Tag Archives: Solar System

Why is The Earth a Life-Sustaining Planet?

landscape-at-sunsetWhat has allowed the earth to maintain stable conditions that are favorable for life? To answer this question we need to look at our planet’s history, and ask why the earth has been habitable for almost 4 billion years. This is an extremely long time, and probably the time needed for intelligent life to evolve. Scientists have second-hand evidence to go by, like entering a crime scene after the fact. But there is plenty of evidence to reconstruct the major historical events of our planet; this comes from a wide range of scientific fields.

The earth is the home of all life that we know about, possibly the only home humans will ever have. However, given enough time, it is possible that much like ancient sailors settled new lands, spaceships will cross space to new worlds. Up until now we should consider ourselves very fortunate that our planet has maintained the conditions necessary for life. For sure, the earth has gone through dramatic changes in its lifespan, but not significant enough to snuff out life. Let us examine a number of plausible reasons why a life-friendly earth has endured for so long:

The Goldilocks Zone

The earth’s location in relation to the sun has been called the ‘Goldilocks Zone’ or ‘Habitable Zone,’ because it is just the right distance from the sun to support life. Specifically, the temperature on earth is within a range that allows for water to flow (life as we know it needs liquid water). The right location is the starting point for a living world.

It is possible that life could exist with other chemicals that are liquids at other temperatures. For example, it has been suggested the liquid methane at extremely cold temperature could support life, such as the lakes of Titan (Saturn’s largest moon). But this is speculative, and that form of life would be unfamiliar to us. Nevertheless, finding evidence for liquid water on other worlds is challenging, as Goldilocks Zones are hard to come by. Although over 2 thousands exoplanets (planets outside our solar system) have been discovered, planets in habitable zones are rare.

The Solar System

solar-system-planetsThe earth is in constant motion and in relation with other celestial bodies in the solar system. Somewhat like a mobile hanging above a baby’s crib, all the bodies have influence on the system. In addition to a habitable zone, a long-term stable system is necessary. At least, the overall effects of the celestial bodies must stabilize the movement and climate of one body (like the earth). Here are 4 earth-friendly characteristics of our solar system:

  1. Earth’s Tilt: The earth is tilted at an angle of 23.5 degrees away from the plane of the elliptical orbit. The tilt gives us our seasons, which allows a greater surface to attract heat from the sun. Without seasons, only the region around the equator would be habitable. This would have drastically changed life on the planet?
  2. The Sun: The sun is 4.5 billion years old and will live for another 5 billion years. Some stars only live for a few million years. For complex life to evolve it takes several billion years, therefore a long-lived sun is needed.
  3. The Moon: The earth-moon system seems to have attained a stable relationship. The moon is just the right size to help prevent a chaotic wobble of the earth’s axis. The moon also aids in creating larger tides, which is thought to have played a role in transitioning life from water to land. And the speed of the earth’s spin has slowed over time due to the moons presence, thus moderating climate extremes.
  4. The Gas Giants: Jupiter and Saturn are the largest of the outer planets. Their orbits outside the earth’s orbit have protected the earth from large impacts. In the early development of the solar system, there were many large moving objects. The gas giants are believed to have ejected some of the large debris out of the system, and aided the inner planets to form sooner. And who knows how many potential collisions with the earth were absorbed by the gas giants.

Climate Stability

It is remarkable that the earth has maintained a stable climate for billions of years. I mean stable in the sense that the climate has not varied enough to wipe out life. Since life has appeared the earth has gone through a number of ice ages and periods of intense warming. Average temperatures may have varied by as much as 100 degrees C. But for reasons only partially understood, the climate has always returned to moderate levels.

Factors controlling the temperature have fluctuated throughout planetary history, such as: the heat generated by the sun, the earth’s heat absorption rate, and the amount of greenhouse gases that trap heat in. Could there be a regulating effect or cancelling-out effect that has prevented a runaway process? The earth has avoided irreversible climate change, unlike our two cosmic neighbors (Venus is to hot and Mars is to cold). Currently the average global temperature is about 15 degrees C.

