Since the launch of the Kepler Mission, you may have seen headlines like "NASA Finds Earth-size Planet Candidates in the Habitable Zone," "Newfound Alien Planet is Best Candidate Yet to Support Life," "NASA finds dozens of planets that might support life," and "Alien Planets: Billions Of Habitable Exoplanets In Milky Way Galaxy." Whenever scientists discover a planet that is similar in size to earth and in the liquid water habitable zone, excitement grows because they do add something to the search for a planet that could possibly host life. It is an exciting time to be an astronomer, to say the least, and the excitement has filtered down into the public because of the frenzy of news articles about the discovery of "earth-like" planets.
However, questions arise with all this excitement. With all the data pouring in from the Kepler Mission, how excited should we really get? Is NASA on the verge of discovering alien life or is that just media hype in order to sell a story? What does it really mean when an "earth-like" planet is discovered? How common or rare is Earth? Are the "habitable planets" really habitable? This last question is the most important because it brings up the possibility of life existing on other planets. Below we will cover these questions and do so with particular focus on the idea of "habitable planets." (Warning, by necessity this is going to get a bit technical. I have done my best to clearly explain these things but if something is not clear, comment and ask me about it.)
Let's talk for a few minutes about what it means to be "earth-like." As we all know, the term "like" is a very ambiguous term and in the astronomical business the situation is no different. There is no consensus on what it means for a planet to be "earth-like." How similar in mass and size does the planet have to be to earth in order to be considered "earth-like"? What kind of atmosphere does it need to have to be "earth-like"? What kind of star does it need to orbit to be "earth-like"? How far from its star does it need to be in order to be "earth-like"? What kind of orbital period (the time it takes for the planet to make one revolution around its star) is necessary for a planet to be "earth-like"? These are all good questions and there is no consensus on the answers. Now, do not be too quick to judge astronomers because they have not narrowed down a precise definition for "earth-like" planets. Think of how long it took to narrow down the definition of "planet" itself. Astronomers have known about most of the other planets in our solar system for hundreds of years, yet a formal definition for "planet" was not agreed upon until the IAU's meeting of 2006! "Why did it take so long?" one might ask. Well, let's use a little thought experiment to demonstrate why. Imagine for a moment that the only other humans you know about are those in your immediately family. For me, that would mean only five people (if I do not include my wife). Now, imagine your family wanted to define formally what it meant to be human. You could look at the family dog and say, "That is not a human." You could look at the family cat (though I cannot understand why anyone would want one) and say, "That is not human." Then you could look at each other, assess the common features of your family, and begin to define "human." But how accurate could you really be? Your sample size is so small that you would likely be tempted to be too narrow in your definition (e.g. perhaps including skin pigmentation or hair color in your definition) or too broad in your definition (e.g. perhaps including only the ability to walk erect in your definition). You need a much larger sample size and a lot of time to deliberate together even to begin to be able to create an accurate definition. The same goes for astronomers. In the case of the formal definition of "planet," astronomers needed a much large sampling of planets than those in our solar system to create an accurate, formal definition. In the case of "earth-like" planets, astronomers are just beginning to discover other planets that appear to them to look like earth. It is going to take a much larger sampling of planets and a lot more discussion for an accurate, formal definition to emerge. So, when you hear "earth-like" take it with a grain of salt and start asking other questions: What do you mean by "earth-like"? How large or small is the planet compared to earth? What kind of star does it orbit? How far is the planet from its star? What kind of atmosphere does it have (if that information is even available)? The answers to all these will be very important, especially when it comes to the possibility of the planet being habitable.
Skipping the first general question, let's take a look at the importance of the second: How large or small is the planet compared to earth? This is a crucial question when it comes to the planet's habitability. "Why is that?" one might ask. Because the size of the planet is related to its tectonic activity: the larger the planet the more tectonic activity; the smaller the planet the less tectonic activity. Tectonic activity is the movement of the tectonic plates on which the surface of a planet sits. This movement is what creates events like earthquakes and geological features like mountain ranges and volcanos. It has a much more important function for life, however. Tectonic activity is crucial for life because it recycles carbon dioxide, which helps regulate the planet's temperature. As plates move apart, slide under one another, and even crash into each other, they also recycle carbon dioxide. This regulation and recycling of this greenhouse gas acts as a thermostat to keep the planet warm (but not scorching) over large geological time scales. Without tectonic activity the earth would look like Venus. It is only slight smaller than earth but it is too small to have sustained tectonic activity (any activity after the planets initial formation), so carbon dioxide has built up to form a thick atmosphere and keep the surface temperature of the planet at about 800 degrees Fahrenheit (far too hot for life). So, being only slightly smaller than the earth is too small to support life because there will be no sustained tectonic activity. If a planet goes the other direction, larger than the earth, the possibility of life runs into another problem: too much tectonic activity. As a planet gets larger the tectonic plates become thinner, weaker, and more easily moved, so there is much more tectonic activity. Any larger than two to three times the size of the earth and the planet would have frequent and massive earthquakes, making it too geologically unstable for any type of advanced life. So, when someone says, "Scientists have found a habitable planet!" ask them how big it is compared to earth. If it is only slightly smaller, it will not be able to support any life, and if it is just a little larger, it also will not be able to support any type of advance life.
