Saturday, March 10, 2018

Are Earth-like Planets Common?

It seems that every few months newspaper and magazine headlines will declare something like "Scientists have discovered an Earth-like planet." Such headlines probably lead the reader to imagine a planet with an environment much like we see on Star Trek or Star Wars where we could land our spaceship, take off our spacesuit, and frolic around the countryside. But what do scientists mean when they say that they have discovered an "Earth-like" planet? How likely is it to find a planet that can support higher life forms (defined as anything more complex than bacteria)? Are planets like the earth common or rare? Let's explore the answers to these questions.

When scientists say that an Earth-like planet may have been discovered, they actually mean one of three things. Either (1) the planet is in such an orbit around its central star that allows the temperature on the planet to possibly harbor liquid water, or (2) the planet is about the same size as the earth, or (3) the planet is solid and rocky rather than gaseous. Of course any one of these criteria, or even all three, does not actually give us a true Earth-like planet. We know that our moon is in the correct location to contain liquid water, but it is not "Earth-like." We know Venus is about the size of the earth, but it is not "Earth-like." We know that Mercury is rocky and not gaseous, but it is not "Earth-like." So none of these criteria really give an Earth-like planet. Headlines and sound-bites are not meant to be precise but to draw attention, and it is much more exciting to proclaim an "Earth-like" planet has been found rather than a "Venus-like" planet (if even that could be claimed).

Some scientists have cautioned their colleagues about making unsubstantiated claims regarding the habitability of planets, including an article by Leonard Davis at on December 13, 2017 or an article by Mike Brown in the October 26, 2012 Wall Street Journal. Nevertheless, you can be fairly certain that such headlines will persist.

An enlightening book on this subject is Rare Earth: Why Complex Life Is Uncommon in the Universe
by Peter D. Ward and Donald Brownlee, published in 2000. One of my favorite chapters in the book is titled "The Surprising Importance of Plate Tectonics" which documents why plate tectonics is required in order for complex life to survive. Having lived 23 years of my life in California, I am well acquainted with the consequences of plate tectonics, but had no idea that such activity was crucial for my survival. Ward and Brownlee document how plate tectonics not only forms and maintains continents, but promotes biological diversity, regulates global temperature, and helps maintain a planetary magnetic field. They write, "It may be that plate tectonics is the central requirement for life on a planet and that it is necessary for keeping a world supplied with water," (p. 220).

The astrophysicist Hugh Ross has done a rough estimate of the probability of finding a single planet that could support even simple unicellular life for a sustained period of time. Including correlations and longevity factors, and assuming there are 10 billion trillion planets in the visible universe, he concludes that the probability of finding a single planet that could support unicellular life for a prolonged period of time is 1 in 10556, (see Part B of this document). If this informed estimate is even close to being correct, then there are no other planets in the visible universe that can support life.

Of course, from a theological viewpoint, if God created one planet capable of supporting life he could have created many more. From a biblical viewpoint we are given no information about whether or not God made other life forms that live within our physical universe. (Although we are told there are other created beings like angels that can interact with our universe.) So, from a theological perspective, these kinds of probability estimations neither limit the possibilities or challenge any biblical doctrines, whether or not other complex life is ever discovered in the universe. But these informed probability estimates do make a strong case that the habitable Earth is not an accident and that there seems to be both a designer and creator behind our unusual planet.

A careful examination of the requirements for an Earth-like planet give more evidence that our universe was designed and created by an intelligent creator who has carefully crafted a home for humanity. Over and over again, the findings of modern science give credibility to the postulate that there is a God who has a plan for humans. Contrary to the rhetoric that claims that because there are a lot of planets many of them must be habitable, the known required characteristics of a habitable planet set stringent limits on the probability that any of the thousands of known extra-solar planets, or the trillions of planets out there, actually support any kind of complex life. So the next time you read that an "Earth-like" planet has been found, you can marvel at the amazing ability humans have to discover the wonders of the universe including the many rocky, earth-sized planets out there—planets that are almost certainly devoid of any life, but interesting places nevertheless—planets that emphasize the rarity and distinctiveness of our home the Earth.


