I have to admit to experiencing a certain impatience with how the media covers science matters, particularly where the search for life elsewhere in the universe is concerned. Oftentimes, the impression is given that if liquid water can exist on a planet’s surface, it must be a good candidate for life. The truth of the matter is much more complicated.
When discussing the potential for life elsewhere in the universe, advocates like to cite statistics such as the possible number of stars in the observable universe, which scientists estimate at 50 billion trillion, and the fact that most stars appear to have planetary systems. With so many other planetary systems out there, they reason that earth-like planets must be rather common, and surely life has developed on at least a few of them.
Going by purely naturalistic assumptions, this might seem like a persuasive argument, but the more we drill down into what we’ve learned about other stars and the planets that orbit them, the more likely it becomes that our earth is truly unique. I could outline several aspects of this issue, but for the moment I’ll concentrate on one statistic that narrows the range of habitable systems dramatically.
Stars come in a variety of categories, and are classified in several ways, including by age, size, color, and mass. For instance, our sun is a class G yellow dwarf star, of moderate luminosity (brightness) and exceptionally stable. Roughly 12.6% of stars in our part of the Milky Way galaxy are like the sun. By far the most common type of star in the universe is the red dwarf (class M). Red dwarf stars are much smaller, cooler, and dimmer than stars like our sun. The closest star to our sun is a red dwarf: Proxima Centauri, which is located about 4.2 light years away. Even though this star is so close to our solar system, because it’s so small and dim, it’s not visible to the naked eye and wasn’t discovered until 1915. Only one red dwarf star (Lacaille 8760) is actually visible from earth—barely—and even it can’t be seen without exceptionally clear, dark skies.
Given that they’re smaller and cooler than our sun, one might think red dwarf stars would be ‘kinder and gentler’ than other stars, serving as ideal hosts for life-bearing planets. Actually, the opposite is true: red dwarf stars are temperamental, and planets in orbit around them would have a difficult time sustaining life. For one thing, red dwarf stars often fluctuate wildly in brightness (a behavior known as ‘flaring’). Flare activity not only changes the amount of light planets receive from their host stars, it can also bombard them with charged particles that gradually strip away their atmospheres, making them uninhabitable. Flaring could also result in dramatic fluctuations in UV radiation that could effectively sterilize the surface of a planet orbiting close enough to receive enough warmth to sustain life.
Because red dwarf stars are so much cooler than our sun, any planets they may have must orbit closer to them in order to receive enough energy to warm them sufficiently, maintain liquid water on their surfaces, and allow plants to conduct photosynthesis. This not only subjects them to significant radiation hazards, as I already mentioned, it also increases the chances for what scientists call ‘tidal locking.’ Tidal locking is when a planet’s rotation rate matches the time of its orbit around its parent star, with the effect that the planet only ever presents one side of itself to the star (just as our moon only ever presents one side of itself to the earth).
When a planet becomes tidally locked to its parent star, one side of it continually experiences day, while the other side experiences an eternal night. As you might expect, the day side of such planets—continually bathed in sunlight—become extremely hot, while the opposite side—continually deprived of sunlight—turns into a deep freeze. The odds are very low that any sort of life could survive such temperature extremes. A planet with a thick atmosphere might be able to maintain high enough temperatures to permit bacteria or certain types of plants to survive, but on the other hand, a thick atmosphere might also produce a runaway greenhouse effect, making the planet too warm for life. Of course, this assumes that the planet is large enough to retain its atmosphere in the first place. Take Mars, for instance. Mars is approximately 10% as massive as Earth, which is why its atmosphere is much thinner than Earth’s: it simply doesn’t have enough gravity to hold on to it in competition with the eroding influence of the solar wind (charged particles streaming outward from the sun). The planet would also need a substantial magnetic field to help hold on to its atmosphere. Mars has a negligible magnetic field, which is another reason for its tenuous atmosphere.
Another problem for life in red dwarf solar systems is the fact that red light has a longer wavelength and lower energy levels than is ideal for photosynthesis in plants. Experiments have been done showing that it is possible for plants and some forms of bacteria to survive and grow in light mimicking the spectrum of a red dwarf, but flaring would be a potential problem here, as plants sensitive enough to make use of the lower energy level of red light might be overwhelmed by sudden, dramatic increases in energy output. Again, atmospheric density and composition would also play a role in how well plants were able to grow in the light of a red dwarf star.
So, yes, in a universe of 50 billion trillion stars, most of which probably have planets, one might think that life-bearing planets could be common; and this is certainly the impression many in the media like to convey when covering news of astronomical discoveries. When you consider that the vast majority of stars fall into a category that is less than ideal for life, however, those numbers look far less impressive. Red dwarf stars are, at the very least, inhospitable hosts, and any planets in orbit around them would have to meet exacting conditions in order to sustain life. I’ve touched on a few of the issues here, but there are many more. For instance: Does the planet’s soil have the right mineral content in the right proportions? Does it have plate tectonics to recycle its soil? Does it have a comparatively large moon to stabilize its axial tilt in order to give it stable seasons? Does it have enough water? What sort of gravitational influence do other planets in the system subject it to? And then there are outside influences to consider as well. For instance, how close is the system to other solar systems, to black holes, or to the energetic core of the galaxy? Conditions might be ideal within the system itself, but if it is constantly being gravitationally destabilized and blasted by radiation from outside sources, the odds against anything beyond perhaps simple bacteria existing there are remote at best.
All of these are factors that weigh heavily on the question of whether a planet is a viable prospect for life, and, to date, no other planet has been discovered that even comes close to the conditions we enjoy on our Earth. Our world is located on an ideal planet, which is itself located in an ideal position within an ideal solar system, in orbit around an ideal star, in an ideal part of an ideal galaxy in an ideal area of the universe.
We occupy the best-known real estate in the observable universe, and I believe that this is no accident.
Proxima Centauri: the closest star to our sun, and an artist's impression of a planet orbiting a red dwarf star.
