In Search of Earth-like Extrasolar Planets: Garden Spots of the Galaxy

October, 1957, the Soviet Union startled the world with the sounds of Sputnik. The political overtones were electrifying. Russia had the bomb, and now they had the ability to place one into orbit.

Nestled in a small, West Texas town, one seven-year-old boy did not care about political or military implications. Such was his excitement that he could not bear to look at Douglas Edwards on the CBS Evening News. The words were all that he could stand, but he soaked them up with the thirst of a man long abandoned in the desert.

Finally, humanity had left the planet. It was a feeble first step, but the stars beckoned ever stronger. The young boy could feel it in his bones.

Why the fixation on space? The seven-year-old boy was not the only one. And such fixation was nothing new. Dr. William Gilbert, of England, published De Magnete, in the year 1600. In that book, he proclaimed that the space beyond Earth was filled with a vacuum and that there were countless unseen stars and planets in that void. Several decades later, Isaac Newton gave us some of the needed tools for exploring that vast, outer realm.

The young boy had recently seen the then new movie, Forbidden Planet, about a follow-up expedition to an Earth-like world in a nearby star system. That ‘island’ in the dark of space had been home to a proud race called the Krell, a million years more advanced than humans.

Imagine all those ‘islands’ in the dark of space—those worlds similar to our own. So many science fiction authors have done just that. And Hollywood has continued to take up that challenge, with Kubrick’s 2001, Roddenberry’s Star Trek and Lucas’s expeditions to a galaxy far, far away.

But not only were the dreamers thinking of other planets, scientists were, too.

In the 1960’s, Stephen Dole, a researcher with the RAND Corporation (a think tank), wrote a paper called ‘Habitable Planets for Man.’ Science and science fiction author, Isaac Asimov, collaborated with Dole to produce a popular book with the shortened title, Planets for Man. The paper and the book gave the recipe for planets suitable for humanity—what kind of star and what kind of planet. Two key ingredients called for star systems of the right age and the right chemistry.

The chemistry is easy enough to garner from the star’s spectrum. The star needs to be relatively ‘metal rich.’ Here ‘metal’ includes elements such as oxygen and carbon, but also iron and other traditionally metallic atoms. Without these, any planet would likely be mostly or totally hydrogen—no land or oceans.

The other key factor—age—was not so easy. Scientists had determined, over decades of study, that stars have a certain maximum age which depends on their mass. More massive stars burn their fuel (hydrogen) much more rapidly, and thus enjoy shorter lifespans. Dim, red dwarfs live far longer, but a planet needs to huddle far closer in order to gain the needed warmth for life to exist. The problem there is that gravity and tides force such planets to stop rotating. Thankfully, the reach of tidal braking expands outward at a much slower rate than does stellar brightness for any given range of star masses. Stars like our own sun have a ‘just right’ Goldilocks zone far beyond the clutches of the sun’s tidal braking zone.

But to pick any star similar to our own sun in chemistry and in mass does not guarantee a habitable star system. The star may be too young. Only in recent years have scientists advanced their theories sufficiently to estimate the ages of individual stars. For those stars under two billion years in age, their planets are likely still undergoing the heavy bombardment of formation. Life may exist there, but precariously.

For stars with at least similar metal content and with an age of at least three billion years, there may be potentially habitable planets. These are the ‘garden spots of the galaxy.’ Alpha Centauri, next door, is one such system. Though we do not yet know if our next-door neighbor possesses planets, if an Earth-like world were to encircle one of the system’s two main suns, and if life were to have evolved at a pace similar to that on Earth, that world may possess a civilization nearly a billion years ahead of our own—not ‘million,’ but ‘billion!’ Alpha Centauri is that much older than our own sun.

Nearly four years after Sputnik, the young, West Texas boy almost forgot his own birthday. One of the teachers had brought a television to the auditorium of his school so that all the students could witness the launch of the first American into space, Alan B. Shepard, Jr. After the event was over and the auditorium had been cleared, the young boy returned to the stage where the television had sat. He touched the spot, feeling his chest rise with deepening breath. This was only the beginning.

Photos courtesy NASA.