From Medium: “A World Apart”

From Medium

Nov 6, 2018
Tony Deller

No image caption or credit.

Why Earth is unique and what that means for the many exoplanets we are finding in this golden age of astronomy.

To most of us, our world is simply the place where we live: you’re born, get an education and/or learn a trade, perhaps start a family of your own, pass on some of the knowledge and wisdom you have gained to others, grow old and eventually die. It’s an oversimplification, but that is the common experience of a human on planet Earth. Earth is just where we do “everything”. If you are very lucky, you have opportunities to actually travel around the Earth and visit continents other than the one you were born on, seeing the true vastness of the planet and the variety of its civilizations and biomes. You realize we are many, but we are also one…all of us together on a single sphere of rock, covered with a thin sheen of water, orbiting a massive ball of fire.

For a long time, the view that humans (and Earth) were the center of the cosmos ruled scientific and philosophic thought. Indeed, great minds like Aristotle and Ptolemy supported this model of the the universe. Though a near-contemporary of those two, Aristarchus of Samos, had proposed a heliocentric view of the universe, his ideas didn’t receive enough support to stick. It took nearly 1800 years for the heliocentric model to become generally accepted, under the scientific leadership of Nicolaus Copernicus.

The Copernican Revolution, as it is known, gained further support over the succeeding century through the work of Johannes Kepler and Tycho Brahe. Galileo’s telescopic observations of Jupiter’s moons definitely put a nail in the coffin of the geocentric model. Isaac Newton then carried forward with the heliocentric model to show that the Earth and other planets in the Solar System orbited the Sun.

As telescopic engineering improved, our view of the local universe grew larger and larger. By 1750, Thomas Wright posited that the Milky Way was a tremendous body of stars all held together by gravity and turning about a galactic center.

Milky Way NASA/JPL-Caltech /ESO R. Hurt

To us then, the Milky Way was all there was — all we could observe — and so the Milky Way was the universe. It took until 1920, though, when the observations of incredibly faint and distant nebulae by Heber Doust Curtis led to the ultimate acceptance that the Andromeda Nebula (Messier 31) was actually another galaxy.

Andromeda Galaxy NASA/ESA Hubble

Milkdromeda -Andromeda on the left-Earth’s night sky in 3.75 billion years-NASA

Optic technology continued to advance, and more and more galaxies were found throughout the 20th century.

The first exoplanets were confirmed in 1992, discovered around pulsar PSR B1257+12. These were terrestrial-mass worlds. The next exoplanetary finding occurred in 1995, a gas giant orbiting 51 Pegasi. Since that time, the rate of discovery of exoplanets has accelerated to the point that we can now detect hundreds within the confines of a single project. In 2016, the Kepler space telescope documented 1,284 exoplanets during one such period, over 100 of which are 1.2x Earth mass or smaller, and most likely rocky in nature.

NASA/Kepler Telescope

As of September 2018, the combined observatories of the world have detected 3845 exoplanets distributed across 2866 planetary systems, of which 636 are multiple planet systems.

These worlds are detected using various methods, including: measuring the radial velocity of the (potential) planet’s host star to get an idea of the planet’s mass by how it affects its star;

Radial Velocity Method-Las Cumbres Observatory

Radial velocity Image via SuperWasp http://

transit photometry which sees a (potential) planet as it moves between our telescopes and its host star;

Planet transit. NASA/Ames

reflection/emission modulations which might show us the heat energy of a (potential) planet; observation of tidal distortions of a host star caused by the gravity of a (potential) massive gas giant; gravitational microlensing in which two stars line up with each other in relation to our observational view from Earth and their gravity distortions act as a magnifying lense that can help us notice planets around one of them; and nearly a dozen other ways.

Gravitational microlensing, S. Liebes, Physical Review B, 133 (1964): 835

There are currently 55 potentially habitable exoplanets out of the thousands of worlds we have thus far detected. These are classified into two categories by the Planetary Habitability Laboratory at Arecibo in Puerto Rico: Conservatively habitable worlds are “ more likely to have a rocky composition and maintain surface liquid water (i.e. 0.5 < Planet Radius ≤ 1.5 Earth radii or 0.1 < Planet Minimum Mass ≤ 5 Earth masses, and the planet is orbiting within the conservative habitable zone).” The optimistically habitable planets “are less likely to have a rocky composition or maintain surface liquid water (i.e. 1.5 < Planet Radius ≤ 2.5 Earth radii or 5 < Planet Minimum Mass ≤ 10 Earth masses, or the planet is orbiting within the optimistic habitable zone).”

Credit: Planetary Habitability Laboratory

If there are so many potentially habitable exoplanets out there, what is it about Earth that makes it so special?

