NASA recently reported that astronomers using the Kepler spacecraft and the Spitzer Space Telescope have "mapped" the clouds on a nearby exoplanet. This research takes the study of exoplanets to a new level, with detailed characterization rather than just measurements of mass and size. Kepler, launched in 2009, has brilliantly demonstrated that the Milky Way is teeming with extra-solar worlds.
The holy grail of astrobiology, currently, lies in identifying Earth-sized worlds within the habitable zone of their star and with the possibility of an atmosphere that might support life. In May, Kepler lost the second of its four reaction wheels, three of which are necessary for precision pointing. Having fulfilled its original 3-and-a-half year mission in late 2012, Kepler was able to detect planets and predict statistically their ubiquity in our galaxy. But, it takes the sensitive instruments of an infrared telescope like Spitzer or the upcoming James Webb Space Telescope (JWST) to better characterize the composition of their atmospheres.
At least 100 to 400 billion exoplanets orbit the stars of the Milky Way and range in size from massive hot-Jupiters to Kepler-62f, perhaps the exoplanet as yet most similar to Earth. Reported in April 2013, Kepler-62f is possibly a rocky world, about 1.4 times Earth in size, capable of harboring oceans. The planet orbits its star every 267 days in the habitable zone. As of this writing, JPL's PlanetQuest website lists 906 confirmed exoplanets, with another 3,588 candidate planets identified.
Many factors determine a planet's habitability, including the type of star an exoplanet orbits. Stars come in various sizes ranging from brown dwarfs, no bigger than Jupiter, to colossal stars topping out at about 200 solar masses that are 10 million times brighter than our Sun. About half of all stars are binaries, in which two stars dance around each other. Some binaries host circumbinary planets with stable orbits around both stars, the first of which, reported in 2011, was Kepler 16b, also dubbed the Tatooine planet for its dual sunsets as depicted in George Lukas' Star Wars. In wide binaries, the orbits of habitable planets would be stable. Most of the habitable "real estate" in the galaxy is associated with M dwarfs, the most common stars in the Milky Way. With less than half the mass of our Sun, M dwarfs emit most of their light at infrared wavelengths.
Life on Earth is keenly adapted to living with our Sun. Human eyes, for instance, are sensitive to the visible band of the Sun's electromagnetic radiation, the most intense radiation emitted by our star. Eyes emerged as a critical innovation for life on Earth. The first sophisticated eyes appeared at the beginning of the Cambrian, about 530 million years ago, making it easier for animals to see and seek prey. Zoologist Andrew Parker refers to the Cambrian period as the "big bang" in evolution "in which the blueprints for the external parts of today's animals were mapped out. Animals with teeth and tentacles and claws and jaws suddenly appeared," as defense mechanisms against other predators with eyes. Parker argues that visible light acted as strong selection pressure for the development of eyes in most species. In what he calls the "Light Switch" theory, Parker theorizes that sunlight has been the primary factor in determining animal evolution and morphology.
Our lives, in fact, are intimately bound up with our Sun. We are well adapted to sunlight in a number of ways, one being that in about 15 minutes of exposure our skin absorbs the daily recommended amount of Vitamin D. Moreover, as the American astronomer Harlow Shapley explained, we are made of stardust. The iron in our blood, the calcium in our teeth, and the oxygen and nitrogen in the air we breathe were forged in the explosions of ancient massive suns. The lead in a pencil, potassium in bananas, nickel in coins, and mercury in a thermometer were produced in spectacular supernovae and then surfed the blast waves into interstellar space. Then, more than 4.5 billion years ago, vast clouds of dust and gas laced with these heavy elements coalesced into our star and its attendant solar system.
Awash as it is in solar systems and exoplanets, the Milky Way galaxy may be teeming with life. As on Earth, life on those worlds would be well adapted to the light emitted by their parent star. Habitable planets are surely out there. Astronomers already have found more than a hundred that are Earth-like. They eagerly anticipate the James Webb Space Telescope (JWST), slated for launch in 2018, to scan exoplanet atmospheres for biomarkers such as ozone and oxygen, volatiles that here are only sustained by Earth's biota. The discovery of extrasolar worlds continues and what we learn has the power to transform our species in as yet unknown ways.