Scientists have narrowed down the parameters of Goldilocks -- or habitable -- zones around distant stars. They've discovered Super-Earths and what might be waterworlds. But a true Earth-like planet -- one that is similar in size and orbits its parent star at just the right distance to sustain life as humans understand it -- has yet to be found. NASA scientists believe this will change by the end of 2014.
"I believe Kepler will find a 'Goldilocks planet' within the next two years," Shawn Domagal-Goldman, an astrobiology specialist and researcher at NASA headquarters in Washington, D. C., said in a statement to Space.com. "We'll be able to point at a specific star in the night sky and say 'There it is — a planet that could support life!'"
Such optimism could prove well-founded.
Earth-sized planets have been difficult to detect due to the their relative size in comparison to their host star. The luminosity of the star all but completely overshadows the dim light reflected from a much smaller object -- say, like an Earth-like world -- that might pass in front of it. Still, with detection methods becoming more accurate, astronomers feel that finding a planet like Earth is inevitable.
Besides, astronomers believe that an indirect method of detection known as transit spectroscopy will eventually produce an extra-solar world similar to Earth.
Transit spectroscopy is a technique whereby a planet's atmosphere's composition can be discerned via its reflected light.
Two planned NASA missions also hold the promise of discovering the first "alien" Earth. Finesse, or Fast INfrared Exoplanet Spectroscopy Survey Explorer, will employ the transit spectroscopy method. It is designed to measure the spectra of a star during two situational periods: while the suspected planet is in front of its parent star and when it is behind it. Differences in the spectral data would indicate the composition of a transiting planet's atmosphere, indicating whether or not the extra-solar world would harbor water and other life-sustaining elements.
The second mission, TESS, or Transiting Exoplanet Survey Satellite, will be charged with observing stars within 50 light years of Earth, its goal to actually find alien worlds.
At present, the Kepler Space Telescope has found at least 2,300 additional planet candidates to add to those already confirmed. In an announcement in January, NASA noted that the Kepler mission had increased the number of extra-solar candidate worlds by five times as many as had been previously found.
Kepler program scientist Doug Hudgins at NASA's Headquarters in Washington said, "Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits."
Nearly a year ago, scientists observing Gliese 581, a star where several planets had been discovered in its train (six to date), announced that Gliese 581d just might orbit the star at just the right distance to harbor liquid water, making it potentially the first water world and, therefore, habitable by human standards. However, Gliese 581d, which is the fourth planet circling the red dwarf star, is dissimilar to Earth in that its gravity is twice that of the Earth. It is also seven times bigger. Still, scientists believe that it could be much like the Earth, exhibiting features like oceans, cloud cover, and rainfall.
(photo credit: Ignacio González Tapia, Wikimedia Commons)