Earth, our blue planet is a tiny oasis in a vast, cold, and dark space. It is the only planet we know of that can support life. The fossil record tells us that life on Earth has lasted at least 3.5 billion years (the Earth is about 4.54 billion years old), with the oldest physical traces of life dating back 3.7 billion years. And, if some kind of disaster doesn’t intervene, our planet should continue to host life for at least another 1.75 billion years. Here are the 8 things that make life on Earth possible.
1. Milky Way galaxy is suitable for life
The safest environments for life similar to that on Earth are the lowest-density regions in the outskirts of large galaxies like the Milky Way. Our knowledge of galaxy types and their distribution suggests that life as we know it can only exist in about 10% of all galaxies.
The main reason is extremely energetic explosions that have been observed in distant galaxies, called Gamma-ray bursts. They are the brightest electromagnetic events known to occur in the Universe.
The gamma-ray bursts are extremely dangerous events. A GRB within a few parsecs, with its energy directed towards Earth, will mostly damage life by raising the UV levels; during the burst itself and for a few years thereafter.
Some types of galaxies are too compact (stars so close to each other). These are not suitable for life, too.
According to a 2015 study, elliptical galaxies are the most habitable in the cosmos, though.
2. Solar System’s location in the Milky Way galaxy is far from many hazards
Fortunately for us, we live in the “boring” suburbs of the Milky Way. Our Solar System is located in one of the spiral arms of the Milky Way (called the Orion Arm) which lies about two-thirds of the way out from the center of the Galaxy.
The location of the Solar System is a safe harbor, compared to many other places in the galaxy. It is located in what astronomers call the “Galactic habitable zone”, the region of a galaxy in which life might most likely develop.
Solar System’s location avoids the galaxy’s perilous inner regions. There are relatively few stars near the Sun. This reduces risks to Earth (or any other planet in the Solar System) from gravitational tugs, supernovae, or gamma-ray bursts (all observed GRBs have originated from outside the Milky Way galaxy, though).
The unusually circular orbit of our Sun around the galactic center also tends to keep it clear of the spiral arms. Most stars the same age as our Sun have more elliptical orbits. Spiral arms are dangerous places because massive star supernovae are concentrated there, and giant molecular clouds can perturb the Oort cloud comets leading to more comet showers in the inner Solar System.
- Metallicity: metals are the building blocks of “rocky” planets. In the Milky Way, or in any other galaxy, the amount of metals in the interstellar medium varies with location. Please note that all elements other than (heavier than) hydrogen and helium are called metals in astronomy. Most metals are produced in nuclear reactions in the cores of massive stars and distributed through supernovae explosions into the interstellar medium. Some heavy elements like silver, gold, platinum, cesium, and uranium are produced by violent collisions like neutron star mergers. According to a study, around 4.6 billion years ago, two neutron stars collided near the early Solar System. 0.3% of the Earth’s heaviest elements have been created by this event.
- Cosmic Threats: Supernovae pose a great danger to the development of complex life. A supernova is a transient astronomical event that occurs during the last stellar evolutionary stages of a massive star‘s life, whose dramatic and catastrophic destruction is marked by one final, titanic explosion. If one occurred within 10 parsecs of Earth the high-energy photons and protons would obliterate the ozone, leaving land animals unprotected from the Sun’s ultraviolet radiation (marine life would be largely unaffected). But, the closer the supernova, the higher the risk. Scientists also associate supernovae with gamma-ray bursts.
3. Our Sun is a stable, long-lasting, and metal-rich star
Our Sun is the most important source of energy for life on Earth. It’s also a stable and long-lasting star.
Stars more massive than the Sun live shorter, usually not long enough for planets to develop life. In general, the larger a star, the shorter its life.
All but the most massive stars live for billions of years. But, the largest stars, known as hypergiants, have lifetimes of only a few million years.
Less massive (such as red dwarfs), or younger stars are often unstable. They are prone to blasting their planets with bursts of radiation.
The Sun is also unusually metal-rich for a star of its age and type. One possibility is that the Sun formed in a part of the Milky Way Galaxy that had an abundance of metals and then migrated to its current position.
