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The Pale Blue Dot Revisited: Appreciating Our Uncommon Place in the Universe

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Origin of the Universe

Dr. Bijan Nemati's talk at the 3rd International Conference on The Origin of Life and the Universe

It's an honor to be with you today. We live in an era where the dominant scientific worldview is based on materialism, a metaphysics that holds aspects of the physical world, such as matter and energy, as self-existent. From the physical universe and its laws, this view claims, we can explain everything we see, everything we experience. The material universe is sufficient to explain the nature of all that exists, and if that is true, in the words of the late Stephen Hawking, “science can explain the universe without the need for a creator.” This or some version of this view is very common in our day, particularly in academic circles.

Yet, this view leaves many questions unanswered. For example, what is the nature of consciousness? What is the origin of morality? Does it even exist? What about love, or mercy? Are these illusions? Are they simply electrochemical impulses? Did they simply arise as encoded reactionary patterns in our brains through evolution? If so, are moral principles simply reducible to mathematics and statistics? And finally, given these questions, can a proponent of materialism live their life consistently with their worldview?

While these profound questions remain unanswered, materialists use ancillary arguments in support of their view. One such argument, that I want to examine here, is the appeal to mediocrity, sometimes called the Copernican principle. To explain what I mean, consider the story of the pale blue dot:

In his book, the Pale Blue Dot, the late astronomer Carl Sagan recounts an event that occurred in the course of NASA’s Voyager 1 mission. In 1990, 12 years after the launch of Voyager 1, the spacecraft had left the outer planets of the solar system, and on February 14, it was commanded to turn around and take a family portrait of the planets in the solar system it was leaving forever behind. When the images were transmitted back, NASA scientists and engineers were having difficulty finding the Earth. Eventually they found it, as a pale blue dot, near a shaft of light that was entering the camera, reflecting from some point on the spacecraft. About this picture, Carl Sagan had this to say:

“Because of the reflection of sunlight . . . the Earth seems to be sitting in a beam of light, as if there were some special significance to this small world. But it’s just an accident of geometry and optics. . . Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light . . .  Our planet is a lonely speck in the great enveloping cosmic dark …” – Carl Sagan

As you can sense here, this is actually not a scientific statement, but a philosophical, actually a theological one. But the claim is not supported by all the science that we have learned over the last few hundred years in fact. In this talk I would like to examine the assertions of this so-called Copernican principle in the light of modern science, and in particularly, my field, which is extra-solar planets (or “exoplanets”).

Brief History

Before we discuss exoplanets, let’s take a brief historical look. It was Aristotle who 2400 years ago argued for a geocentric model of the Universe. He envisaged concentric crystalline spheres in which the heavenly bodies resided. Most nearby where the moon, Mercury, Venus, and Mars, then Jupiter and Saturn. Further out where the firmament, which are the stars, and these were all set into motion by a prime mover. Aristotle said reason and common experience confirmed this view, and no one doubted it.

But there were problems with this simple view. One was the retrograde motion of the planets. For example, if you look in the night sky at Mars, which comes near us every two years, it comes to a point called “opposition” (which is the point on the sky opposite to the direction of the sun at that time) and if you watch its location relative to the background stars you will notice a retrograde motion: over the course of a few months, you see the planet moving one direction then backwards, and then forward again in the same direction relative to the stars. This was difficult to explain in the basic geocentric view of Aristotle.

Five hundred years after Aristotle, in his great work called the Almagest, Ptolemy provided an explanation. He agreed with Aristotle that the perfect, heavenly bodies have to have perfect motion, which meant they were going in circles, but the planets are actually traveling in smaller circles called epicycles, and the epicycles are centered on something we call deferents, which in turn are centered on the earth. In this way the planets can have retrograde motion. By setting the diameters and rotation rates just right, Ptolemy was able to make very accurate predictions of the locations of the planets, so good in fact, that for the next 14 centuries there was no rival to this picture.

It was not until the 16th century that a serious alternative was proposed, and here it was Nicolaus Copernicus, who in his own magnificent work, called “On the Revolutions of Heavenly Bodies” detailed a much more elegant system: the sun-centered (or “heliocentric”) system. The earth, he suggested, was not the center of the universe, and the heavenly bodies do not all revolve around a central point. With these and a few other axioms, he was able to explain the same observations more elegantly and simply.             

