Yes, Earth Is Unique, But This Study Doesn’t Demonstrate That!

The earth as seen from space

The earth as seen from space

The earth is sometimes called “the blue planet.” Just as Mars looks red when viewed from the earth, the earth looks blue when viewed from space. Why? Because of all the water. About 71% of the earth’s surface is covered by water, and that’s one of the many, many factors that allows life to flourish on this planet. Based on the limited observations we have, there is simply no other planet like earth. To me, that’s not at all surprising. God created this earth as a haven for life, so it makes sense that there isn’t another planet like it.

Based on my news feed from a few days ago, you would think that a recent scientific paper confirmed this idea. The Daily Mail, for example, proclaimed:

Earth really IS special: None of the 700 million trillion planets in our known universe are similar to our own, study finds

Other sources, such as Science Alert and Discover agree. The latest, most cutting-edge physics demonstrates that earth is unique.

Because these headlines peaked my interest, I decided to look at the scientific paper that describes this cutting-edge research. When I did so, I learned that once again, the media doesn’t bother to try to understand the science that they report. In fact, the researchers who wrote the paper didn’t find the earth to be unique. They estimate that there are about 2×1018 similar planets in our observable universe.

Now before I describe the results presented in the paper, I have to make it clear that this isn’t a conclusion based on observations. It is based on a computer model whose development was guided by current observations. Thus, it has all the uncertainties that one would expect for a computer model that is trying to simulate things we don’t understand very well. The authors, of course, admit this:

A study such as this, which attempts to extrapolate results on the [terrestrial planet] population from our local neighbourhood to the whole Universe obviously carries substantial uncertainties.

Given the uncertainties, what did their model reveal? They estimate that there are about 700 quintillion terrestrial planets orbiting stars that emit light similar to the light emitted by our sun. They estimate a further 20 quintillion terrestrial planets that orbit M stars, which are cooler and much more prevalent in the universe than our sun. They then go through a series of steps to eliminate the terrestrial planets that cannot support life. In the end, they decide that about 2 quintillion of them might be habitable.

How did the news media go from 2 quintillion habitable planets to earth being “unique?” I can’t say for sure, but I have a hunch. There is a point where the researchers discuss the Copernican Principle, which assumes that there is nothing really special about earth. It is best described by something Carl Sagan once said:

We find that we live on an insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people.

This principle is terribly misnamed, because Copernicus moved the sun to the center of the solar system specifically because he thought the earth was special. He thought of it as a unique creation made by God in a very orderly universe.

What did the authors say about this misnamed principle? When comparing the earth to the other habitable terrestrial planets in their model, they say that it represents:

a mild violation of the Copernican/mediocrity principle

I think that reporters decided this “mild violation of the Copernican/mediocrity principle” was enough to make the statement that earth is unique. However, that’s clearly not the case.

In the authors’ model, 0.5 quintillion of the habitable planets are found in galaxies that are like our Milky Way galaxy. The other 1.5 quintillion reside in spheroid-dominated galaxies, which are not like the Milky Way (a disk-dominated galaxy). This represents “mild violation of the Copernican/mediocrity principle,” since random chance would favor earth being among the 1.5 quitillion terrestrial planets that are found in spheroid-dominated galaxies. This, of course, is why the authors use the term “mild.” While random chance would favor earth being in a different type of galaxy, there is still a 25% chance for it to be where we find it. Those aren’t prohibitively small odds!

Now, of course, I don’t put much stock in the paper at all, since it is dependent on a model that attempts to simulate things we barely understand. Nevertheless, if you believe the model, there is no indication that earth is unique. The study shows that earth isn’t the “insignificant planet of a humdrum star” imagined by Carl Sagan, but it also indicates there might be as many as 2 quintillion other planets like it in our observable universe.


  1. Dana says:

    I so appreciate your analysis of these things. It’s so important to make sure we are teaching truth and that when we discuss things, that we actually understand the arguments and evidence we present. There are so many times I’ve gotten excited about something and when I tracked down the paper, it’s results are inconclusive and sometimes even are opposite of what the article claims. Then I wonder if it gets passed around do to lack of understanding about how to read a scientific paper (and how rare it is to conclusively prove anything)or because of a willful attempt to deceive.

  2. Jake says:

    Since you always give a source for the images you use, I’m assuming you took this one yourself.

    1. jlwile says:

      Hehe. Like all NASA images, it is in the public domain and doesn’t require a credit.