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by Liam Kelley
figures by Alexandra Was

Are humans as special as we think we are? We certainly like to believe so, at least on this planet. However, the question remains as to whether humans, or our planet, are unique in the universe. Are there other planets teeming with life, and is that life intelligent? Given the vast distances involved and the question of what forms alien life could take, finding the answers to these questions is an incredibly daunting challenge.

On February 22, NASA announced the discovery of a solar system containing seven rocky planets, three of which may contain liquid water. This system was named TRAPPIST-1, after the telescope that discovered it (which itself was named after a type of beer brewed by monks). TRAPPIST-1 is located 40 lightyears away from our solar system, making it one of the closest solar systems to our own. However, it is so far away that radical advances in technology would be required to even send a man-made probe to the system. If we are not alone in the universe, space is large enough that exploring alien worlds filled with life will likely never be achievable.

It can be far too easy to look at the colorful, fantastical artistic renderings released when we discover new planets and feel that we are close to finding strange new lifeforms in the galaxy. But how close are we to finding alien life? It can be useful to take a step back and look at what can be feasibly investigated by man-made probes or directly by humans. So, let’s examine where current efforts are looking for life, and what specifically they are looking for.

Recent Cassini images of Saturn's moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. This image was acquired by NASA's Cassini spacecraft on Nov. 27, 2005.
Recent Cassini images of Saturn’s moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. This image was acquired by NASA’s Cassini spacecraft on Nov. 27, 2005.

Finding the Pieces of the Puzzle

Looking for life on other planets is difficult because we’re not certain what to look for. All known life on Earth is water-based, composed of molecules predominantly containing carbon, hydrogen, oxygen, and nitrogen. It is reasonable to assume that conditions similar to those found on Earth would likely lead to life developing with similar characteristics. However, even with the same basic ingredients, our world could have turned our quite differently. For example, scientists have generated two new DNA bases that could have evolved alongside the four found in Earth’s organisms. The possible forms of life that could develop from these same starting blocks are hard to fathom.

To simplify the search for extraterrestrial life, the main compound of interest has been liquid water, given how vital is was for the development of life on Earth. In our own solar system, both planets and their moons have been investigated using spacecraft that either study from afar or land directly on the surface. Outside the solar system, the Kepler telescope looks for evidence of other planets by searching for dimming stars as planets pass in front of them. Using this technique, Kepler has identified over 2,400 exoplanets (planets around other stars). The atmosphere of these exoplanets is inferred from spectroscopy, where compounds in the atmosphere block specific wavelengths of light as the planet passes in front of its star. Several of these identified planets are about the size of Earth, and reside in the zone around their star where liquid water could be present based on their stars’ heat output.

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Looking in the Neighborhood

Due to its proximity, the planet that has been most closely researched for signs of life is Mars, which has been studied using orbiting satellites and rovers, and probably humans themselves in the near future. Though Venus is closer on average to Earth than Mars is, Venus’s 800 degree Fahrenheit acid-laden atmosphere makes it far less likely to sustain life, making Mars our closest plausible planetary neighbor with signs of life. Researchers look for geological features on Mars suggesting the presence of liquid water in its past or present, such as dried-up river basins or underground water, and complex organic molecules that can only be formed by life. So far, small streams of salty water have been observed running down Martian slopes, but the vast majority of water on Mars appears to be in the form of ice.

Three other sites in the solar system considered candidates for life are Europa (a moon of Jupiter), and two moons of Saturn: Enceladus and Titan. There is widespread scientific consensus of an ocean of salt water on Europa, hidden by a thick layer of ice. Even though the surface of Europa is incredibly cold, it is predicted that Jupiter’s gravity continually warps the moon, heating up its core and allowing for liquid water under the surface. Water has been observed venting into space via jets from the surface, suggesting the moon is geologically active.

Similarly, the Saturn moon Enceladus was recently discovered to release hydrogen from its subsurface ocean, shooting out of geysers and providing evidence of a potential energy source for alien microbes. Enceladus has not been as well studied as Europa, but offers similar potential for life in its frigid subsurface ocean.

Titan, the second largest moon in our solar system, seems like a strange candidate for life to develop. Its surface is marked by rivers and lakes of liquid methane and ethane, which are gaseous on Earth but liquefy at the frigid temperatures of -290 degrees Fahrenheit on the surface. If life were to develop on Titan, it would be drastically different from any water-based life we could envision. However, because it has the clearest evidence of liquid on its surface, it remains a potential site of extraterrestrial life.

Artist's Concept of Mars Reconnaissance Orbiter
Artist’s Concept of Mars Reconnaissance Orbiter

Current and Future Missions

There are several current missions looking for evidence of potential life, both in our solar system and in others. Mars is currently being studied both on the surface and from above. The Curiosity Rover has been studying the surface since 2012, while the Mars Reconnaissance Orbiter studies the planet from orbit. Another rover, planned as a successor to Curiosity, will land in 2020. Other places in the solar system being studied by probes include the dwarf planets Pluto and Ceres, along with Saturn’s moons. Looking beyond our solar system, the Kepler telescope will continue its search for exoplanets, and will be joined on its mission by a new telescope, TESS, which will launch later this year.

Some of the search for life will also be done by humans. A current NASA proposal intends to land humans on Mars in the 2030s. With current technologies, manned missions further than Mars would not be feasible, as it would simply take too long to reach the farther planets. As a result, humans will continue to rely on robotic probes and rovers to search for other life in our solar system.

While we still don’t know how life might manifest on a different world, the search continues. There are several current and future missions that will shed light on this subject for years to come, while the search for planets outside our solar system continues to ignite the imagination. It is human nature to explore new frontiers, and finding life on other planets may just be the greatest frontier of all.

Liam Kelley is a first-year graduate student in the Biological and Biomedical Sciences program at Harvard University.

For more information:

Exoplanets 101 (NASA)

Journey to Mars Proposal (NASA)

NASA Image and Video Library

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