Earth through the agesImage credit: NASA/JPL-Caltech/Lizbeth B. De La Torre

Once seen as the center of the universe, Earth has suffered a series of demotions over the past few hundred years. Now, in an age of fast-paced discovery, we’ve learned we’re likely just one of trillions of planets in the Milky Way galaxy – and among the smaller ones at that.

Yet Earth remains a standout, and so far, one of a kind. Of the thousands of exoplanets – planets around other stars – confirmed by our increasingly powerful telescopes, and despite extensive probing of the solar system, ours is still the only planet known to host life.

In some ways it’s an embarrassment of riches. Earth’s abundant, persistent, and pervasive lifeforms seem to fill almost every nook and cranny, from the boiling, caustic pools of Yellowstone National Park to the Dry Valleys of Antarctica. Life might well have gotten its start only a few hundred million years after Earth formed from a swirling disk of gas and dust – an eyeblink in geologic time.

But Earth hasn’t always looked like the blue orb we know so well. The variety of contending creatures that have come and gone over billions of years, in a sense, paints a picture of the many planets Earth has been: a lava-covered rock with a poisonous atmosphere, an ocean world with the bare beginnings of microbial life, a steaming tropical riot of earth-shaking dinosaurs, or an Ice Age expanse where cave-dwelling humans hunted mammoths.

“We tend to talk about Earth-like planets as a planet like ours is today,” said Doug Hudgins, program scientist for NASA’s Exoplanet Exploration Program at NASA Headquarters in Washington. “The planet has been radically different at times in the past.”

Compare Earth’s lively past to a dazzling cosmos stretching endlessly in all directions, so far silent on the question of life. Not even crickets.

So, are we really alone?

Finding an answer is a high priority for NASA, but even posing the question – that is, interrogating the universe – quickly becomes an inventory of ingredients from astrobiology: chemistry, planetary science, and cosmology, strongly seasoned with statistics.

Searching for life: What we know and don’t know

The odds seem a bit better now that we’ve confirmed more than 4,000 exoplanets in our galaxy, about a fifth of them in Earth’s size-range. We know the building blocks of life are present throughout the solar system and the cosmos, and that includes water.

We don’t know how readily life begins, whether it’s common or rare, how long it endures. Up the ante to intelligent life, and the questions only multiply.

That includes one of the most famous questions of all: Where is everybody? Physicist Enrico Fermi posed it back in 1950 during lunch with fellow physicists, igniting decades of debate. Even at a leisurely, slower-than-light pace, the reasoning went, our galaxy could be easily crossed by a spacefaring civilization within a few million years. The Milky Way galaxy is presently pushing 14 billion. And while it took 4-billion-plus- years for technological intelligence to develop on our planet, the galaxy contains plenty of planetary systems of comparable age, as well as others much older.

We hope you’ll join us for a trip through our solar system, and the planets and stars beyond. Through stories and visuals, we’ll take stock of where the search for life stands and get a glimpse of the future – the space telescopes, instruments, probes, landers, rovers and advanced technology NASA plans to deploy in coming decades. The goal is to find that hidden blue and white marble, or perhaps even an orange one – another living, breathing planet.

“We only have our planet’s example to follow,” said Nancy Kiang of the NASA Goddard Institute for Space Studies, who studies how plants might adapt to exoplanet environments. “There could be a similar evolutionary pathway (on other worlds) – convergent evolution on a planetary scale.”

Maybe not quite like looking in a mirror, but not too far from looking like home.

By Pat Brennan

NASA’s Exoplanet Exploration Program