The Life of Super-Earths (16 page)

Stars form out of low-density gas, which must cool while being compressed (under its own weight) for a star to be able to condense. Because hydrogen and helium are terribly bad at such cooling, the first stars must have been super-size only—hundreds of times larger than our Sun. The first stars were massive, had short lives, and produced some heavy elements that were dispersed inside the nascent galaxies and made it possible to form smaller, less massive stars. The addition of even a sprinkling of elements to the hydrogen-helium gas helps it cool, so the next generation of stars can be formed from a wider range of gas clumps. With each generation, progressively smaller stars can form—and they do. Today our Galaxy has many stars smaller than our Sun. Partly this is due to the fact that smaller stars live longer by burning their nuclear fuel slowly, but mostly because small stars have formed in increasingly larger numbers as the Universe has evolved.

Small stars disperse a portfolio of heavy elements when they die, so the enrichment of the Universe with heavy elements continues at a slow, steady pace. In fact, after 13 billion years only about 2 percent of the original mixture has been transformed to heavy elements; the enrichment, as astronomers like to call it, is very slow indeed.

The brief story of the Universe, then, looks like this: from just hydrogen and helium about 13 billion years ago, generations of stars made enough iron and oxygen, silicon and carbon, and all other elements, to be able to form Earths and super-Earth planets. There are at least two important morals to this story regarding life.

First, it took a long time before stars anywhere in the Universe could have planets. Stable environments in normal galaxies that were enriched enough to have planets became available about 9 billion years ago.
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If you ask about large terrestrial planets, such as rocky super-Earths and Earths, then it is more like 7 billion to 8 billion years ago. We can imagine that the emergence of life had to wait until that time in the history of the Universe, if not later.

Second, the enrichment continues to this day, and we have a fairly clear idea of how our Universe will be transformed in the eons to come. For example, we see that massive stars have been forming less frequently for the past 5 billion years, so the small stars will dominate element production and enrichment in the future. Generally, that means more carbon than oxygen. Today there are three times more oxygen atoms than carbon atoms in most of our Galaxy, but eventually a point will be reached when carbon and oxygen exist in equal abundance. When this happens, the mineralogy of rocky planets changes. Carbides dominate silicates, and there will be important implications for the origins of life on such planets, as the carbon planets described earlier in the book go from being rare to being common.

In general, though, the future of life looks excellent. Unless life is an exceedingly rare phenomenon, there should be more of it, and more diverse forms of it, in the future. Planets may be just a tiny fraction of the Universe because they are so small, yet there are so many of them that there are plenty of places for life. We now know that our Universe is passing
through its peak of forming stars (known as the stelliferous era), but it appears that it is still peaking in terms of forming planets.
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This implies that the Fermi paradox, which is about the past, is the wrong way to look at the question of whether there is life elsewhere. The paradox assumes that there was enough time before us for others to emerge and develop. The new evidence does not support such an assumption easily. Of course, when it comes to technology, not microbial life, we can only speculate–our own technological capabilities have grown exponentially recently, and if such growth were used as a basis, then the Fermi paradox remains strong statistically. But for life, the logical sequence I follow is: (1) complex chemistry is necessary for life to emerge—enough heavy elements are needed; (2) stable environments that allow chemical concentration are also necessary—terrestrial planets (Earths, super-Earths) are needed. When in its past did our Galaxy (and our Universe) fulfill these requirements?

The answer is, Between 7 and 9 billion years ago. I arrive at this answer via two independent paths. The first path, much of which relies on what we've been considering, is to observe the stars and gas in distant galaxies, measure their abundance in heavy elements (the ones needed for life and planets), and thus see how their abundance grows with time. When we begin seeing stars with just enough heavy elements to allow forming Earths and super-Earths, we have pinpointed the time in the past we are looking for. The only problem is that we need to know how much heavy elements
are enough to form big terrestrial planets. That's a tough question. If our computer models for planet formation are accurate, then a solar system requires at least 1/1,000 of the proportion of heavy elements that our Sun has. Our Galaxy reached this state about 9 billion years ago.
¹⁰

The second path to answering the above question goes directly to the planets. Do we observe a decline in the number of planets around stars that are poor in heavy elements? Yes. This evidence surfaced early on in the planet-hunting game. It was so pronounced that most teams were tempted to select stars rich in heavy elements in order to discover more planets. Nobody was surprised that such a trend—more metals, more planets—existed, but the strength of the trend was surprising. The trend drops off to practically no planets so fast that even the proportion I mentioned above—1/1,000 of the heavy elements of the Sun—seems too generous. It comes out to something like 1/100 of heavy elements compared to the Sun.
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This would put the time in the past when planets that were capable of cradling life could form at just about 7 billion to 8 billion years ago.

A word of caution is due here. The “more metals, more planets” trend is currently only observed for Jupiter-like and Saturn-like planets, and for hot Jupiters in particular. I have to assume for now that it holds for terrestrial planets, but the Kepler mission is working to answer that question accurately.