But it goes even further. In the 20th century it was the astronomer Harlow Shapley of Harvard University who discovered that the sun is not at the center of our galaxy, which is called the Milky Way. He did this by measuring the locations of globular clusters on the sky, including their distances. Globular clusters are mini-galaxies with hundreds of thousands of stars. Shapley noticed that they are orbiting an area of our galaxy that is many thousands of light years from us. So, he famously concluded: “The solar system is off center, and consequently man is too…

So, you see this pattern. Copernicus showed us that the Earth is not at the center of the solar system, then Shapley showed us that the Sun is not at the center of the galaxy, and later, although I didn't mention it earlier, Edwin Hubble discovered that the Milky Way is only one of hundreds of billions of galaxies. Pointing to these observations, the so-called “Copernican” principle seems to show us that man does not hold a central position in the universe, and by extension, it must be true that we are not here for a purpose.

The claim seems to be well supported, when expressed as I just did. But the claim gets some of the history wrong (for example neither Copernicus nor Galileo would consider the Earth’s removal from the center as a demotion!) but also, the claim actually completely missed much of what we have learned in the last century about habitable planets. These are planets where life could at least survive if placed there.

Habitable Planets

So now, we consider the question "what is a habitable planet?" What do we need for a habitable planet?

There are many requirements. But at the minimum, a habitable planet must be a terrestrial planet that supports complex carbon and water-based life. It needs to be a planet in what is called a Circumstellar Habitable Zone. And finally it needs to be a planetary system in the Galactic Habitable Zone.

The first requirement, mainly that it should be a terrestrial planet, meaning a rocky planet, is already limiting. This is because much of the matter in the universe consists of Hydrogen and Helium. It takes complex processes in the stars to generate the heavy elements that make up a planet that is rocky like the earth. Beyond being rocky, however, it must also have water. And these are just minimum conditions.

There is also the location of the planet. The Circumstellar Habitable Zone (CHZ) is defined as that region around a star where water can be liquid at some part of a rocky planet that is situated there. Since the surface heat of a planet is from the sunlight it absorbs, around a cold star the habitable zone is close in, while around a hot star, it has to be further out. If a planet is located closer to its host star than the inner edge of the HZ, a runaway greenhouse effect will raise the temperatures, causing the water to evaporate into the atmosphere, and be carried away by the solar wind, making the planet dehydrated.  At the other extreme of the HZ, there will be precipitation in terms of ice and snow, and that will make the planet absorb less of the star light and become colder still. This leads to an uninhabitable “snowball” planet. In our solar system, only the Earth is inside the HZ. So, the planet has to be the right distance from the star.


As for the star, are all the stars equally suitable? It turns out that the answer is no! Many astronomy textbooks refer to our Sun as an “average” star. This is true only in a limited sense: there are certainly stars that are more hot than our star, and there are stars that are cooler than our star, than our sun. But the sun is actually within the 10% most massive stars in the Milky Way. And in fact, stars that are much more massive than the Sun are actually too unstable to be producing habitable zones. And then stars that are less massive than our star, they are cooler, so they have habitable zones that are closer in. But when a planet gets that close to a star, it suffers from an effect called tidal locking. So for the cool stars, tidal locking problem occurs: the spin of the planet becomes equal in duration with its orbital period, and as a result, one side of the planet becomes permanently “day” while the opposite side becomes permanently “night.” The day side becomes hot and the moisture is transported to the other side, where it snows down and stays permanently frozen. Tidally locked planets are poor choices for life. Cool stars also have more frequent life-threatening events called coronal mass ejections. In the end, only 4% of the stars are main sequence G stars like our Sun.

What about the location of the star within the galaxy? To get heavy elements, from which a rocky planet can form, you have to be closer to the center of the galaxy. On the other hand, if you get too close, life threatening events like super-novae become more frequent. A supernova can sterilize all life within many light years around it. These are not only more frequent near the center but also within the spiral arms. So, the planetary system, to stay safe, needs to be at the right radius from the center of the galaxy. Not too far and not too close, and also not within a spiral arm.

What about the galaxy itself? Here too, we find ourselves in a privileged place. Our galaxy, the Milky Way, is among the 3% most massive galaxies in the nearby universe. Because it was so massive, it was able to accumulate heavy elements more quickly, and planet formation started earlier around the stars in the Milky Way. Almost two-thirds of the age of the universe had gone by by the time there was enough material that a planet like the earth could be formed. So there was a brief window.

There are in fact many parameters that we could discuss; the list is very long. Very briefly, at least we need a planet with a magnetosphere. Our magnetosphere on the Earth protects us from cosmic rays and occasional solar bursts that would otherwise dehydrate our atmosphere. We need a large moon. The Earth’s moon is unusually large relative to the Earth's size. This is important because our massive moon stabilizes the axial tilt of the Earth's rotation, and that helps to stabilize our climate. To support large living beings like animals and like us humans, a planet needs to have high enough oxygen content, but then if it has too much oxygen there will be rapid fire growth. There needs to be a neutral gas as well, to avoid devastating fires. Our oxygen-nitrogen dominated atmosphere is the perfect balance of these requirements. Finally, the Earth’s planetary neighbors play important roles. Jupiter, the largest planet in our solar system, has 300 times the mass of the Earth. It has a near-circular orbit five times farther from the sun as the Earth. This combination of being massive and having a large circular orbits makes Jupiter a benevolent agent in the solar system, absorbing to itself, like a massive vacuum-cleaner, comets and asteroids that could potentially threaten life on the earth: many of these eventually crash into Jupiter or Saturn. 

