Why We Search

Whether life exists beyond Earth is one of the most profound questions of all time. The answer will change us forever, whether it reveals a universe rich with life, one in which life is rare and fragile, or even a universe in which we can find no other life at all.

The hunt for an answer also is revealing important details about our own place in the universe – where we came from, how life came about and, perhaps, where we’re headed.

Earth and Kepler-452b
This artistic concept compares Earth (left) to Kepler-452b, which is about 60 percent larger. The illustration represents one possible appearance for Kepler-452b – scientists do not know whether the planet has oceans and continents like Earth. Both planets orbit a G2-type star of about the same temperature; however, the star hosting Kepler-452b is 6 billion years old, 1.5 billion years older than our sun. As stars age, they become larger, hotter and brighter, as represented in the illustration. Kepler-452b's star appears a bit larger and brighter.
NASA/Ames/JPL-Caltech/T. Pyle

The years and decades ahead will bring us ever closer to the ultimate in self-reflection: a mirror image of our own planet Earth. A small, rocky world with clouds, oceans and an atmosphere bearing signs of possible life. This might be a combination of gases – oxygen, carbon dioxide and methane – that, seen by themselves, don't tell us very much, but together speak volumes.

Such a world might be hundreds of light-years away, perhaps forever out of reach. But the molecular evidence we read in its atmosphere, using ever more acute technology, could give us the answer we've awaited since the dawn of humanity: No, we are not alone.

Pale Blue Dot
A photo of Earth taken Feb. 14, 1990, by NASA’s Voyager 1 at a distance of 3.7 billion miles (6 billion kilometers) from the Sun. The image inspired the title of scientist Carl Sagan's book, "Pale Blue Dot: A Vision of the Human Future in Space," in which he wrote: "Look again at that dot. That's here. That's home. That's us."

NASA's search for life

The ultimate goal of NASA's Exoplanet Program is to find unmistakable signs of current life.

Exoplanets’ own skies could hold such signs, waiting to be revealed by detailed analysis of the atmospheres of planets well beyond our solar system.

When we analyze light shot by a star through the atmosphere of a distant planet, a technique known as transmission spectroscopy, the effect looks like a barcode. The slices missing from the light spectrum tell us which ingredients are present in the alien atmosphere. One pattern of black gaps might indicate methane, another, oxygen. Seeing those together could be a strong argument for the presence of life. Or we might read a barcode that shows the burning of hydrocarbons; in other words, smog.

Spectroscopy lines of an exoplanet
Light from exoplanets, if passed through a prism, can be spread out into a rainbow of colors called a spectrum. Different colors correspond to different wavelengths of light. Missing colors show up as black lines, indicating specific gases are present, because each gas absorbs light in a specific wavelength (or color).

Finding another blue and white marble

For a world to have life as we know it, we understand that it would need liquid water on the surface, however, it might not look anything like Earth.

The planet would most likely exist in the “habitable zone” of the star it orbits, where it is neither too close nor too far from its star. Also called the Goldilock’s zone, this is the area around a star in which liquid water could exist on planets over geological timescales and where its atmosphere could contain the right balance of gases that could support life.

Partners in the search for life

NASA scientists hunting for life beyond Earth form a broad coalition: those investigating our solar system, ancient or extreme life forms on Earth, and even our Sun. Signs of life might be found on Mars, Jupiter's moon Europa or Saturn's moon Enceladus, and potential future missions are in the conceptual or planning stages. Better understanding of early Earth life, or even living "extremophiles," could inform our attempts to detect life beyond our planet. And truly knowing distant exoplanets requires knowledge of the stars they orbit; greater understanding of our Sun will help us to know other stars.

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