Today astronomers know with certainty that less than 13 billion years have passed since our Universe was capable of having stars and planets. This makes the stellar, planetary
Universe very young. (Because we see that our Galaxy and the rest of the observable Universe, and its 200 billion galaxies, show a clear potential to continue on as we see them today for hundreds of billions of years, if not much longer, I feel that the words “very young” describe the Universe adequately.) The anthropomorphic analogy to parent-daughter, when we talk about a Universe with planets and Earth life, is then pretty good, as well. Life on Earth could really be among the first older siblings in the family.

So far I have been talking about microbial life. But what about the bigger question: Are we humans alone? That is a far more difficult question to answer. However, if planets and life are so young in our Universe, perhaps we are not latecomers to the party. We may be among the early ones. That could explain why we see no evidence of “them.” This does not necessarily mean, however, that no one is there.

By all accounts, today the Fermi paradox remains unresolved and allows for a fascinating range of possible solutions—from the very deep to the very entertaining, all of them worth more attention than I plan to give them here, but for recommending the rich literature that does.
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With this answer to the Fermi paradox in hand, we can now estimate just how big the family of life—the census of habitable planets—is. The answer is, Pretty big. Consider this: there are more stars in the Universe than there are grains of sand in all the beaches on Earth.
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And there are equally as many planets (see
Figure 11.1
). Of course, as I noted at the beginning of this book, those astronomical numbers do not imply inevitability, no matter how good we feel about our models. We have to go
and find out for ourselves. The survey by the NASA Kepler mission will accomplish that. In the meantime, we can use the current discoveries of extrasolar planets to make a preliminary estimate.

 

FIGURE 11.1
.
There are as many grains of sand in all the beaches on Earth as there are stars and planets in the Universe. The bright dots in the vicinity of our Sun denote some planetary systems we have already discovered.

To begin, the number of known planets to date, summer of 2011, is in the mid-hundreds (about 600), and most of them are in our neighborhood of the Galaxy (see
Figure 11.2
). They will be a useful reference.

First, I need to know the number of stars in the Galaxy. This number is being constantly updated but has not changed much in the past decade, and is based on many different surveys. Many millions of stars, of different types and in different parts of our Galaxy, have now been counted. With these counts and a measure of the extent of the Galaxy, I multiply to obtain the total number of stars: about 200 billion stars in total. Of these only 90 percent are small enough and long-lived enough to develop and have planets. In addition, only 10 percent of these smaller stars were formed with enough heavy elements to have Earth-like planets. So far, our estimates are very secure and robust. But now I need to know how many of that 10 percent of stars actually harbor Earth-like planets.

I turn to planet counts, just as with the stars. I count how often Earths and super-Earths pop out in the planet surveys done so far. This is a difficult task because few super-Earths have been discovered (no Earths to date); as we've seen, they are much more difficult to find, compared to the large and heavy giant planets, so any census of them needs to take this difficulty into account. One way to do that is to compare two different methods of planet discovery and see if they lead
to the discovery of different ratios of super-Earths to giants. Comparing the Doppler shift method with the gravitational lensing method suggests that about half of the stars with heavy elements should have at least one Earth or super-Earth planet. If we assume that there are no privileged orbits—that all else being equal, planets can form and remain in any orbit around a star—then only about 2 percent of these Earths and super-Earths will happen to be inside their star's habitable zone. The remaining 98 percent will orbit too close or, more often, too far from their star. The ultimate answer will come from the Kepler mission, near its nominal end in a few years, but the preliminary data already is consistent with this estimate.
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FIGURE 11.2
.
Most extrasolar planets discovered so far are close to our Solar System; about 80 percent of all known planets are within 500 light-years and are around stars from the Orion arm of the Milky Way.

Now I am ready to sum up the numbers. With all these fractional reductions to the population of 200 billion stars in our Galaxy, I end up with 100 million planets with habitable potential today.

This number is not precise, but there is no escaping how big it is. On the other hand, the majority of these planets are of an age similar to our Earth's. Some are younger, but only a few, given the recent advent of heavy elements, should be much older. With such time constraints, we need to be cautious about drawing conclusions of inevitability. All we can be certain of is that life is not an impossibly rare phenomenon—it definitely has odds of 1 in 100 million. These odds aren't so bad; events of such rarity do happen. As of May 2011, the US Megamillions lottery jackpot had been won four times that year, with odds of 1 in 176 million. In fact, using the binomial distribution, we can see that there is an 18 percent chance of two successes in 100 million trials.

As I explained in the previous chapter, we live in a very young and
changing
Universe, so the estimate of 100 million planets with habitable potential is just a snapshot of our Galaxy now. A real estate developer once asked me about the trend in the number of habitable planets—are they growing or diminishing in number? Good question!