For most of human history, we had no idea if there were other worlds beyond our own. We gazed at the stars and wondered: are we alone in this vast and awesome cosmos? Or is there life out there, perhaps even intelligent life, with whom we could someday communicate?
We still do not know the answer to that question, but we have made some progress. We have explored our own solar system, a family of planets and other bodies that orbit our Sun. We have discovered eight major planets, some with moons of their own, and many smaller objects like asteroids and comets. We are even searching for a possible ninth planet, far beyond the orbit of Neptune. But these are all our neighbors, our kin. We share a common origin and a common destiny with them.
Exoplanets are different. They are worlds that orbit other stars, far away from our Sun. They are strangers, unknown and mysterious. And they are very numerous. In the past few decades, we have detected over five thousand of them, using various methods and instruments. And we suspect there are many more, perhaps billions or even trillions in our galaxy alone.
Exoplanets come in all shapes and sizes. Some are huge and hot, like Jupiter but much more so. They circle their stars so closely that they are scorched by their heat and radiation. Some are smaller and colder, like Neptune or Uranus. They may have thick atmospheres of gas and ice. And some are rocky and terrestrial, like Earth or Mars. They may have solid surfaces and perhaps even oceans and continents.
The most intriguing exoplanets are those that resemble Earth in some way. They are not too big or too small, not too hot or too cold. They orbit their stars in a zone where liquid water could exist on their surface. Water is essential for life as we know it. And so we wonder: could these exoplanets harbor life? Could they be oases of green and blue in the desert of space? Could they be home to creatures that think and feel and dream?
We do not know yet. But we are trying to find out. We are building new telescopes and instruments that will allow us to study these exoplanets in more detail. We will look for signs of water, oxygen, methane and other molecules that could indicate the presence of life. We will listen for signals that could be sent by intelligent beings. We will try to make contact with our cosmic cousins, if they exist.
We are on the verge of a new era of discovery and exploration. We are about to learn more about our place in the universe and our connection to other worlds. We are about to answer one of the oldest and most profound questions of our species: are we the only ones?