Exo-Earths—the search for worlds like our own

Focus is on finding planets with water and air at the right distance from a stable parent star.

By Amir Alexander
Science Writer, The Planetary Society

Earth as seen from space by the astronauts of Apollo 17.

Even now, somewhere in the vastness of space, in our galaxy or far beyond it, a planet much like our own is orbiting a distant sun.

It is a rocky planet, like Earth, rich in water and minerals, its surface composed of vast oceans and the large tracts of dry land that we call continents. This world, furthermore, carries life.

Living beings large and small roam its lands and fill its oceans—creatures that, had we seen them, would be strange to us, but still familiar enough to be called animals. It is even possible that this world is home to what could be the rarest thing in the universe—intelligent beings like ourselves.

How do we know that such a world is out there? Truth be told, technically we don’t. No such planet has ever been found, and scientists concede that we are still years or decades away from being able to detect or find such a world.

To make things even more challenging, even if an Earthlike planet were detected today, scientists would find it difficult to say what the actual conditions are on its surface. It will be years before astronomers possess the technology to differentiate between a dead planet and one teaming with life. As a result, the list of known “Earths” is still the same as it has been throughout history. It comprises one planet—our own familiar home world.

A question of numbers

And yet most scientists are confident that exo-Earths—worlds like our own orbiting distant suns—exist, and that it is only a matter of time before we find them. To scientists, the very size of the universe and its staggering richness make the existence of exo-Earths almost inevitable. Think about it this way.

Our galaxy—the Milky Way—contains between 200 and 400 billion stars, and the observable universe (the region where light has had time to reach us since the Big Bang) contains at least 170 billion galaxies.

Taking the lower estimate for the number of stars in our modest-sized galaxy, one arrives at the inconceivably large number of 3.4 x 1022, or 34 billion trillion stars! This, to be sure, is only a rough figure, and scientists believe that the true number of stars could well be as high as 1024 (a trillion trillion stars).

But it is enough to give one an idea of the staggeringly large number of stars in the universe. If even 1% of these stars have planets, then we are still dealing with something in the range of billions of trillions of planetary systems. Is it conceivable, given these numbers, that our Earth is absolutely unique?

Is it plausible to assume that it is the one and only planet in the universe rich in water and carbon and sustaining complex life? Hardly. The laws of probability practically compel us to believe that there are other “Earths” out there, waiting to be discovered.

One world or many?

The idea that there exist other worlds like our own, inhabited by living creatures, is not new. Long before humans had any notion of the true size and complexity of the universe, there were those who argued that our world is not unique.

In the 5th and 4th centuries BCE, the Greek philosophers known as “atomists” argued that the universe was composed of an infinite number of minuscule atoms, whose random combinations and collisions make up all that exists.

And since the number of these particles was infinite, whereas the number that made up our world was finite, there were more than enough atoms available to create an infinite number of worlds. The atomists’ view was, to be sure, a minority position in the ancient world. Indeed, the notion that all the complex structures we see around us, including life itself, are the result of random collisions of atoms seemed highly dubious.

Most ancient scientists were convinced that our world was unique, and the center of the one and only “Cosmos.” This view was given its highest scientific form by the Alexandrian astronomer Claudius Ptolemy (c 90–168 CE), who developed a detailed and complex mathematical system that placed Earth at the center of the universe, with the Sun, Moon, planets and stars all revolving around it. Ptolemy’s Earth-centered (or “geocentric”) system held sway in the West for an astonishing 1400 years.

Spiral galaxy M81 is similar in structure to our own Milky Way.

It accorded well with the Christian belief that humans were the central actors in a divine plan of fall and redemption. The notion that there were other worlds, and perhaps other “men,” in the universe, sat uneasily with the belief in Christ’s sacrifice in our own world.

With both science and religion supporting the centrality of the Earth, the unique status of our one and only world seemed secure. Then, in 1543, an obscure Polish astronomer named Nicholas Coper­nicus published a long and highly technical book called De revolutionibus orbium coelestium (On the revolutions of the heavenly spheres).

Copernicus’s main contention was that the Earth, far from being at the center of the cosmos, was in fact a planet orbiting the Sun. To support this radical view Copernicus developed a detailed mathematical model that equaled, and in fact surpassed, the work of Ptolemy and his followers.

Scientists, philosophers, and theologians would now have to take seriously the possibility that the Sun, rather than the Earth, stood at the center of the universe. And with that, everything was changed. If Earth was not the center of all things, but one of several planets, then why should we assume it is different from other planets?

And since Earth is home to living creatures of all kinds, including human beings, is it not reasonable to conclude that other planets are also home to living beings? Copernicus himself did not think so.

A conservative in all respects except for his single radical idea, he would not entertain the idea of other planets as their own distinct worlds. But the cat was out of the bag, and future generations of astronomers and philosophers would never cease speculating about other worlds beyond our own.

The list of scholars who speculated about the existence of worlds like our own reads like a who’s who of the greatest scientific and philosophical minds. There was the German Johannes Kepler (1571– 1630), the first to correctly describe the elliptical orbits of the planets and the mathematical relations between them.

His essay Somnium (Dream), published posthumously, described a voyage to the moon and an encounter with its inhabitants. There was René Descartes (1596– 1650), France’s leading philosopher and the inventor of analytic geometry. He argued that the universe was infinite, and filled with solar systems just like our own.


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