How special is Earth?

Joe Silk, a famous astrophysicist, interviewed planetary scientist Mario Livio about the specialness of the Earth and solar system. Mario Livio would like to believe that there is nothing special about the earth or solar system. In 2000, Ward and Brownlee wrote a book called Rare Earth in which they argued that intelligent life in the universe is rare. it was controversial at the time, but more and more scientists are beginning to agree with them.

They discuss the Fermi paradox, which is that we should have seen signs of other civilizations in the universe, but we haven't seen any evidence of other civilizations on other planets or colonizing the galaxy. Livio thinks that there might be bottlenecks that prevent evolution of intelligent life, or that intelligent civilizations might normally destroy themselves. Silk thinks that the probability of life evolving on a planet might be extremely low, and Livio agrees that this is a possibility. Silk thinks supernovae or meteorites might normally cause destruction of life on a planet. Livio thinks that gamma ray bursts might sterilize planets. On the other hand, they could open the gate to more intelligent animals if they eliminated stupid large creatures such as dinosaurs, such as in the End Cretaceous Extinction. They wonder whether clusters of stars are good places to look for habitable planets and consider the importance of the galactic habitable zone. They stipulate that metallicity (elements heavier than helium) is an important part of habitable planets. Livio concludes by stating that we should look at similar environments when looking for exoplanets with life.

Livio wrote a book in which he tried to answer the question, How Special is the Solar System? [1] He identified two features that distinguish the solar system from other exoplanet systems: “lack of super-Earths, and absence of large close-in planets” to the star, such as hot Jupiters.” Livio made the following statement that indicates the difficulty of calculating the probability of life on other planets.

“The bottom line from the discussion so far is simple. There may be many factors that are necessary, but maybe not sufficient, for life to emerge and evolve on a planet. If we were to multiply the probabilities for all of these factors (in a Drake-type equation) together, we could end up with a very small probability for life in the Milky Way. Without concrete knowledge of which ones of these factors are truly essential for life, however, such an exercise would merely represent our ignorance. If we blindly consider every single aspect of the solar system, we will find it to be unique. From the parameters that I have considered here, however, I have not identified any feature that would argue for the Earth being exceptionally rare. The eccentricities of the solar system, while on the low side, are not exceptional, and it is at the expected level for an eight-planet system. The main difference between earth and other systems is lack of super-earths and lack of planets, having migrated inward from the outer solar system.”

Livio believes in the Copernican or mediocrity principle, which is the opposite of the concept the earth is special (unusual). The mediocrity principle has been defined as follows and means that the solar system should be typical of normal planets in the universe:

“If an item is drawn at random from one of several sets or categories, it is likelier to come from the most numerous category than from any one of the less numerous categories.”

The Copernican principle was described by Hermann Bondi. He was one of the proponents of the Steady State Hypothesis that was proposed as an alternative to the Big Bang) as follows:

“The properties and evolution of the solar system are not unusual in any important way. Consequently, the processes on Earth that led to life, and eventually to thinking beings, could have occurred throughout the cosmos.” [2]

Until the time of Copernicus, most people in Europe generally aligned with Aristotelian beliefs, and thus rejected the concept of humans on other worlds;[3] After Copernicus, Giordano Bruno (see chapter 2), inspired by Lucretius, promoted the concept of extraterrestrials on other planets, but this was considered heretical, and he was condemned to death by the Inquisition. There were sporadic discussions of extraterrestrials in the next few centuries. Kepler speculated about life on the Moon in a fictional book published after his death. The disbelief in other civilizations began to change with Swedenborg and Kant in the 18th century. Swedenborg stated that he talked with aliens on other planets in the solar system. He must have influenced Kant because Kant bought Swedenborg’s set of theological commentaries, and Kant then used the concept of aliens on other planets as a justification for philosophical naturalism: if all of the other planets had other angles of obliquity, then this meant that God did not adjust each planet to make it optimal for human life. Thus, God does not intervene in the universe, etc… Kant later denounced Swedenborg and left this argument out of his later rewrite of Universal Theory and Natural History of the Heavens.

The belief in extraterrestrials received a boost at the end of the 19th century when astronomers discovered what looked like canals on Mars. Although these turned out to be optical illusions, this was enough to get the wheels turning. H.G. Wells published War of the Worlds in 1897, which depicted Martians attacking Earth. In the 20th century, proponents referred to supposed UFO sightings as evidence of extraterrestrials.

In 1959, scientists began to try and detect signals from civilizations from other planets with radio telescopes.[4] This led Frank Drake to listen for signals from other planets with a 25 m radio telescope. In 1961, he developed an equation designed to assess the possibility of life on other planets.

In the year 2000, Ward and Brownlee published The Rare Earth Hypothesis, which contends that our planet and solar system, with all of the characteristics that support life, are unusual. Rare is the opposite of mediocre. They contended that we are extremely lucky to have all of the characteristics that support intelligent life on Earth. Ward and Brownlee were careful to distinguish between characteristics of planets that might support microbial life and the additional characteristics that are needed for intelligent life. Microbial life can live in extreme environments on earth and thus could possibly evolve and live in other planets in the solar system.

Ward and Brownlee argued that our solar system is particularly conducive to intelligent life, having a star that produces primarily visible light (no gamma rays) for billions of years, circular orbits of planets, minimal asteroid and comet activity in the solar system, and in a remote location in the outer galaxy that has minimal gravitational force or gamma rays from other stars. Likewise, our planet has many inherent and evolved characteristics that have contributed to making earth habitable. Earth is unique in the solar system in that it has atmospheric temperatures that range from below freezing to midway between freezing to boiling. Plate tectonics, unique to the earth in the solar system, releases essential gases through volcanoes into the atmosphere. Our one large moon keeps the earth’s axis at a stable angle and generates much larger tides than the gravity of the sun would generate. Tides may have been key to the origin of life and the movement of sea animals to land. The earth’s magnetic field prevents solar wind from blowing away the hydrogen and oxygen (water) in the atmosphere. The earth’s percent of water, 0.03%, allows for a distribution of continents and oceans and a hydrologic cycle. Oxygen and carbon dioxide in the atmosphere allow for photosynthesis and biological respiration, and soil facilitates high plant production rates.

Ward and Brownlee’s book was extremely controversial with approximately half of scientists supporting Ward and Brownlee and the other half supporting the Mediocre Earth hypothesis (the Earth, which is that Earth and the solar system are normal occurrences in the universe. There might not be too many Tucsons out in the universe.

References

[1] Livio, Mario. "How Special Is the Solar System?." arXiv preprint arXiv:1801.05061 (2018).

[2] Encyclopedia Britannica, Extraterrestrial Intelligence. 2017. Accessed February 5, 2017 at <britannica.com/science/extraterrestrial-intelligence#ref959787>

[3] Crowe, Extraterrestrials

[4] Cocconi, G.; Morisson, P. (1959). "Searching for Interstellar Communications" (PDF). Nature. 184 (4690): 844–846. Bibcode:1959Natur.184..844C. doi:10.1038/184844a0. Retrieved 2013-04-10

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The city of Tucson in 1909