Science Uncovered
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Science Uncovered

Expansion Everlasting; Exoplanets and Beyond

Image credit: bright center star cluster by NASA. Public domain via Unsplash.

In this following extract, Steven E. Vigdor author of Signatures of the Artist asks: how many more habitable planets are we likely to find?

Astronomers have been searching for planets orbiting stars beyond our solar system for nearly 30 years now, with results to date dominated by the recent burst of discoveries by the Kepler Space Telescope. At this point, nearly 3500 such exoplanets have been discovered and confirmed, largely by monitoring the temporary dimming they cause in light from the stars they orbit, when their orbit crosses between that star and the detecting telescope. More than 2300 of these were discovered by the Kepler mission alone. More than 30 of these exoplanets exist within the habitable zone surrounding their stars, with at least 20 more candidates identified, but not yet confirmed Based on results to date, astronomers have estimated that our Milky Way galaxy contains at least as many planets as it does stars, namely, some 100–400 billion.

Of these, roughly 1%, or 2–3 billion exoplanets, are estimated to surround sun-like stars in the habitable zone.

We of course do not yet know that any of these other candidate planets are actually inhabited. But the results to date suggest that the odds against finding potentially habitable planets in our universe, while high, are not mind-blowingly so.

It is more difficult at this point to estimate the likelihood that pocket universes within a multiverse driven by the string theory landscape can support life. The string theory landscape is not sufficiently quantified to allow a detailed estimate. But we can begin to get an idea by combining the simplifying guess that string theory vacua occupy the full spectrum of energies allowed up to the Planck scale, more or less uniformly, with Weinberg’s estimate of anthropically allowed values of the cosmological constant.. The odds against universes forming within the potentially habitable regime of small cosmological constants would then be 10118 to 1. And these astronomical odds do not yet take account of all the other fine-tuning needed for life. The odds would undoubtedly be better if we could restrict the sample to observable surviving universes, as was done for the planetary sample taken by the Kepler Space Telescope, but we have no way of doing that for the foreseeable future.

Odds of 10118 to 1 are hard to get one’s mind around. To provide a more visceral feeling, imagine a hypothetical situation in which the United States, at some future time, gets public support for increased taxes by instituting a daily lottery. The government uses a random number generator to assign distinct numbers to each of the 100 million tax-paying households. They then have a nightly drawing to select at random just one of those numbers, with the winning household receiving a million-dollar prize. Each household has an equal one-in-a-hundred-million chance of winning on any given night. Winning once does not, in any way, improve or reduce your odds of winning on any other night. To beat odds of 10118 to 1, your household would have to win the lottery on 15 consecutive drawings!

After your first win, you would feel extremely lucky. By the second, you would begin to wonder if this were all a dream, something that doesn’t happen in reality. By the fifteenth win, you would feel pretty special and wonder if the designer of the random number generator used by the government somehow had you in mind. Meanwhile, the government would be trying to quell the rioting crowds of losing families by insisting that, if the lottery drawing were held an infinite number of times, each of their numbers would also eventually show up 15 times in a row.

Steven Vigdor is a well-known experimental physicist with more than 45 years of experience in cutting-edge research in nuclear and particle physics. Currently Professor Emeritus in the Department of Physics at Indiana University, he was formerly the Chair of that department and the Associate Laboratory Director for Nuclear and Particle Physics at Brookhaven National Laboratory (BNL). In the latter role, he oversaw in 2007–2012 operations of the largest U.S. facility for nuclear physics research — the Relativistic Heavy Ion Collider — as well as all of BNL’s research in nuclear and high-energy physics, and most of its efforts in accelerator science research. Since returning to Indiana in 2013, he has started a small business to develop state-of-the-art instrumentation for next-generation proton radiotherapy treatments of cancer, in parallel with the writing of this book. He is the author of Signatures of the Artist: The Vital Imperfections That Make Our Universe Habitable.



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