Moderate and Gradual Change

Changes to the climate and environment are essential for the evolution of life, provided that the changes are moderate and gradual. Evolution is a multi-generational process, in which individuals that are better suited to their environments survive longer and reproduce. Beneficial genes are passed on to future generations; however, what constitutes beneficial genes is unstable, because the earth is constantly changing.

As a result of moderate and gradual changes species evolve into other species. If the planet was unchanging, the earliest life forms would not have evolved into more complex forms. On the other hand, if the changes were too drastic life could not have adapted successfully. Earth’s history shows that environmental changes have caused some species to go extinct, while others have evolved and branched out into new species.

The Gaia Hypothesis

James Lovelock, a NASA chemist in the sixties, proposed the Gaia Hypothesis when he was searching for life on other planets. While comparing the atmospheres of Earth, Mars and Venus he noticed that the earth was chemically in a state of flux. Conversely, Mars and Venus were chemically unchanging and predominately composed of carbon dioxide. The fact that the earth’s atmosphere was an active mixture of gases and still retained its overall composition, suggested some form of planetary regulation. His conclusion was that life regulated the atmosphere by its many processes.

earthLovelock expanded the Gaia Hypothesis (also called Gaia Theory) to include the whole biosphere (climate, rocks, oceans, biology, etc.) and described the earth as a self-regulating system. In other words, the earth acted as one organism. Gaia was controversial as a scientific hypothesis when first proposed. The main objection was evolutionary theory, as organisms are not believed to act in concert with their environment (sometimes supportive and sometimes destructive). The argument against Gaia Theory was that organisms would somehow have to communicate with each other, and act altruistically towards the planet. This was impossible.

Lovelock’s counter-argument was that Gaia was not intentionally achieved, yet that natural selection was critical in shaping the regulatory patterns of the planet. Gaia did not need a controlling center; it was a consequence of natural selection. Nevertheless, loosely applied it points to life processes as being critical in creating and maintaining living conditions. Over time Lovelock’s idea gained more popularity as evidence grew for an ever more interconnected and interdependent biosphere.

It could be that natural selection allows life to adapt to whatever conditions arise, giving the impression of Gaia. Or possibly, that long-term climate and atmospheric stability is in large part due to the existence of life.

Good Luck

It could be that the earth is a rare and unique planet, which has benefited from an extraordinary amount of good luck. Evidence for planets outside our solar system is mounting. There are a number of earth-like candidates, but the odds are stacked against finding a place just like earth. This does not mean that other earths don’t exist, just that they would be extremely far away. Paradoxically, the unfathomable size of the universe could mean that life is both rare and plentiful.

Anthropic reasoning would suggest that the earth has endured through a long succession of fortunate events. Intelligent observes are the result of anthropic selection, of which other lifeless worlds have no one to observe them. If events had not worked out just right for us, we wouldn’t be here. Still, it is difficult to comprehend the many unlikely phases of earth’s evolution. For example:

  1. The emergence of life.
  2. Multi-cellular life.
  3. Atmospheric transformation from carbon dioxide rich to available oxygen.
  4. Life moving from water to land.
  5. The rise of consciousness and intelligence.

These examples are major thresholds that were crossed, yet countless other variables could have changed the course of history. Life could have taken a completely different direction, even to the point of total extinction. Obviously, this has not happened, either from cosmic events or global catastrophes. When life began there was no guarantee that it would survive for nearly 4 billion years. And the specific circumstances that led to human beings were even more tenuous. We should consider ourselves very lucky to be here, on such a special planet.


References: David Waltham, Lucky Planet (New York: Basic Books, 2014).

Beautiful Minds – James Lovelock – The Gaia Hypothesis / Gaia Theory, Published on Sep. 12, 2013.

