The search for life beyond Earth is one that has fueled science and the imagination. Planet-hunting spacecraft, such as the Kepler Space Telescope and the Spitzer Space Telescope, have been able to identify thousands of new planets beyond our solar system, but only a few are within the habitable zone of the star they orbit. Finding alien life is not an easy task, but it is one researchers are well-equipped to pursue.

The Search for Earth 2.0

An incredible discovery using the Spitzer Space Telescope made headlines and sparked the imagination of people around the world. A small dwarf star approximately 40 light years from Earth is host to not one, but seven planets. Even more fascinating in the search for potential alien life in the universe, all planets in the TRAPPIST-1 system could potentially support liquid water.

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To date, the Kepler Mission, including the extended K2 mission, has identified 5,216 potential exoplanets, or planets beyond our solar system, with 2,514 confirmed new planets. The search for planets outside our solar system started relatively recently with the first confirmed exoplanet being announced in 1995, but astronomers have made great strides since then in identifying potentially habitable planets.

Of the thousands of confirmed exoplanets discovered by Kepler, only 21 exoplanets have the size and location to be considered potential candidates for Earth 2.0. Astronomers are trying to find habitable planets based on the ingredients needed for life on Earth. That means finding a rocky body located in the "Goldilocks zone" of its host star, where the temperature is just right for liquid water. Three of the TRAPPIST-1 exoplanets discovered by Spitzer are all located in the "Goldilocks zone." The upcoming James Webb Space Telescope will be another powerful tool in the search for life.

In August 2016, astronomers using the European Southern Observatory's telescope in Chile discovered an Earth-like planet orbiting Proxima Centauri, our nearest neighboring star after the Sun. Whereas Kepler's potentially habitable planets can be located hundreds of light-years from Earth, Proxima b is located approximately four light-years away. An actual spacecraft would take thousands of years to reach the exoplanet, but a miniature probe could take only 20 years to reach Proxima b.

How to Find Life

With space telescopes such as Kepler, astronomers use the transit method to identify potential planets. Astronomers are looking for small dips in brightness that are the result of a planet crossing in front of its host star. Another way to search is through the radial velocity method, which measures shifts in the observed light spectrum of a star caused by the gravitational tug of a planet.

Even though the two methods are indirect observations, astronomers can gain plenty of insight into these exoplanets. The planet's size affects the dimming of its host star. The bigger the planet, the more light that's blocked by the exoplanet as it passes in front of its host.

On Earth, oxygen and methane are strong indicators of life and the same applies for an exoplanet if we are looking for life similar to that found on our planet. The atmosphere of a planet acts like a prism, explains NASA. Chemicals and gases in the atmosphere absorb different wavelengths, which creates a pattern that astronomers can decipher. Based on the black bands in the light spectrum, we can determine if an exoplanet has oxygen, methane or even pollution.

Without this vital research, the search for life would stall before it could even start. "If we can understand the properties of exoplanetary atmospheres from a theoretical perspective we can more effectively and efficiently develop the technology for actual missions," says Dr. Trisha Hinners, an astrophysicist conducting basic research on exoplanet atmospheres at Northrop Grumman. "We perform interdisciplinary research in astrophysics, astrochemistry and astrobiology built on a foundation of computational physics and data science. It takes a terrifically gifted team here on Earth to look for simple signs of life out in the cosmos."

The search for life beyond Earth isn't limited to new planets. In fact, there may be life found much closer to home.

Moons of Possibility

Jupiter's moon Europa may contain a liquid water ocean underneath its thick ice shell. This ocean could have the right recipe for microbes or more complex life. Water vapor plumes shooting from Europa's surface were first detected in 2012 and again observed by the Hubble Space Telescope. The James Webb Space Telescope could help confirm these plumes and an upcoming NASA mission to the moon could sample the water vapor to determine its chemical composition.

Like Europa, Saturn's moon Enceladus is believed to have a subsurface ocean that could potentially harbor life. Enceladus's geysers have been observed by the Cassini spacecraft and could be sampled by a future mission. Astronomers could determine if there are any building blocks for life, such as amino acids, or if there's hydrothermal activity similar to that found on Earth.

Of course, alien life does not have to abide by Earth's rules. So far, we're trying to find life based on terrestrial life, but other planets could harbor organisms that previously have only existed in the realms of science fiction.