which create proteins, which themselves are organic chemicals.

I read about another goldilocks zone concept in an article titled “Alone in the milky way” in Sept 2018 issue of Scientific American. Called the galactic habitable zone, our solar system is at the right distance from the center of the universe. It is about two thirds the distance from the center, and one third from the outer edge. Since our galaxy is a spiral galaxy, we are situated on one of the spiral arms. If the star is too far from the center of the galaxy, metal formation does not happen. If it is too close, there are violent phenomena like getting sucked into the supermassive black hole at the center.

So what next ? Is our milky way in a goldilocks zone of a higher cosmological structure, which makes life possible ? Is our universe itself in a goldilocks zone, amongst the multiple universes of which we have no evidence, of right parameters to support life ? Looking forward to future discoveries to throw more light.