The hunt for a second Earth: What conditions are required for an exoplanet to support life?
In 1950, Italian physicist Enrico Fermi posed an interesting question to his colleagues: If sophisticated alien societies existed somewhere in the galaxy, where were they?
His remark, known now as the Fermi Paradox, has been the driving force behind decades of research within the fields of astrophysics and astrobiology, and scientists are closer than ever to finding the long-sought-after answer to his inquiry.
According to NASA, the first exoplanet, a term for a planet orbiting a star other than our sun, was confirmed in 1995 by NASA’s Kepler Space Telescope. Since then, somewhere in the range of 4,000 other exoplanets have also been discovered orbiting stars in our galaxy.
This artist's rendering made available by NASA on Thursday, July 23, 2015 shows a comparison between the Earth, left, and the planet Kepler-452b. It is the first near-Earth-size planet orbiting in the habitable zone of a sun-like star, found using data from NASA's Kepler mission. (NASA/Ames/JPL-Caltech/T. Pyle via AP)
In fact, scientists estimate that these exoplanets are so numerous, that it is likely that every star in our galaxy has at least one planet orbiting around it. But how many of these planets are capable of supporting life?
“The real answer is we don’t know,” said Eric B. Ford, a professor at The Pennsylvania State University’s Eberly College of Science and a researcher at the Center for Exoplanets and Habitable Worlds.
“If instead of asking how would you find any conceivable type of life, which is a very hard question, you instead ask what would be required for Earth-like life, then you can look at the life on Earth and make observations,” he said. “All known life on Earth uses liquid water at some phase in its life, so it seems likely that Earth-like life requires liquid water.”
Planets that are the right distance from their sun to support liquid water are known by scientists as habitable zone planets, but this name can be rather misleading in some circumstances.
Ford stressed that just because a planet may be in this so-called habitable zone, it doesn't necessarily mean that the planet harbors life or even that it could harbor life in its current state. It just means that its relative distance to its star is similar to that of Earth.
In our solar system, both Mars and Venus can be considered part of the habitable zone, but both planets lack the atmospheric conditions to maintain a supply of liquid water on their surface. Without water, the chemicals needed for the function and growth of even the most simplistic of cells are unable to be dissolved or transported, and no life can emerge.
According to the Lunar and Planetary Institute, the presence of water, and thus the habitability of a planet, relies on a few factors beyond just the planet’s distance from its sun.
The first of those factors is the availability of nutrients. Biotic organisms require certain chemicals to sustain life, and without an abundance of those chemicals and a means of circulating them around the planet, complex life will never be able to emerge.
On Earth, these chemicals are things like proteins and carbohydrates, circulated by systems like volcanic activity and the water cycle. In order for Earth-like life to be present on an alien world, comparable alternatives to these chemicals and processes must somehow take form.
Snow-covered Mount Etna, Europe's most active volcano, spews lava during an eruption in the early hours of Tuesday, April 11, 2017. (AP Photo/Salvatore Allegra)
A planet’s atmosphere is also an incredibly important determining factor in assessing that planet’s ability to support life.
“We typically look for planets that are able to have a rocky surface, or an icy surface, as opposed to a liquid hydrogen surface, and have an atmosphere that’s thick enough so that there’s some atmosphere, but also thin enough so that things would not be crushed by the pressure,” Ford said. “And also, the atmosphere can’t cause a runaway greenhouse effect, like on Venus.”
The proper composition and consistency of a planet’s atmosphere is vital in keeping the surface the right temperature, protecting against harmful radiation and shielding the planet’s surface from small- to- medium-sized meteorites.
In order to undergo chemical reactions and perform biological functions, lifeforms require a source of energy to power themselves.
Without abundant amounts of sunlight and energy-providing chemicals like iron and sulfur, a planet cannot be habitable. This can be a limiting factor for planets that are tidally locked or have one side that is always facing their sun.
In the case of tidally-locked planets, the illuminated side can receive far too much sunlight, rendering the temperature too high to support life, whereas the dark side can receive far too little, rendering it much too cold. Some scientists believe, however, that a small sliver on the border of the two sides, called the “terminator zone”, may be inhabitable in some cases.
With thousands of exoplanets already discovered and millions more waiting to be uncovered, the chances that a planet fitting these qualifications exists seem pretty high, but researchers like Ford warn against impatience.
“It’s not like you can just pull an Earth-like planet out of a hat; it’s a long, gradual process,” he said. “One thing I like to tell people is that when we first discover an Earth-like planet, we won’t know it, and by the time we know it’s Earth-like, you’ll be so bored of hearing about Earth-like planets that you won’t even notice.”
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