Hunting For Exoplanets
July 19, 2013

Inside NASA’s Search For Habitable Exoplanets

redOrbit Staff & Wire Reports - Your Universe Online

The hunt is on for planets that, like Earth, have the conditions necessary for life to survive on their surfaces. But what exactly are scientists looking for? Earlier this week, NASA officials shed a little light on precisely what conditions they are seeking on worlds located beyond our solar system.

These worlds, which are also known as exoplanets, have to meet a handful of different criteria in order to be considered potentially habitable, the US space agency explained. Ideally, these planets should be just like Earth, since it obviously is capable of supporting life, but at the very least they need to orbit their suns at just the right distance - a region astronomers refer to as the habitable zone.

Defining exactly what comprises an exoplanet's habitable zone is the topic of ongoing research. While it is defined by NASA as "the belt around a star where temperatures are ideal for liquid pool on a planet's surface," scientists are still investigating where a star's habitable zone begins and ends.

Too far away from the star it orbits, and a planet would probably be too cold and frozen for life to exist; too close to the star, and it would be too hot for that world to support living organisms. Conditions have to be just right, which has led to worlds existing in the habitable zone to be nicknamed "Goldilocks planets" after the well-known fairy tale.

Scientists are being assisted in their search for these Goldilocks planets by NASA's Kepler mission, as well as other telescopes. Thus far, officials at the agency report these instruments have confirmed "a handful" of candidates, each of which are slightly larger than Earth and thus have been dubbed "Super Earths." The search for Earth's so-called twin, a habitable zone planet of roughly equal size, continues.

In one new study, researchers working out of the NASA Exoplanet Science Institute at the California Institute of Technology (Caltech) closely studied the location of a planet identified as Kepler-69c and its habitable zone. Based on their analysis, they have determined that the Kepler-69c is 1.7-times the size of Earth and is located just outside the zone's inner age, making it closer to being a Super Venus than a Super Earth.

"On the way to finding Earths, Kepler is telling us a lot about the frequency of Venus-like planets in our galaxy," explained Stephen Kane, the lead author of the new study, which appears in the Astrophysical Journal Letters. "Understanding the properties of the star is critical to determining planetary properties and calculating the extent of the habitable zone in that system."

The process of determining the location of a star's habitable zone begins with determining how much total radiation it emits, according to NASA. Stars more massive than our sun are hotter, have more radiation, and have habitable zones that are farther out from ours. Conversely, those that are smaller and cooler have closer and smaller habitable zones than our solar system's, and Super Earths in these systems orbit closer to their stars than Earth.

"Knowing precisely how far away a habitable zone needs to be from a star also depends on chemistry," the agency explained. "For example, molecules in a planet's atmosphere will absorb a certain amount of energy from starlight and radiate the rest back out. How much of this energy is trapped can mean the difference between a turquoise sea and erupting volcanoes."

Flares, (eruptions from the surface of stars), can also impact whether or not organisms can thrive on a planet, according to Kepler science team member Lucianne Walkowicz of Princeton University, if a planet is constantly bombarded by flares, the radiation can make the conditions on the surface less than ideal for life. However, worlds with liquid water capable of providing a shield from the radiation could make it possible for living beings to thrive in the oceans, she added.

"Ideally, astronomers would like to know more about the atmosphere of potentially habitable planets," NASA said. "That way they could look at the planet's molecular makeup for signs of runaway greenhouse gases that could indicate an inhospitable Venus-like planet. Or, future space telescopes might even be able to pick up signatures of oxygen, water, carbon dioxide and methane - indicators that the planet might be somebody's home."