Interplanetary exploration, often referred to as the “final frontier,” is a subject science has only begun to scratch the surface of.

For some, discovering and studying planets—which are so unfathomably far away no human mind can even comprehend the distance—may seem like a waste of time. There is hardly a soul who can argue that upon hearing that some of these planets have atmospheric and environmental conditions capable of sustaining intelligent life that he or she does not get just a little bit excited by the prospect.

Utilizing advanced technology such as deep space telescopes and light-analyzing computer programs, astronomers and physicists have been able to gather data on planets lightyears away. One major topic has scientists and star-gazers alike buzzing with excitement: the possibility of other planets similar to Earth.

“Similar” may seem like too broad of a word, but in fact there is a scale used to determine the likeness of distant worlds to our own, and it is called the ESI, or Earth Similarity Index.

The University of Puerto Rico at Arecibo Planetary Habitability Laboratory “The Habitable Exoplanets Catalog,” states that “the ESI is a measure of Earth-likeness for exoplanets as a number between zero (no similarity) and one (identical to Earth).”

There have already been a number of planets identified as being candidates for having atmospheric, climate and surface quality which could potentially support life. For these conditions to be met, a planet must lie within a certain distance of its star, and this distance varies based on the size and type of star.

According to Planetquest author Pat Brennan, “This is the band of congenial temperatures for planetary orbits — not too close and not too far. Too close and the planet is fried…too far and it’s in deep freeze. But settle comfortably into the habitable zone and your planet could have liquid water on its surface. The zone can be a wide band or a narrow one, and nearer the star or farther, depending on the star’s size and energy output.”

When it comes to supporting human life on another planet, Dr. Keith Conover, division chair of the biology department at GCC, believes that oxygen, water and photosynthesis are some of the vital, overarching concerns.
“The temperature probably couldn’t be over 50 degrees celsius,” he said. “People would need a source of liquid water as well as oxygen.”

Even some of these conditions have conditions. “If you have too much oxygen it becomes toxic, and too little then you can’t breathe,” he added.Sustainable habitation is another issue, because the difference between a visit to a place and living there is monumental. Aside from liquid water and oxygen, people would need a food source and that only comes from the previous two conditions having been met.

“To support plant life, a planet would need carbon dioxide. Almost all oxygen is produced through photosynthesis, a process which needs light for energy,” Conover said. “There was an oxygen concentration of almost zero on Earth before plant life developed and photosynthesis began.”

Some popular, potentially habitable candidates include Kepler-22b, Kepler-452b, Gliese 667 Cc, and many more. Based on current data, these planets likely have rocky surfaces with climates similar to Earth. Numerous questions remain, however, such as the presence of water on these planets or the chemical makeup of their atmospheres.

One optimistic view comes from Mike Wall, senior writer for Space.com, who said, “Recent discoveries suggest that the solar system and broader Milky Way galaxy teem with environments that could support life as we know it… Further, NASA’s Curiosity rover has found carbon-containing organic molecules and “fixed” nitrogen — basic ingredients necessary for Earth-like life — on the Martian surface.”

As astrobiologists continue their search for habitable exoplanets, the data currently available seems almost as vast and unexplored as space itself.
Elizabeth Howell, science journalist and communications professor, in the Space.com article “Exoplanets: Worlds Beyond Our Solar System” talks about this.

“It has helped define a whole new class known as “super-Earths”: planets that are between the size of Earth and Neptune… but astrobiologists are going back to the drawing board to consider how life might develop on such worlds,” Howell said.