Three weeks ago, a Harvard astronomer named Dimitar Sasselov gave an 18 minute TED conference talk that caused quite a stir. His contention is that recent research suggests there could be 100 million "earth-like" (read: earth-size) planets in the Milky Way galaxy alone! This is the perfect topic for a blog about learning new things - there is arguably nothing I know less about than planetary science. But given the press coverage and controversy resulting from Professor Sasselov's talk - oh yeah, and that whole nagging issue of "are we alone in the universe" - I've decided to take a stab at the topic. I will start with a very brief overview of what he said, quickly explain the controversy and finish with a few big picture thoughts. For those who don't want to read on, I'll make it simple for you: I don't think life exists only on earth.
Sasselov starts with Copernicus, from whom we learned that planet Earth is not the center of the universe. This realization led to 400 years of curiosity and scientific study focused on the possibility that other forms of life exist. Indeed, every new, more powerful telescope has shown us a bigger and bigger universe. Put another way: the more we learn about what is out there, the smaller we realize we are on a relative basis. The latest and greatest telescope - Kepler (think souped up Hubble) - is at the center of a $600mm NASA project to identify other planets in our galaxy. More specifically, the goal is to find other planets orbiting stars in our galaxy that are similar to Earth.
The process by which the Kepler does this is fascinating. It monitors over 100,000 stars in the galaxy, and tracks the dimming of spots of light on the stars. This small dimming of light can potentially represent a planet orbiting that star - as the dimming mass moves, you can deduce things like planet size, rotation, orbit, and closeness to the star around which it is orbiting. The Kepler has identified many potential planet candidates, and the scientist teams follow up on promising leads from there. Perhaps the most important planet characteristic sort is by size. If the goal is to find earth-like planets, this makes sense. For a variety of scientific reasons, small planets are better suited for water and for life (or at least life as we know it). In our solar system there are four planets besides Earth that fall into the "small" category: Mars, Venus, Mercury and Pluto. Looking out to our galaxy, we had previously found a disproportionate number of planets bucketed in the large planet category (think Jupiter). We now know this was because our telescopes caught only the biggest masses. The latest Kepler data have found about 1160 planet candidates. Compositionally, this group of planet candidates seems to have significantly more smaller planets relative to larger planets! This is hugely important; for the first time we have real scientific data verifying the proportionally large existence of small, potentially earth-size planets in our galaxy.
Now, there were a number of blogs and articles that reacted somewhat negatively to this talk. One objection was that he used "planets" to describe the newly found masses, and not the scientifically correct "planet candidates." There is apparently a scientific process that must be followed before something can be officially called a "planet." A second objection was that he used the term "earth-like" to describe the planets they found, when they are not necessarily earth-like, but rather are earth-sized. Both of these are fair contentions, but in my mind they do not take away from the significance of the research being done or how exciting this is for all of us. If after all the analysis is done there are only one hundred thousand truly "earth-like" and not merely "earth-size" planets in our galaxy, that would still be an incredible scientific finding.
Of course a tremendous amount of work remains: the scientists must now study the existing sample size to determine the habitability of some of these planet candidates, and there is a constant stream of additional data to be reviewed and analyzed. What's REALLY cool about this is the overlap between the Kepler project and Harvard's Origins of Life Initiative. This is an interdisciplinary institute with all types of scientists (biologists, chemists and astronomers among them) whose goal is to learn about the beginning of life on Earth and perhaps in other places. This gets at the still unanswered question of: if there is life on other planets is it similar to life on earth? Is life as a chemical process universal - like gravity - or is it tailored to a specific environment? One example of how biology experiments can help astronomers is a recent finding that some clay and liquid water, when mixed, can produce naturally available molecules that form spontaneous bubbles, whose membranes are similar to every living membrane on Earth. As Sasselov puts it, this interdisciplinary group at Harvard is building a bridge from two sides of the proverbial river. There are those like him who are looking at planets (think pond scum) and those on the other side who are in the lab (think DNA/RNA). Both groups are trying to get to the foundation and formation of life, and they hope to do so by meeting somewhere in the middle. Right now it is not a full bridge, but if you believe Sasselov it is an important stepping stone to what he says is science redefining life as we know it.
Copernicus helped us realize our spatial insignificance relative to the universe around us. It is possible that through our evolving discoveries about life we will again change our thinking. As Sasselov points out, life may be insignificant in size but it is not insignificant in time. The Earth's biosphere (i.e., life) has existed almost one third of the time the universe is predicted to have been around. And the importance of life relative to the universe increases even more if earth-like or even earth-size planets are nearly as prevalent as they seem to be. Perhaps, then, we aren't alone... and perhaps we will soon learn more about our intergalactic partners in this funny thing called life.