Life on Earth Can Thank Its Lucky Stars for Jupiter and Saturn, By Sarah Lewin, Staff Writer | January 12, 2016 07:30 am ET, http://www.space.com/31577-earth-life-jupiter-saturn-giant-impacts.html

What Makes Earth So Perfect for Life? Dec 13, 2012 03:00 AM ET, http://news.discovery.com/human/life/life-on-earth-121019.htm.


The Moon: Our First Satellite

moonWhen one thinks of a satellite it is usually in the form of a man-made object orbiting the Earth. However, by definition a satellite is a moon, planet or machine which orbits a planet or a star. From our vantage point here on Earth, the Moon is the predominant satellite. Long before Sputnik 1 (the first man-made satellite launched by the Soviet Union) the Moon was our one and only satellite.   

Long, Long Ago

When the Solar System first formed it consisted of a star surrounded by a disk of gas. Eventually this gas gathered into dust, rocks, asteroids and finally planets. Each planet also had its own disk of gas, which in turn would follow a similar process. Some of the debris was pulled into the planets, but not all. Over time some of the gas eventually turned into moons. Some moons could have formed independently from their host planet, and later were captured by gravity as they drifted through space.

Our Moon is believed to have been created by a different manner. The early Solar System was a very violent and chaotic place. As planets and moons were born, they were bombarded by asteroids and small planets. The Moon’s many craters is clear evidence of this early chaotic period. In the 1970s a theory was proposed: about 4.5 billion years ago the Moon was formed by a gigantic collision between the early Earth and another planet. Recently a new theory has surfaced which tweaks the original 70s theory. I’ll begin with the established theory first, then get back to the revised theory later.

earth moon collisionThe original theory states that a mars-size planet on a similar orbit as Earth struck the Earth on an angle. The collision created the Moon and quite possibly the tilt of the Earth’s axis of 23 degrees. This rouge planet is sometimes referred to as Theia, named after the mother of the ancient Greek moon goddess, Selene. The impact generated intense heat in both planets. The Earth absorbed part of Theia along with her heavy iron core, the lighter rocky material ended up in a ring around Earth’s orbit. From this debris our first satellite would from. Interestingly, the Moon may have been intact after only several decades. Over billions of years both bodies cooled, but not entirely; the Earth still has a largely molten core. The smaller Moon may have completely cooled or perhaps still retains a tiny molten core.

 Evidence for the ‘Giant Impact Theory’ (70s theory)

  • The Moon is large for a satellite in comparison to the size of the Earth. Most moons are much smaller in ratio to the planet they orbit. Models for how moons are usually formed place a limit on how big a moon can be in relation to its host planet. Our Moon appears to be too big to have formed by surrounding gas in the early solar system or captured by the Earth’s gravity.
  • By examining the surfaces of both Mercury and Mars we are able to see what the early solar system must have been like. Virtually unchanged for about 4 billion years, these planets are dotted with craters. Some of which are as large as six hundred miles wide. The Earth has no such markers due to climate and erosion, but by deduction, we can assume that the early Earth was also hit by large objects (such as Theia).
  • In six trips to the Moon the Apollo astronauts collected rock samples and for the first time they were able to see what the Moon was made of. Remarkably, the Moon samples were found to have a similar chemistry to Earth. This discovery is in line with a Theia and Earth collision. Such an event would have blasted parts of the Earth into space which coalesced with bits of Theia to form the Moon.
  • The impact hypothesis was also put to the test with computer simulations. The impact suggested, was applied to software that recreated the conditions of the early Solar System. After running several simulations of a Mars-size object colliding with the Earth at the angle predicted, everything worked. The end result was the Earth/Moon system we have today.

The ‘big Whack’ (new theory)

This is where the new revised theory comes in; modern computer simulations suggest a much more intense collision at a significantly sharper angle. Such an intense impact would have vaporized Theia and much of the Earth. This  accounts for why the Earth and Moon are so similar in their chemistry. In fact, new research is finding increasing chemical similarity. This points to a much more violent impact which would have thoroughly mixed both bodies before they separated.