Metal-rich stars are more likely to have planets orbiting around them. Observational data suggest that the Earth-like planets likely formed from circumstellar disks with metallicities Z > 0.1 Z_Sun (study).
Furthermore, all life forms require certain core chemical elements needed for biochemical functioning. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, often represented by the acronym CHNOPS. Our solar system contains a lot of these metals (all elements heavier than hydrogen and helium are called metals in astronomy).
4. The Earth is just at the right distance from the Sun
The Earth receives enough energy from the Sun to allow water to exist as a liquid on the surface.
If it was too close, the Earth would be too hot. It would lose all the oceans. They would basically boil and steam away.
If it is too far, then the oceans would freeze over. You want that big sort of ocean reservoir at the surface which happens when you’re kind of in the middle.
The Earth is in the middle of which is called the habitable zone or Goldilocks zone.
The Goldilocks principle is named by analogy to the children’s story, The Three Bears, in which a little girl named Goldilocks tastes three different bowls of porridge, and she finds that she prefers porridge that is neither too hot nor too cold but has just the right temperature.
The Goldilocks principle states that something must fall within certain margins, as opposed to reaching extremes. In planetary science, the “Goldilocks zone” is the terminology for the band around a sun where temperatures are neither too hot nor too cold for liquid water to exist.
5. Gas giants Jupiter and Saturn protect Earth from bombarding asteroids
According to simulations, without the two biggest gas giants in the Solar System, Jupiter, and Saturn, life may not have been able to gain a foothold on Earth.
Tom Barclay of NASA and his colleagues suggest that massive impacts like the one that was responsible for the formation of the moon 4.5 billion years ago would happen more frequently, and for longer time periods, without Jupiter and Saturn.
Scientists say because of the giant planets like Jupiter and Saturn, the remaining small solar system bodies were either ejected out of the system more quickly or became a part of the existing planets sooner, thanks to the angular momentum the gas giants add to the Solar System.
6. There’s water on Earth
Earth is a blue marble in space: the water gives our planet its blue color: about 71 percent of the Earth’s surface is covered with oceans and lakes.
Without water, most probably life couldn’t emerge on Earth.
Why water is important for life?
Water’s extensive capability to dissolve a variety of molecules has earned it the designation of “universal solvent,” and it is this ability that makes water such an invaluable life-sustaining force.
On a biological level, water’s role as a solvent helps cells transport and use substances like oxygen or nutrients. Water-based solutions like blood help carry molecules to the necessary locations. Thus, water’s role as a solvent facilitates the transport of molecules like oxygen for respiration.
7. Earth has a protective magnetic field
Our planet has a strong magnetic field that deflects most of the solar wind, whose charged particles would otherwise strip away the ozone layer that protects the Earth from harmful ultraviolet radiation.
8. Earth has a big moon to stabilize its axial wobble
The tilt of our planet’s axis would vary over time dramatically without a big moon. This could create some very extreme seasons and weather conditions. Thanks to our Moon, the tilt of the axis is very stable – it only varies between 22.1 and 24.5 degrees during a cycle that averages about 40,000 years. Currently, it is 23.5 degrees.
Without that giant impact that created the moon, we may not be in the same place in our orbit around the Sun. Our planet might be out of the Goldilocks Zone. If this was the case, life on Earth would not be possible.
- “Milky Way and Our Location” on the NASA website
- What is Earth’s location in space? on caltecth.edu
- History of Earth on Wikipedia
- How long will life survive on planet Earth? on the BBC website
- How Much Longer Can Earth Support Life? on Live Science
- “Galactic habitable zone” on Wikipedia
- “Galactic Habitable Zones” on the Astronomy Education at the University of Nebraska-Lincoln website
- Gamma-ray burst on Wikipedia
- Galactic Habitable Zones on the Astrobiology Magazine website
- Sun on Wikipedia
- “Life on Earth Can Thank Its Lucky Stars for Jupiter and Saturn” on Space.com
- Earth’s magnetic field on Wikipedia
- Life on Wikipedia
- Biological Roles of Water: Why is water necessary for life? on the Harvard University website
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