Back to the Moon: the Next Giant Leap for Humankind by Joseph Silk

Author:Joseph Silk
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2022-07-21T00:00:00+00:00

Exoplanets

Even if life tracers, such as microbial fossils, were found by our future missions to Mars or Europa, this does not really inform us definitively about the ubiquity of life. There is too great a risk of terrestrial pollution. After all, a few terrestrial meteorites have been identified as having a Martian origin.

We need to search farther afield than the solar system. Observations made primarily with the Kepler space telescope have shown that for every cool dwarf star in the Milky Way, there is a significant chance of finding at least one Earth-sized planet orbiting its so-called habitable zone. The habitable zone is the “Goldilocks” region that allows liquid water to exist on the planet’s rocky surface because the planet is neither too far from the star, where it’s too cold, nor too close, where it’s too hot.2

Adding possible options as we extrapolate the Kepler statistics, we arrive at an expectation of more than a billion such Earth-like planets in our Milky Way galaxy. And adding M-dwarfs as likely exoplanet hosts sends this number through the roof. The Kepler telescope search statistics alone uncover thousands of promising exoplanets that are the closest and the easiest to study. Perhaps something interesting is lurking in this horde—unless, that is, the probability is exceedingly low that a habitable-zone planet would develop a technologically capable species of life. In this case we might need to search a million candidates or more. Even then, we will find no advanced biotracers if life is truly rare.

How rare would life have to be if we are truly alone? We can estimate that life would need to break out less than once on a billion trillion exoplanets. This is just the expected number of terrestrial-like planets in habitable zones in the observable universe. These odds are very low indeed, but perfectly compatible with our ignorance about the origin of life.

But perhaps this line of thinking is too pessimistic. Unless we are very unlucky, it is not unreasonable to conclude that Earth’s humanity should not be the only technological civilization to have ever existed in the Universe. Certainly there has been lots of time. But we must also acknowledge that such advanced entities may be not only really rare, but simply very far away, perhaps beyond our current reach.

Having a chance to answer at least some of these questions is the prime reason why constructing bigger and better telescopes could bring immense rewards. We simply need to look farther afield. Key to our future efforts to extend our reach will be the Moon.

Looking nearby is certainly a useful pilot strategy. To more reliably assess the odds of life existing elsewhere, we desperately need more examples of Earth-like exoplanets. Let’s start with the closest candidate. At a distance of 4.2 light-years from Earth, Proxima Centauri is the Sun’s closest stellar neighbor. It is a red dwarf star, with a mass of only 12 percent of the Sun’s. Its luminosity is 17 percent of the solar luminosity. The habitable zone is consequently much closer to the star than is the case for the Sun.

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