The second question, "What kind of star does it orbit?" I have discussed before here. This is important because of what is known as the planetary/circumstellar habitable zone. This zone is a band that circles around a star (circumstellar) which defines the minimum and maximum distance a planet can be from the star even to be considered "habitable." The type of star that a planet orbits is crucial to the circumstellar habitable zone because, as I discuss in my earlier post (please see it for more detailed information), there are really two zones around each star--a liquid water zone and a UV radiation zone--that must overlap for life to be possible. This overlap is a band around the star where liquid water could possibly exist and there will be enough UV radiation (but not too much) to give life the energy it needs. If the star has an effective temperature below 4,600 K or above 7,137 K, the zones will not overlap and life will not be possible. This rules out 80% of all stars as possible candidates for life-supporting planets! The Extrasolar Planet Encyclopedia has a list of all the confirmed and verified planets, and it gives you the effective temperature of the host star. When someone says, "This planet is habitable!" check it out. Go to the list, find the planet, click on its link, and check out its host star's effective temperature. If it is not within the above range, it will not be habitable.
The third question is "How far is the planet from its star?" This is crucial because distance from the star affects the planet's rotation (the planet's spinning on its axis). If the planet is too close to the star then it will be "tidally locked." A tidally locked planet is so close to its star that the star's gravity only allows it to rotate once per revolution, meaning the same side of the planet always faces the star. For example, the moon is tidally locked to the earth, i.e. the same side of the moon always faces the earth. This creates a serious problem for the possible habitability of a planet. No matter what kind of star the planet orbits, if it is tidally locked the side facing the star will become incredibly hot (well above boiling temperature) and the side facing away will become incredibly cold (well below freezing temperature). So, even if a planet is in the habitable zone (both the liquid water and UV radiation zones), a tidally locked planet could not support life. This is an important feature of the recent headline "Alien Planets: Billions Of Habitable Exoplanets In Milky Way Galaxy." This research claims that there could be billions of habitable planets orbiting M-dwarf stars in our galaxy (M-dwarf stars make up about 80% of our galaxy's total number of stars). (They have not, of course, detected that many planets; it is a statistical prediction.) The problem with this claim is that it is simply looking at whether or not the planet is in the liquid water habitable zone. What it does not point out is that M-dwarf stars are so dim/cool that any planet in the liquid water habitable zone will be tidally locked! Even though the star is dim, if one side is always facing it that side will be far too hot for life and the other side will be far too cold. Again, when someone says, "Scientists have discovered habitable planets!" ask how far it is from its star and whether or not it is tidally locked. If it is, then, while it may be in the habitable zone, the planet itself is in no way habitable.
Finally, one must inquire as to the atmospheric characteristics of the planet, "What kind of atmosphere does it have?" Unfortunately, rarely can this question be answered. Our detection methods are not refined enough to give much information about an extra-solar planet's atmosphere. The point of asking the question, however, is to point out that even if all of the above qualifications are met (proper size, host star has a good temperature, and the planet is not tidally locked) the atmosphere could still easily rule the planet out as habitable. Since we have no idea what its atmosphere is like, it is dubious to claim it is habitable. Let's do another thought experiment. Say you were looking at our solar system from 100 light years away. You would see it has eight planets and, to your delight, you would discover that three of the eight are in the habitable zone (both liquid water and UV radiation)! Why? Because Venus, Earth, and Mars are all in that zone, yet we know that two out of the three cannot support life (Mars is debated, of course, but as it stands there is zero evidence for present or ancient life there). Why can't they? Venus' atmosphere is so thick that its surface temperature is about 800 degrees Fahrenheit. Mars' atmosphere is composed mostly of carbon dioxide, nitrogen, and argon and its atmosphere is so thin that it has lost all its liquid water. So, if you were looking at our solar system from far away you would think there were three habitable planets, yet when you got here you would only find one. The point is that being in the habitable zone, being the right size, orbiting the right kind of star, and being the right distance from the star does not at all mean the planet must be habitable.
So, what should we make of all this verbose (probably too verbose) explanation? How excited should we get? Are the planets really habitable? How common or rare is Earth? Well, in answer to the last question, Earth is still without an equal. Of all the claims for discovering habitable planets, none meet all the criteria I have named above. Furthermore, the criteria I have gone through above is a small sampling of the many characteristics necessary for life to exist on a planet (for a large working list, see this article by RTB), and all the planets discovered do not even meet the criteria on my small list. For more information on the rarity of Earth, check out this book (by two atheists!): Rare Earth. Are the planets really habitable? Most of the articles you will read in the popular media are just looking at whether or not liquid water could exist on the planet. Certainly liquid water is a necessary condition for life but it is in no way a sufficient condition for life. Furthermore, the list I linked above from RTB shows that there are dozens of necessary conditions for life and liquid water is only one of those. Finally, how excited should we get? I am very excited for two reasons: first, each new planet tells us a little more about how planets form, what most planets are like, and how solar systems form, all of which expand our knowledge of the universe; and second, each new planet shows us how fine-tuned our planet is for life, particularly for human life. It is no coincidence that no other planet is like ours. God chose to create humans, the apex of His creation, here on Earth and He made sure it was perfectly suited for our needs. No other planet even comes close. Far from showing God is not necessary or life is abundant throughout the universe, all the new planetary discoveries show that God's fine-tuning is absolutely necessary for life and life only exists where He wants it to be.
By His Grace,
Taylor
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