Aside from us, that is?

While the exoplanets we have found so far that exist within the confines of what we have deemed “potentially habitable” may indeed be rocky and orbit at just the right distance from their host stars to maintain liquid water and atmosphere, that doesn’t mean they are habitable, or possess the potential to support Earth life or any life, for that matter. They may not even be suitable candidates for terraforming. That is because the conditions that made and preserved Earth as a safe harbor for life are many and seem to have occurred at precisely the right times throughout the 4.543 billion year history of the planet.

The factors that allowed life to evolve steadily on Earth — “Goldilocks” factors — include the ones that we use to designate exoplanets as potentially habitable: like them, Earth orbits at just the right distance from the Sun to allow liquid H2O, and Earth formed with such a mass and composition that it became a rocky world as opposed to a gas giant.

Beyond those primary characteristics, though, Earth possesses other traits that, for the most part, we are still unable to detect on exoplanets.

Our molten, mostly iron core spins to create a magnetosphere around the planet that deflects excessive solar and cosmic radiation. Our single, relatively large Moon stabilizes our rotation, gives us a 24-hour day, and creates tides that scientists believe were a large driver of evolution.

We have the ozone layer which adds another protective shield for life against UV light. We have two gas giant worlds in the outer Solar System that have been pulling in a majority of asteroids and comets for billions of years, long before they make it into the inner Solar System to possibly impact Earth.

We are located at the edge of the Orion spiral arm of our galaxy, far from the much denser, crowded center of the milky Way where asteroids, comets, stellar collisions and supernovae are much more common. The Late Heavy Bombardment, which pounded the Earth with comet impactors roughly 4 billion years ago, seeded our world with just the right amount of water ice to give us vast oceans.

Our Sun is also quite stable for a star, and luckily isn’t part of a binary star system (which may account for up to 85% of all stars!), which would certainly offer difficulties in the form of gravitational pull from 2 stars and more asteroid activity. The Earth has also been remarkably consistent and stable for billions of years, from its atmospheric and chemical composition to its temperature variations.

Of all the exoplanets discovered, Earth and its ilk can only exist within a rather narrow band of possibilities:

Credit: Planetary Habitability Laboratory

All of these Goldilocks factors added up to a world that has remained a viable habitat for billions of years. Our mineral-rich oceans became a veritable Petri dish in which trillions of generations of single-celled life could mingle and evolve until two such forms merged in a symbiotic relationship that resulted in the first multicellular organism. From there, the diversity of life blossomed uncontrollably.

That diversity would be one of the reasons life on Earth continued to survive through multiple mass extinction events:

Cretaceous–Paleogene extinction event — 65 million years ago, 75% species loss

Triassic–Jurassic extinction event — 199 million to 214 million years ago, 70% species loss

Permian–Triassic extinction event — 251 million years ago, 96% species loss

Late Devonian extinction — 364 million years ago, 75% species loss

Ordovician–Silurian extinction events — 439 million years ago, 86% species loss

In a strange bit of irony, the earliest two of these great extinctions may have been caused rather directly by the power of evolution on Earth. It’s believed by many scientists that in both of these cases, an extreme amount of plant growth led first to the removal of too much CO2 from the atmosphere and a reverse greenhouse effect, and in the second great extinction to mega algae blooms that depleted the oceans of oxygen.

The most recent 3 mass extinctions seem to have been caused by a supervolcano eruption, and two massive asteroid impacts.

There is a sixth mass extinction, generally agreed upon by most paleontologists, that is currently happening: the “holocene extinction event”. It is thought this extinction began at the end of the last ice age (roughly 12,000 years ago) and vastly accelerated with the rise of agriculture, large human civilizations and the Industrial Revolution. Data points to at least 7% of all holocene-era species having already gone extinct directly due to human interaction with our world. Species come into being and go extinct naturally, of course, and this is known as the background rate of extinction. Scientists believe that humans have increased the occurrence of extinctions to possibly as high as 500–1000 times the background rate.

Reversing this trend needs to be a priority — As the most intelligent species on Earth, we should see ourselves as caretakers of a multi-billion year legacy. We should not, and must not, allow Earth to become a barren hunk of rock due to our inherent drives that often do more harm than good. We are smarter than that. But even if this most recent mass extinction event snowballs and becomes unfixable, it is likely that life will continue to thrive on Earth, whether it be beneath ice or in the ocean’s deepest corners. We need to keep in mind that it is always the creatures at the top of the food chain that die off first in any great extinction.

And if (and when) Earth does become unlivable for us humans, we should be capable of finding and reaching exoplanets that might become a new home. Hopefully by then we will have become wiser.

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