Are we alone, or is life a common feature of many worlds in the cosmos?
One of NASA's important objectives is to explore our solar system to understand its story - its origin and evolution - and the potential for life elsewhere. But NASA also seeks to understand how systems of planets form around other stars, and under which circumstances those planets are most likely to have life on them.
Terrestrial Worlds (Artist Concept) (2008-02-17)NASA
In the past couple of decades, scientists with NASA and other organizations have begun discovering these exoplanets, using a variety of observatories on the ground and in space.
Kepler K2-138 Planetary System (Artist Concept) (2018-01-11) by NASA/JPL-CaltechNASA
NASA's Kepler mission alone has discovered thousands of new planets, with thousands more candidate planets that may be confirmed in the next few years. Whether you realize it or not, you are living in a golden age of exploration.
Hubble image of the star-forming Lagoon Nebula (2015-05-30) by NASA, ESA, J. Trauger (JPL)NASA
The general study of the origin, evolution, distribution and future of life in the universe is known as astrobiology.
Astrobiology research sponsored by NASA focuses on three basic questions: How does life begin and evolve? Does life exist elsewhere in the Universe? And what is the future of life on Earth and beyond?
Early Earth (Artist Concept) (2014-11-18) by NASA Conceptual Image LabNASA
Astrobiologists have uncovered myriad clues to answering these big questions. Much of the research focuses on what makes a world habitable, that is, able to support life or its origin.
This term, habitability, has become a major buzzword in astrobiology as researchers have learned more about extraterrestrial environments in our solar system and beyond and deepened their understanding of how and when the early Earth became habitable.
Planet Earth From Space (2015-07-06) by NOAA/NASANASA
Why is Earth habitable? How, when, and why did it become habitable? Are, or were, any other bodies in our solar system habitable? Might planets orbiting other stars be habitable? What sorts of stars are most likely to have habitable planets?
These are just a few of the questions that astrobiologists are trying to answer today.
Many of the thousands of exoplanets (and candidate planets) found so far are giant worlds made mostly of gas. (Think alien analogues to Jupiter and Neptune in our solar system.) While it's unlikely these planets could host water and the right conditions for life, some of them could harbor ocean-bearing moons.
The Habitable ZoneNASA
We've also found some rocky worlds in Earth's size-range, and scientists expect many more will be discovered in the coming decades.
Much of the search is concentrated on seeking planets that orbit in a star's habitable zone--the right distance from a star for water to be liquid on its surface.
Seeing Starlight Through a Planet's Rim (2014-09-24) by NASA/JPL-CaltechNASA
But in seeking signs of life on these distant alien worlds, what do we look for? We're unlikely to know the precise nature of any life we might detect, be it crusts of algae or more complex organisms, but we could catch tantalizing glimpses of the right conditions for life by studying the planets' atmospheres.
With advances in observing technology expected in years to come, it's likely we will begin to study the chemical makeup of exoplanet atmospheres in great detail. (In fact, early work in this area is already underway.)
Spectra Reveal Gases in a Planet's AtmosphereNASA
It's a pretty neat trick. A star's light can be spread out into its component colors to create a spectrum -- much like how a prism creates a rainbow from sunlight.
It turns out that, if a star's light passed through the atmosphere of a planet orbiting the star on its way to the telescope, that light will carry information about the gases and other chemicals present in the planet's atmosphere. These gases absorb slices of a star's light in specific places, leaving behind narrow black gaps in the spectrum.
The effect looks something like a bar code.
A Small Habitable-Zone World (Artist Concept) (2013-04-18) by NASA/Ames/JPL-CaltechNASA
One pattern of black gaps might indicate methane. Another, oxygen. Seeing those together could be a strong indicator of possible life on an exoplanet. We might even detect the burning of hydrocarbons by more advanced alien life. Even with no detectable radio transmissions, the aliens' presence could be revealed by their effects on their planet's atmosphere.
The ultimate goal of NASA's exoplanet research is to find unmistakable signs of current life. How soon that can happen depends on two unknowns: the prevalence of life in the galaxy and how lucky we get as we take our first exploratory steps.
Our early planet finding missions, such as the Kepler space telescope and Transiting Exoplanet Survey Satellite (TESS), could yield basic evidence of potentially habitable worlds.
If an Earth-sized planet is discovered the habitable zone of a star not too far from Earth, follow-up investigations could look for signatures of oxygen, water vapor or some other powerful indication of possible life.
How to Find a Living Planet (Video) (2017-11-15) by NASA Goddard Space Flight CenterNASA
Unless we get lucky by finding it early, the search for signs of life on distant planets could still take decades. Discovering another blue-white marble hidden in the canvas of stars, like a sand grain on the beach, is an enormously ambitious task.
But the results are certain to be worth the wait.
For more information about NASA's Exoplanet Program, visit https://exoplanets.nasa.gov.