NASA’s Kepler space telescope is finding lots and lots of extrasolar planets. But how many might support intelligent life? And, is there a “sweet spot” in the galaxy where SETI astronomers should aim their telescopes?
I’d say we have already stumbled across that sweet spot three decades ago, but more on that later.
First, we need to understand what are the prerequisites for the evolution of life beyond microbes. Because we only have one example — Earth — extrapolations are dicey. For example, if completely water-covered worlds are more common than estimated, chances for advanced surface life go down. What’s more, only surface dwellers could develop technological civilizations.
Fundamentally, we do know that heavier elements, such as carbon, oxygen, and nitrogen, are needed for building planets and the complex molecules for life. And, we need a star — as well as a planet — that is stable over billions of years to allow for evolution.
Factoring this in, along with a host of other variables, Michael Gowanlock of the University of Hawaii and co-investigators ran an extensive computer model of our galaxy to look for the legendary habitable zone where intelligent life has time enough to evolve without being destroyed by cosmic catastrophes. They made a three dimensional model that estimated habitability as a function of distance from the galaxy’s center, as well as stars above and below the galaxy’s pancake-shaped disk.
Previous models have touted the idea of a narrow galactic habitable zone. This has been and embraced by Rare Earth hypothesis supporters. In particular, the idea that the habitable zone is a 6,000 light-year diameter ring roughly halfway between the center and rim of our galaxy. It is hypothesized to be a circular lane, much like the habitable zone around a star where planetary oceans could exist.
Any closer to the hub or our galaxy and the radiation environment from supernovae sterilizes the planets by destroying their ozone layer and irradiating the surface. Stars farther out are less impacted by supernovae, but there is a deficiency of heavier elements for making planets.
Previous observations have shown that the more heavy elements a star has, the more likely it is to possess a planetary system. 25 percent of the heavy element-rich stars harbor planets, while only 3 percent of stars that are almost entirely hydrogen and helium do.
But contrary to earlier studies the new galactic model predicts that the inner hub of galaxy should be hospitable and be loaded with planets. The heavier elements are more concentrated here, as has been found in studies of the core of the neighboring Andromeda galaxy as well.
However there are more supernovae in the center of our galaxy, given the abundance of massive stars, the vigorous star formation rates, and the closer proximity of stars.
But the expected preponderance of planets in the core outweighs the supernova risk say the researchers. What’s more, a planet could be irradiated several times but evolve life at a later stage. For example the heavy asteroid bombardment of Earth 4 billion years ago may have wiped out emerging life, but it managed to resurface again at 3.7 billion years.
The team calculates that across the galaxy, 1.2 percent of all stars have planets that could support advanced life. That’s over 1 billion worlds. According to this study the hub of the galaxy contains the most planets suitable for life. The team calculates 2.7 percent of the planets in the core are habitable, while only .25 percent in the outer rim are habitable. Therefore, the core is a likely place to look for advanced civilizations.
In the 1985 Carl Sagan novel Contact, a wormhole interstellar transportation system built by an ancient extraterrestrial civilization carries Earthlings to the core of the galaxy, where they encounter aliens who make use of this legacy transit system.
Science fiction aside, the new galaxy habitability model bolsters my belief that the legendary “WOW signal,” a mysterious minute long burst of radio waves detected in 1977, was a form of an interstellar Twitter. It came from the direction of the heart of our galaxy in Sagittarius. I’ve always held that the location is more than coincidental in this cosmic ghost tale.
Sagittarius could be the most promising sweet spot for SETI surveys — though the WOW signal has never returned and therefore remains inconsequential. Nevertheless, it has been speculated that civilization might chose to send a transmission in the exactly opposite direction from the galactic core. This would be a logical sightline for anyone inhabiting the galaxy to monitor for a beam from E.T.
Image credits: ESO, NASA