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Frank Drake: Habitable Zones

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Frank Drake gave a public lecture on Kepler and Its Impact on the Search for Extraterrestrial Intelligence at Kepler Science Conference II,  NASA Ames Research Center, Moffet Field, CA. He gave the lecture on November the 5, just a few days short of November 11. At 1700 GMT on that day in 1974, the giant Arecibo 1,000 foot radar/radio antenna emitted a signal that any Galactic civilization in its path could detect using a similar antenna. The effective radiated power of the signal was three million megawatts in the direction of transmission, greater than the total installed capacity of electric power stations on Earth. Its message describing life on Earth has now traveled 39 light years towards its intended recipient, the Great Cluster in Hercules, some 300,000 stars that sit within the transmitted beam width at a distance of 25,000 light years. The binary message consisted of1679 bits sent sequentially with one frequency representing "0" and another "1". Here black indicates "1" and white indicates "0". The sequential message has been reformatted into 73 consecutive groups of 23 characters, stacked one above the other. (Image Credit: National Science Foundation)

Professor Drake has been engaged with the Search for Extraterrestrial Intelligence (SETI) for the past 60 years. He was the first to attempt to detect extraterrestrial signals, with Project Ozma in 1960, developed the equation that goes under his name (for calculating the number of extraterrestrial civilizations), and helped compose the message described above that was transmitted at high power towards distant star systems. At the conference he talked about the habitable zone where liquid water could exist and life might evolve. He noted that defining the habitable zone is important in the search for life outside the solar system but that it is a very fuzzy concept.

Drake pointed out that whereas a nominal habitable zone would extend from 0.5 AU to 2.0 AU in the solar system, there was potential for life at 5 AU in the clouds of Jupiter. It could occur at the level where temperatures were benign in the clouds of water vapor, methane and ammonia clouds and might evolve as floaters from life forms analogous to the bacteria floating high in the Earth' atmosphere. Saturn, Uranus, and Neptune could also have microbial life at some level in their atmospheres, so that the habitable zone might extend much further than was previously thought.

Satellites of planets could also be abodes of life; Jupiter's satellite Europa, completely covered with water ice 10 km thick, has a liquid water ocean 100 km deep underneath the ice. Hydrothermal vents could provide a region for life. Some terrestrial species, such as the comb jelly, would be happy on Europa. Enceladus, another moon of Jupiter, shows water geysers erupting into space. If these have a long life, there may be living microbes in a subsurface ocean. So whether a zone is habitable or not depends on the organism. Another Saturnian moon, Europa, has lakes of methane, ethane, and butane and is a perfect place for the origin of life, said Drake.

He also saw planets of M-stars as a potential region for life. These stars are very much smaller and cooler than the Sun, but there are many more of them. Their planets move in orbits close enough to the star for tidal lock to occur, like that of the Moon, which therefore turns the same hemisphere always towards the Earth. A planet tidally locked to its star would have one side heated by the star, while the other faced the coldness of interstellar space. However, calculations have shown that cloud cover could partially equalize the temperatures between the two hemispheres, making life possible over a broad region. [More . . .]

Drake pointed out that tidal lock would not occur if the planet's orbit had an eccentricity greater than Mercury,'s. Originally thought to be in tidal lock, that planet gradually rotates to present a changing face to the Sun. At least half of the M-dwarf planets that have been detected have an eccentricity greater than that of Mercury. The other concern for M-stars is that the closeness of the star would cause its surface to be blasted by radiation from stellar flares. To avoid this hazard, as well as cosmic rays and the stellar proton wind, life would probably evolve in the shelter of an ocean.

Drake suggested the ultimate limit to the habitable zone is displayed by rogue planets that have been ejected from the vicinity of their star by a gravitational interaction. There are thought to be billions of these wandering through the Galaxy. Analysis indicates that if ejected with an atmosphere, the planet could be kept warm enough for life for billions of years by radioactive decay in its interior. So here are planets with no star to be in the habitable zone of.

Drake argued that the habitable zone is defined by the type of animal, as well as by the type of star. He concluded that in the search for life, emphasis should be given to M-stars: search where there are most planets (in the Galactic plane) and don't ignore empty space. The recommendation holds both for visual astronomy and for SETI. As regards the latter, Drake pointed to the very large increase in the efficiency of wireless, television, and radar systems on Earth, with a corresponding decrease in the emissions they inadvertently send out to the Galaxy. At the same time, there has been a massive increase in the light emitted by cities at night. These trends might influence the way SETI searches (and those of other planetary civilizations) are carried out.

While advanced civilizations might detect the radiation leaking continually from civilization on Earth, Drake thought it inconceivable that they would find any value in an expedition to Earth that would match the gigantic cost. Instead, he envisaged the development of channels of electromagnetic communication between civilizations, setting a challenge for Earth to join this Galactic internet.

An unacknowledged but nevertheless detectable assumption at the conference was that the type of habitable zone preferred was one where a planetary civilization would be able to evolve.