  1. Dr. Strauss, I have to firmly push back on this human-centric point of view. It just feels inauthentic to predict whether life exists on other planets or not and I've expressed this to you before. Maybe a year ago or a little less. I looked at the Part B link and even saved it for future reference for when I understand some of the data points in it better. But even knowing what I know now, postulating that Earth is so unique, God would need the rest of the universe for it to exist just defies reason. Just to give a little example, what if the bacteria on another planet (as you admit there most likely are a good amount of planets with bacteria on them) didn't use adenosine triphosphate for energy, respiration, or metabolism? Theoretically they could still be carbon based. Still have DNA. Still evolve to their surroundings. If their energy on a cellular level was based on something entirely different, all the assumptions in Part B of what we understand our world needs to support life would break down. I don't think I need to point out the evolution of our understanding of our place in the universe to show you the pattern of the most firmly held beliefs based on their own sets of data points they KNEW to be true at the time (i.e the world is flat, the sun revolves around the earth blah blah blah) get smashed over and over. These truths were shattered but with much resistance. We haven't even been to the next planet over from us with anything but a couple robots and some satellites (not discounting that feat or their discoveries, just saying there's an infinite amount of exploring in our own solar system we still need to do). So by your own logic and argument that you have consistently made throughout this blog, why would we assume to know that which we cannot possibly. What did you call it? The logic fallacy? I could be getting the use of that term totally wrong but you understand what I'm saying. We know life in one form. The form it takes on earth. For all the variables Hugh Ross accounts for, he does not account for the ones we don't even know we don't know. You work at CERN. How much about the universe do we know for sure and how much are we assuming because we need it to fit into the laws of physics and chemistry we theorize to be correct for the sake of another theory. That's obviously rhetorical. I would make the argument that the universe is SO finely tuned that to make such a blanket assumption is preposterous. Earth is pretty firm evidence a creator designed the universe. That does not mean the rest of the universe is devoid of Earths. Again, I'll just throw out there that it feels like this argument is rife with confirmation bias because again, you are making an assumption about the universe based on what we think we know and deducing a belief that you cannot even come close to scientifically proving or disproving, even if it is based on a wealth of data. How is that any different from multiverse? I'm not assuming there is life. I'm not making the argument that there's all this space so how could there not be life. I'm making the argument it is impossible for us to know either way and to speculate is somewhat irresponsible of any scientist, religious figure/community, or the media. Either to assume there's life or assume there isn't. How and why can someone like you, who is 100x smarter than I will ever be, come to this conclusion and try to use science to do so?

    1. The discussion in this post is based on the opinion of the experts who study this question. Within our universe almost all experts agree that the only viable element on which to base life is carbon, which then highly constrains the possible environments. I'm not saying the rest of the universe is devoid of earths. If a creator made one he could have made a lot. I am saying that scientists understand enough about the necessary requirements for an earth like planet that (a) we have not found any yet, (b) there may be none to find. I'm not making any claims based on ignorance but rather what is known about what is necessary for carbon-based life to survive.

      Despite the fact that there are many things we don't know, there are also many things we do know, and those we do know can often rule out many possibilities. That is the scenario when it comes to the requirements for life in our universe.

    2. I apologize Dr. Strauss, I have a hard time articulating my thoughts into written word sometimes. I did not mean to imply you were basing this post on ignorance. I understand what you mean. I've learned how unique carbon and water are. I think I was trying to say something similar what your reply said. Except this, that if the universe is fine tuned and we are a part of that, that means there is a high likelihood that (and I guess I would argue more likely than not) that if God created Earth and humans to evolve into it, that God created others to support intelligent life too. And that the chemistry and physics of how unlikely it is for life to evolve are not what matters because those are the tools God uses to create, and not the reason or the purpose.

  2. Dr Stauss, you may be interested in this comment I posted yesterday at another blog:

    1. Thanks for pointing me to this very details comment.