The Search for Planets
de Grasse Tyson, Neil, Natural History
Want conclusive proof that planets exist in the universe? You are standing on one. But before you complain about what constitutes admissible evidence, consider that it's easier to find something for which you already have an example than to find something that is predicted solely by somebody's theory.
A pessimistic estimate for the number of planets in the universe is nine. A more realistic estimate is about one sextillion (one followed by twenty-one zeros). If, for some reason, you have a problem visualizing the magnitude of this number, consider that it's a million times larger than the total number of sounds and words ever uttered by all humans since the dawn of the species. You can, of course, search for these planets just for the sake of finding them. You will probably find some. But why search for planets unless you can simultaneously ask whether the planets you find resemble the ones you know and if any of them sustain life. Indeed, these types of questions guide most planetary searches.
We often make the implicit assumption that our solar system (which we know contains life, whether or not the life can be considered intelligent) is neither special nor peculiar. A more troublesome thought is that our solar system is not ordinary precisely because it possesses characteristics that led to the evolution of beings who ask these questions-a point made recently by George Wetherill, a Carnegie Institution planetary astrophysicist, at a conference on the search for exosolar planets. Wetherill's comments notwithstanding, I remain in the "we're ordinary" camp and proceed with the next step: creating a catalog of nearby stars that would make good hosts for planets.
Theories about other worlds were not always greeted with praise. In 1600, at the Square of Flowers in Rome, the Catholic Church burned Giordano Bruno (naked) at the stake-not for being an ordinary heretic, but for being an "impenitent and pertinacious" heretic. Bruno's crime? He had reasoned that the universe must be infinite, because otherwise it would have to exist in one place rather than another, a notion that conflicted with his philosophical sensibilities. From the vastness of this infinite universe, Bruno then concluded that there must be many other worlds beyond Earth. Fortunately, scientists who hold such views today are allowed to live. And their discoveries are heralded with front-page headlines.
The discovery of planets around other stars ought to be a cinch. We first look in the galaxy for stars whose temperature, size, and age are similar to the Sun's. We call them solar-type stars. We then look for large, gaseous planets. These are Jupiter-like planets. We then strain to find smaller, rocky planets that resemble Earth. And of course Earth happens to be a very good example of an Earthlike planet.
The problem is that the simple act of looking does not always reveal what you seek. Your ability to resolve information about a distant object is limited by several factors, including the stability and transparency of the medium you are looking through and the size of your eye's pupils (the bigger the better). For these reasons, the widely repeated claim that an astronaut can resolve the Great Wall of China from orbit is highly dubious. For example, Interstate 10-from Florida to California-is several times as wide and 50 percent longer than the Wall, yet nobody in orbit waxes poetic about the visibility of the freeway system in the United States. With no clouds or other obscuring atmospheric conditions, an astronaut trying to resolve the Wall would need pupils about the diameter of the average human head. The lesson: using large telescopes in orbit above Earth's atmosphere will always improve your vision of the rest of the universe.
The most obvious way to discover a planet around another star is by direct detection. But planet detection remains one of the most challenging things you can do with a telescope. Heroic efforts have been made by persistent astronomers armed with clever techniques and state-of-thescience hardware. …