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

Milky Way over the Lake Tekapo
Our knowledge of galaxy types and their distribution suggests that life as we know it can only exist in about 10% of all galaxies. Our galaxy, the Milky Way is one of them, it is a galaxy that is suitable for life. Image: “Milky Way over the Lake Tekapo” on Deposit Photos.

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.

Things that make life on Earth possible: Sun's location in the Milky Way galaxy
Things that make life on Earth possible: The Sun’s location in the Milky Way Galaxy

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.

Things that make life on Earth possible: Sun in space
Things that make life on Earth possible: The Sun is a G-type main-sequence star (G2V) based on spectral class and is informally referred to as a yellow dwarf. It is stable (even less active compared to its siblings), long-lasting, and metal-rich. Image: NASA

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.

Things that make Life on Earth possible: Earth is at just the right distance from the Sun
Things that make life on Earth possible: Earth is at just the right distance from the Sun. Image: NASA – ISS013-E-78960 (9 Sept. 2006) – A rising sun and Earth’s horizon are featured in this image photographed by an Expedition 13 crew member on the International Space Station.

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.

Saturn from Cassini, 2016-10-28
Life on Earth may not be possible without the two biggest gas giants in the Solar System, Jupiter, and Saturn.

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.

The Blue Marble
Apollo 17 spacecraft took a photo of Earth from space, at a distance of about 45,000 kilometers (28,000 miles). This image, with the official NASA designation AS17-148-22727, became known as “The Blue Marble“, one of the most iconic photos of Earth from space. In fact, it was not the first clear image of Earth taken from space – similar photos had already been taken as early as 1967. But, the 1970s were the scene of a big surge in environmental activism. For example, on April 22, 1970, the first “Earth Day” was organized by Gaylord Nelson, former senator of Wisconsin, and Denis Hayes, a Harvard graduate student. Millions of people gather in the United States for the event. So, in today’s terms, image AS17-148-22727 went “viral” and became a symbol of the environmental movement, as a depiction of Earth’s frailty, vulnerability, and isolation amid the vast expanse of space. According to NASA archivist Mike Gentry, it is among the most widely distributed images in human history.

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.

Without it, solar winds (charged particles that flow from the Sun) would strip away Earth’s oceans and atmosphere. The lack of a magnetic field on Mars caused a near-total loss of its atmosphere.

Mars - Valles Marineris Hemisphere Enhanced
Mars may once have been more hospitable to life than it is now. Scientists think the magnetosphere of Mars collapsed around 3.7 billion years ago, and it eventually lost its atmosphere. Now, Mars has a very thin atmosphere: the atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi; 6.0 mbar), about only 0.6% of Earth’s mean sea level pressure of 101.3 kilopascals (14.69 psi; 1.013 bar). Photo: NASA

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.

Furthermore, the tides would be only about one-fourth of their current size. By driving the tides, our lunar companion may have jump-started life on Earth, or at least accelerated its progression.

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.

Things that make Life on Earth possible: Full moon over a forest
Without the Moon, life on Earth may not be possible. Source: Deposit Photos


M. Özgür Nevres

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