Another Earth?

Introduction:

Exoplanets are planets with host stars that are not our sun, in other words any planet outside of our solar system. While exoplanets, or extrasolar planets, were undetectable to us until recently (the first was detected in 1992), today almost 1,000 have been recorded (1). It is also estimated that generally all stars have at least one planet in orbit (2). For this reason, exoplanets play a large roll in the search for extraterrestrial life, though candidates are severely narrowed by stellar diversity.

There are many ways of detecting exoplanets, all of which fall into two categories: direct imaging, and indirect. While direct imaging can tell us more about a planet’s characteristics, because of light pollution from host stars, indirect is far more common. Aside from capturing visible light / infrared spectra, there are a number of indirect detection techniques. These include: Astrometry, Doppler shift, Transit/Eclipse, Gravitational microlensing, and Pulsar timing. Astrometry is a method in which one precisely measures the location of the star in an attempt to observe the wobble that results from orbiting the center of mass. A Doppler shift is when the stars orbital disk is in a similar plane to Earth and therefor during the times when the star is traveling toward or away from our observation the wavelength detected is either elongated or compressed. These alterations are referred to as red shift and blue shift depending on which direction. The Doppler effect is a common phenomenon and therefor a versatile detection method. Transit and Eclipse methods both take advantage of either the star or planet blocking the other from our observation. Each method measures the total light (visible or infrared) and how it changes over time. In Transit detection, when the orbiting planet comes in front of its star, it blocks some percentage of emitted light therefor our graphs demonstrate a regularly shaped dip in light detection. In Eclipse detection, there is a drop in infrared light when the planet moves from the side of the star to behind it since the planet emits infrared light. Gravitational microlensing detects the light of an object hidden from the observer’s field of view by an extremely massive body, as that light is bent around the body. Finally, pulsar timing utilizes the regular and precise movement of neutron stars to detect an orbit from imprecision. The very first exoplanet was discovered by pulsar timing detection.

Kepler-22b was discovered transiting its host star in 2009, and then confirmed with a third transit detection in late 2010. The planet’s radius is approximately two and a half that of Earth, though its mass and composition are still unknown (3). It orbits a G5 spectral type star very similar to our own sun. Found 600 light-years away, the host star (Kepler-22) has a mass of about 0.97 solar masses, a radius of about 0.98 solar radii, and a luminosity of about 0.8 times the luminosity of our sun (4). Kepler-22b was an important discovery for the Kepler mission as it was the first exoplanet found orbiting a star similar to our sun within the specific habitable zone.

Habitability:

Though the potential habitability of Kepler-22b is still largely unknown, from the information we have gathered so far, it seems to be a promising candidate for sustaining life. The planet orbits Kepler-22 at an average distance of 0.85 AU (1), which correlates with a similar position within it’s habitable zone as Earth. Earth is located at 1 AU and our sun’s habitable zone is between 0.95 and 1.4 AU. The habitable zone of Kepler-22 is located between 0.82 and 1.21 AU as calculated using the following equations: D_inner = 0.95 AU √(L/L_sun) and D_outter = 1.4 AU √(L/L_sun)

(http://kepler.nasa.gov/images/Kepler22bDiagram.jpg)

The bond albedo and atmospheric conditions of Kepler-22b are unknown; therefor the actual temperature is incalculable using the following equation: T_equilibrium = 255K   D^-1/2

However, models have estimated the planet’s equilibrium temperature under 3 different atmospheric conditions. The first of which is an atmospheric greenhouse gas effect similar to Earth’s, which results in an average temperature of 295°K. Should the planet have experienced a runaway greenhouse gas effect similar to that of Venus, the planet should have an average temperature of about 733°K. Lastly, if the planet has been stripped of its atmosphere completely, it likely has an equilibrium temperature of 262°K. These values are very earth-like and appear to be potentially supportive of life. (5)

            The planet was detected and confirmed using the method of Transit. Since Transit detection measures the decrease in light picked up as the planet crosses in front of its host star, the dip in luminosity can be described as:

(πr_star² – πr_planet²)/(πr_star²) in terms of the percentage of total light still detectable when partially blocked by planet. For Kepler-22b, the light from Kepler-22 should decrease from 1.00 to 0.999337.

            Since Kepler-22 is a G5 spectral type star, it is slightly less massive and luminous than our very own G2 spectral type sun. Kepler-22 is young at only 1 billion years old, compared to our sun’s age of 4.57 billion years. Since it is a much younger star, it has more stellar life left than our sun, though it can be expected to follow the same fate as our sun once it is old enough. The habitable zone will continue to widen and distance itself from the star before fusion is complete, at which point it will expand and cool before evolving into a giant.

Science Fiction

            Kepler-22b is the closest you can find to home, away from home! If you aren’t already dying to take a trip with the family to Kepler-22b, you’re about to be! While 22b is home to tropical warm weather and white sand beaches, there are entirely new aspects to the world that will excite the whole family. First of all, the beaches are made of crystal sand instead of boring old earth minerals! Second of all, the beach water isn’t even water, it’s methane! What could be better than a decade long getaway to swim in the methane oceans of 22b with the kids? The only thing I can think of is CALLING NOW to get 15% off on your next seat booking. Survival not guaranteed.

Works Cited:

1. http://exoplanet.eu/catalog/
2. http://www.nature.com/nature/journal/v481/n7380/full/nature10684.html
3. http://www.nasa.gov/mission_pages/kepler/news/kepscicon-briefing.html#.UmUyFJTwLtg
4.  Kepler-22b, NASA Ames Research Center, retrieved 2011-12-06 (http://en.wikipedia.org/wiki/Kepler-22#cite_note-nasa_ames-2)
5. http://en.wikipedia.org/wiki/Kepler-22b#cite_note-bbc20111205-9