Also, the impact forced the Earth to spin much faster (about once every 2 hours) and tilt as much 60 to 80 degrees on its rotational axis. The Earth’s present rotational tilt of 23 degrees is though to have been arrived at later by complex interactions with the Moon and the Sun. Another interesting fact, which the original theory left unexplained, is the 5 degree tilt of the Moon’s orbital plane. The Moon’s orbit is tilted 5 degrees in relation to the Earth’s orbit around the Sun. The Earth orbits the Sun on what is called the ecliptic plane; this plane is where most bodies orbit the sun. The early Moon’s orbit is though to have matched the severe tilt of the Earth and did not transition smoothly to match the ecliptic plane. The revised theory proposes that the 5 degree orbital tilt of the Moon is but a relic of a much steeper orbital tilt from the distant past.

A Match Made in Heaven?

earth and moonThe two prominent heavenly bodies are the Sun and the Moon. Much of the Sun’s influence on the Earth is clearly recognizable; the Moon, however, affects us in more subtle ways. The warmth of the Sun (or lack of it on some days) is an everyday experience. In ancient times the Sun was worshiped by some cultures as godlike. It would have been clear then, as it is now, that without the Sun the Earth would be void of heat and most likely without life.

As it turns out the Moon’s presence might also be fundamental to life. However, for a large part of human history the Moon remained mysterious. Today scientists speculate that the Moon may have contributed to life in various ways. What follows are plausible explanations for how our Moon influenced life:

  • When the Moon was first formed it was much closer to the Earth than it is today. It is still receding by a minuscule amount every year. Over 4 billion years ago the Moon exerted a greater gravitational pull on the Earth, which may have set plate tectonics in motion. Plate tectonics are believed to be necessary for a living planet.
  • Shortly after the Earth’s post impact formation it rotated about once every 5 hours (70’s theory) or once every 2 hours (new theory). Either way, the Moon’s presents gradually slowed down the Earth’s rotation, diminishing the severity of the weather. The Moon may also have stabilized the earth’s rotation on its axis.
  • Nocturnal animals behave differently at various times during the monthly lunar circle, depending on the brightness of the Moon. If not for the influence from varying moon light, who knows how the course of evolution would have been altered.
  •   The greatest influence the Moon has on the earth is in generating tides. This would have allowed life from the ocean (where life began) to spend short intervals of time on land. This may have provided the ideal training ground for life to gradually adapt to the land.

ocean tidesThe Earth and the Moon have been united by gravity for over 4 billion years. It is hard to know for sure what the Earth would be like without the Moon. Would there be life? If so, what would it look like? Nevertheless, if there was no Moon and life did manage to evolve, it would almost certainly be different.


References: Jim LeBans, The Quirks & Quarks Guide to Space.

Did We Need The Moon For Life? Fraser Cain, Published on Nov 20, 2015, https://www.youtube.com/watch?v=KulEmr7X1HM

Origin of the Moon, tonyweston9, Uploaded on Nov 27, 2008, https://www.youtube.com/watch?v=m8P5ujNwEwM

What is a Satellite? Dan Stillman, Feb 12, 2014, http://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-satellite-k4.html

Scientists propose new theory about how Earth got its moon, By Sheena Goodyear, CBC news, Posted: Nov 1, 2016. http://www.cbc.ca/news/technology/moon-theory-1.3830623

Violent Impact That Created Moon Mixed Lunar and Earth Rocks, By Charles Q. Choi, Space.com Contributor | January 28, 2016 02:28pm ET. http://www.space.com/31763-moon-creating-impact-mixed-lunar-earth-rocks.html

Did Early Earth Spin On Its Side? Monday, October 31 2016. http://www.seti.org/seti-institute/press-release/did-early-earth-spin-its-side

New Model Explains the Moon’s Weird Orbit, October 31, 2016, http://cmns.umd.edu/news-events/features/3680