We now move from theory to experiment: over the last two decades there have been many discoveries of exoplanets, planets around other stars. What have we learned from them?

First, very briefly, I'd like to point out some of the techniques. In the first, very important technique, called Radial Velocity, the planet is not detected directly, but the wobble of the star in reaction to the pull of the planet is detected, from the red and blue shifts of the star’s spectrum. Another highly successful technique, called Transit, looks for the very small drop in the light from a star when a planet transits, when the planet comes in front of the star. A third one that is special to me because this is the area in which I work is one of direct imaging where a technique is used to actually image the planetary system directly. Here you see a montage of many years of images of an exosystem and the planets moving around that.

Now, what have we learned from these? From the radial velocity measurements, we have discovered one very important lesson: that Jupiters, like our Jupiter, are very uncommon. Most gas-giant planets (and Jupiter and Saturn are examples of gas-giant planets) have elliptical, rather than circular orbits, it turns out. Over time, the elliptical orbit means they migrate towards the star, eventually settling from an elliptical orbit to a circular orbit very tightly around the star. Along the way, they can knock off other planets in the solar system; they are dangerous when they do that. This kind of planets are called "hot Jupiters", very dangerous.

The other technique was the transit technique. And it has also provided us a picture of thousands of other planetary systems. But here too, the results show that in the great majority of the planets discovered they are closer to their host star even than our innermost planet – Mercury. So our solar system by that comparison is exceedingly unusual.

So, we see that a very large number of conditions are necessary for a life-hospitable planet, and when we look at the universe we see that the usual condition, the usual situation is that these conditions are not all present at the same time. In fact, one can make a statistical estimate of the expected rate, following the approach of Francis Drake from the 60's. He used a very simple calculation, and he estimated the number of planets in the Milky Way that could host advanced life, such as they could give us a radio signal. Considering a few conditions, he estimated that there should be on the order of a million other planetary systems in our galaxy that could send us a signal.

But half a century later, the list of conditions is actually quite a bit more. And when you actually make the same type of calculation now you expect much less than 1 in 10,000 Milky Ways, where you could expect to see a planet like the Earth. So up to now, it's been an argument to say that the Earth is a very rare planet. But there is again a more profound aspect to our existence here, and this is a notion of what is called the 'priviliged planet'. This was first pointed out by astronomer Guillermo Gonzales and philosopher Jay Richards. Their point was  that the requirements for habitability appeared to be overall coinciding with the requirements for discovery. That is, the same conditions that make a planet habitable are altogether what we would need for that habitable planet to be a place where intelligent beings could study and learn about the universe.

A beautiful example of this is the example of the perfect eclipse. Did you know that from the Earth's point of view the moon on the sky and the sun on the sky are exactly the same diameter? And as a result, the Earth is the only place where we can have what is called perfect eclipses. If the moon was a little bit closer, it would cover too much. If it was a little bit farther, it would not cover enough of the sun. As it is, we have perfect eclipses and we can study aspects of the sun that were otherwise unavailable to us.

For example, historically, it was from observing the spectrum of the atmosphere of the sun during a perfect eclipse that astronomers first discovered that the sun is a hot ball of gas. It also enabled them to know what elements are present in the sun. This in turn opened the door for studying other stars.

It was also during an eclipse that Einstein’s theory of gravity, which eventually told us about the beginning of the universe, was put to its most famous test. The sun’s mass bends light from distant stars, but the bending is so small that only the stars seen very near the sun show an effect. But you can only see this effect during a perfect eclipse.

Many of the other factors also facilitate discovery. Because we are not at the center of the galaxy, our night sky is dark, it's not bright; we can actually do astronomy. Remember we said that we need an oxygen-nitrogen atmosphere; that's a clear atmosphere. By comparison, Venus has a carbon-dioxide atmosphere, which is opaque. You couldn't do astronomy from Venus. It's also 400 degrees Celsius on the surface of Venus, so it is not a good platform.

The Heavens Declare the Glory of God

The list of interconnected requirements for habitability and discovery is long. But the picture should by now be clear.

There is abundant, solid evidence that the world is designed, and we can infer as well that God wants us to study this world and see its design and realize the glory of its Creator.

So, far from being a mere pale blue dot, our planet was not only made for supporting life, but it was also made to support knowing about the universe, and extending from that observation, to know, and be amazed by, its Creator.