Within our solar system quite a bit of exploration has been done by spacecraft including orbiters and landers. Planets outside our solar system will be difficult or impossible to explore with spacecraft in our lifetimes, so astronomers and astrobiologists use other techniques to search for signs of life on these planets. In particular, astronomers are interested in planets within the habitable zone of stars. Some exoplanets transit their stars as seen from Earth, and astronomers can attempt to detect and analyze atmospheres of these planets. During a transit, some of the light from the star will travel through the planet’s atmosphere. Astronomers can use spectroscopy to compare the light from the star during a transit to the light when no planet is present and detect the components of the atmosphere.
Some atmospheric components would strongly hint at the presence of biological sources on the surface of a planet. Ozone in our atmosphere, for example, was formed when large amounts of oxygen were added to the atmosphere with the development of photosynthesis. Astrobiologists hypothesize that if levels of ozone comparable to those on Earth were found in another planet’s atmosphere, it would be a strong indication of the presence of organisms on the planet.
CoRoT and Kepler are two space telescopes whose goals are to search planets around sun-like stars using the transit method. They are observing small sections of sky, studying over 100,000 stars, and attempting to determine the frequency of Earth like planets around sun-like stars.
SETI stands for Search for ExtraTerrestrial Intelligence. This differs from the traditional astrobiological approaches to studying and searching for life in the Universe that involve searching for habitable environments and searching for signs of present or past organisms. SETI is the search for communication from intelligent life forms in the Universe.
The SETI Institute and other groups typically study radio frequencies and analyze them, looking for non-random signals that could be communication from other civilizations in our galaxy.
Enrico Fermi posed the idea, now known as the Fermi Paradox, that if a civilization with the technology could travel through space at a fraction of the speed of light and begin colonizing other planets, then every 500 years or so each new colony set up two new colonies of its own, the whole galaxy should be populated with more colonies than there are stars in the Milky Way within about 20 million years. The fact that we do not see this, he said, implies that we are alone in the Universe.
Frank Drake was much more optimistic and developed what is now called the Drake Equation. He said that the number of extraterrestrial civilizations in our galaxy we can communicate with is given by the equation:
N = R*fpneflfifcL
R* = the rate at which sun-like stars are formed in our galaxy per year
fp = the fraction of those stars that have planets
ne = the number of those planets that could support life
fl = the fraction of those planets that actually do develop life
fi = the fraction of planets with life that develop intelligent life
fc = the fraction of intelligent life forms that are willing and able to communicate
L = the average lifetime of a communicating civilization
The CoRoT and Kepler’s missions will hopefully get us closer to a reasonable prediction for R*. All of the other variables in this equation remain guesses however. Frank Drake uses numbers that give a value of about 10,000 for N, but at the moment all we know for sure is that N≥1.