With the discovery of Kepler 452b, the search for an "Earth-like" planet has made a major advance, but let's keep our expectations in check: There is only one Earth. Our galaxy and our universe may abound with planets, some of which may be habitable, but nothing that we shall find will ever closely resemble this planet that we call home. Inevitably, we will find planets the same size as Earth, and some of them will have a star and an orbit that closely resemble our own. In decades to come we may even discover that some of these Earth-like planets have an atmosphere containing telltale signs of life. And it is possible that some of those exoplanets will harbor intelligent life. But these planets will not be Earth. They will be alien worlds in every sense of the word.
Conditions on even the most hospitable of the "close cousins" discovered thus far are likely to be hostile to human life. The gravitational pull of Kepler 452b may be twice as strong as Earth's, and NASA experts believe that the planet may have a high level of volcanic activity. The air on Kepler 452b is probably too hazardous to breathe, and if a greenhouse effect has taken hold, it could be exceedingly hot.
When, and if, we discover that there are sentient beings out there, we may rejoice, but for all practical purposes, we will still be alone, and likely so for many millennia to come. The interstellar distances are just too vast. Even at the highest speeds yet achieved by our space probes, it would take over 30,000 years to reach the nearest star system, and several million years to reach Kepler 452b.
Our descendants may, one day, set foot on another planet readily capable of supporting human life, but if so, it will not be ours for the taking. It will be populated by other creatures with a superior claim to its bounty. We could, in theory, lay claim to a lifeless world and terraform it, but no matter how much we transform it, it will never in a million years approximate Earth. Our world is, and will remain for all practical purposes, irreplaceable. By dent of God or nature, it is uniquely formulated to support the creatures that currently occupy it, including us, and there are no other worlds in our galaxy or beyond that can serve as an adequate substitute.
In decades to come space explorers may set foot on Mars and become the first to witness an alien sunset. What a thrill. But if the first settlers on that bitterly cold and lifeless planet have no realistic prospect of every returning to Earth, they will develop the solar system's worst case of homesickness.
Even if we were to discover, by some cosmological stroke of fortune, a planet that could readily sustain human life, and even if it were relatively close by astronomical standards, only a select few of us would ever make that journey. For the great bulk of the 7.3 billion people who now occupy Earth, there will be no escaping to another habitable world. Earth is, and will ever remain, our home. And the same will go for our descendants. There will be no mass migrations to another planet.
The preservation of the human species may require us, some day, to explore and settle other worlds, but we should be concerned, for the moment, about the preservation of the only planet that we have ever set foot on: Earth. Our collective impact on the Earth's atmosphere, oceans and forests is now so impactful and so pervasive, that leading scientists assert that we have entered a new geological epoch, the Anthropocene, in which humans have become the major driver of environmental change. That's not a good thing. Our world is fine-tuned. Change it in any material way and you diminish its capacity to support existing lifeforms, including human life, and that is precisely what we are doing today by altering the climate, changing the chemistry of the oceans, and destroying entire ecosystems.
NASA's discovery of Earth-like planets in distant solar systems is exciting, but NASA this past week also unveiled a dazzling photograph of the most valuable piece of real estate in the entire galaxy: Earth. Treasure it.