Exo-planets and the Search for Extraterrestrial Life
by Dale Bryant
Dedicated to the memory of Neil Armstrong, the first human being to walk on another world...
Though the figures are tough to keep up with as they are changing almost daily, the Kepler space telescope has discovered over 2,300 exo-planets in all. Those are just the few in existence whose orbital planes lie at a favorable incline from our perspective. Of those 2,300+, at least 207 are Earth-sized and at least 48 lie in the so-called 'Goldilocks' or habitable zone. The way Kepler achieves this miracle of detection of extra-solar planets is by measuring the varying light curves of their parent or host stars. As a planet transits or passes in front of a star, as seen from our neighborhood, there is a barely measurable but significant decrease in the star's light, on average about 2%, with another .03% decrease in the presence of a planetary atmosphere. This is roughly equivalent to the amount of light lost by an observer of a housefly passing in front of a car headlight as seen from several miles away. When a regular pattern of dimming and brightening can be determined by astronomers, they can deduce that the star has at least one planet in orbit around it. Stars vary in their brightnesses for other reasons, as was seen earlier, but this may the most exciting one.
In 1952, Otto Struve suggested that extra-solar planets might be detected by dips in a host star's light during a planet's transit. Even then, techniques were available to detect such a drop in light but it was forgotten about for decades. In 1999, two professional astronomers using a 10-centimeter telescope discovered the first tell-tale signs of such a transiting extra-solar planet. Amateur and professional astronomers have since detected countless candidates.
One other technique for detecting extra-solar planets is through the measurement of the radial velocity (RV) of a host star, producing changes in the Doppler shift of the stellar lines by the tugging of the planet on the star. By this technique, Right Ascension, Declination, transit depth and ephemerides can be determined by amateurs possessing modest CCD equipment.
NASA's Ames Research Center lists a table of 70-plus confirmed exo-planets discovered by Kepler as of May 2012 and designated by the name 'Kepler' followed by a letter. Planetary charcteristics in the table for each planet include the following headings: Jupiter Masses, Earth Masses, Jupiter Radii, Earth Radii, Density, Temperature, Transistion Duration, Period, Semi-Major Axis (UA), Eccentricity, Inclination (in degrees) and Distance (in parsecs). The table also lists characteristics of the host star. It should be noted here that Kepler 23b - Kepler 30b (15 planets) are planets that are within just a few Earth radii, though they are several hundred times more massive and their orbital periods seem much too short (just a few days) to be within the habitable zone. But it tells us that exo-planets, roughly the size of Earth, are indeed detectable and are out there.
If life is discovered elsewhere in the universe, is it likely it will be found on Earth-like extra-solar planets - planets that fall into that comfy, cozy distance from their host stars that we refer to as the Habitable Zone? We humans like to think so, and it may very well be, but life may also be found in much less likely environments. The diversity of life on Earth itself is staggering in that it can be found in the deepest ocean trenches in waters above 600 degrees Fahrenheit, and pushing well north and south of the Arctic and Antarctic Circles, respectively, on land, in temperatures well below freezing. It must be remembered that we are products of our environment in the struggle for life. Nature has tried out many kinds of organisms through the process of natural selection and most of those organisms were ill-suited to the task of survival. In fact, more species of living things have become extinct than have survived. But some of them were very well suited, indeed, and lived to write about it!
Life "as we know it", would require that it evolve on a planet with the exact same physical make-up as Earth. Life here on Earth is carbon-based, but we shouldn't necessarily exclude, say, even silicon-based life on other worlds. Our body chemistry is that of the Earth. But life will come in many forms. It may be possible to detect life on Earth-like exo-planets possessing an atmosphere by measuring gaseous emissions in its atmosphere by spectroscope, such as the oxygen given off by vegetation here on Earth. Other forms of life give off carbon dioxide and even methane into the atmosphere. Luckily, the universe operates the same everywhere else as it does locally, so we can know what signs to look for. The presence of such gases can be determined by measuring a planet's transmission spectrum during its transit across the face of a star. If a planetary atmosphere is not present, the light fall-off will be the same at all wavelengths. If certain elements are present in the planet's atmosphere they will absorb some of the star's light. In one case, sodium present in the atmosphere of a planet made the planet appear to be six percent larger than at other wavelengths. Another way that exo-biologists expect to be able to detect the presence of life is by spectropolarimetry, or, looking for biosignatures in the reflected polarized light of a host star by one of its planets.
One method of detecting life elsewhere in the universe not mentioned thus far is, of course, through the long-hoped for detection of artificial radio signals, which is what the SETI (Search for Extraterrestrial Intelligence) Institute hopes to do. As early as 1896, Nikola Tesla suggested that radio could be used to contact extraterrestrial life. The first modern SETI experiment was conducted by Cornell University astronomer Frank Drake and was called Project Ozma. Drake used a radio telescope 26-meters in diameter at Green Bank, West Virginia to examine the stars Tau Ceti and Epsilon Eridani near the 1420 gigahertz marker frequency, a frequency called "the water hole" due to its proximity to the hydrogen and hydroxyl radical spectral lines. With the exception of a couple of minor false alarms, no one has been heard from to date.