THE SOUTHWORTH PLANETARIUM

70 Falmouth Street      Portland, Maine 04103

(207) 780-4249      usm.maine.edu/planet

43.6667° N    70.2667° W 

Founded January 1970

2022-2023: XIII

Sunrise: 6:20 a.m.

Sunset: 6:51 p.m.

Civil twilight ends: 7:20 p.m.

Sun's host constellation: Leo ==> VIRGO

Moon phase: Waning gibbous (73% illuminated)

Moonrise: 9:31 p.m.

Moonset: 1:15 p.m. (9/16/2022)

Julian date: 2459838.16

"Other intelligent life forms will differ greatly in appearance - they may resemble the creature in ET or startle us with their beauty- but life, itself, is common, I'm certain."

-Frank Drake

THE DAILY ASTRONOMER

Thursday, September 15, 2022

Drake's Silence

When SETI (Search for Extraterrestrial Intelligence) pioneer Frank Drake died on September 2, many people wondered perhaps if his last feeling before departing the coil was one of profound disappointment. He had, after all, devoted the majority of his professional life to the search for "alien signals," transmissions originating from other inhabited worlds within the Milky Way Galaxy. Despite decades devoted to what many deemed this vain, if not comical, effort, he heard nary a bleep or whisper. He put his figurative ear to the sky, which responded with an abundance of silence.

We'll never know Frank Drake's final thought. However, we do know that he knew the Herculean task he set for himself when, in 1960, he began Project Ozma at the National Radio Observatory at Green Bank, West Virginia. In this beautifully bucolic and conveniently radio quiet setting he started his quest to find aliens out there...somewhere. His initial aim was to direct the 85-foot Howard Tatel radio dish toward two Sun-like stars, Tau Ceti and Epsilon Eridani* while trying to detect signals along the 1,420 Hz frequency, equivalent to the 21 cm wavelength, the now famous "water hole." As this is the frequency of natural hydrogen emitting radiation into space and since hydrogen is one of the two elements comprising water, it is believed that aliens would choose this frequency for their broadcasts. That assumption is predicated on our belief that water is as essential for extra-terrestrial life as it is for Earth-based life. It is also assumed that aliens would conclude that other life is similarly water-dependent. The cosmos could ultimately confound both presumptions.

Drake intermittently monitored these stars for a total period of 150 hours spread out over four months. Despite having detected no signals, except for a "promising" signal later found to have originated from a high-altitude aircraft, Dr Drake would go on to establish the SETI program: a methodical search for signals that would ultimately expand the survey along a wide array of wavelengths and include thousands upon thousands of stars. This effort would also incorporate radio telescopes from all around the globe. And,between 2000 - 2020, home computers were employed to process data with the now-defunct SETI@home program. Because of Frank Drake's early efforts, Earth, itself, is now pressing its ear to the firmament.

An image from the "SETI@ Home" screensaver.   Millions of people around Earth downloaded a program which crunched data directly on their systems.     However, SETI@home was discontinued in 2020 so as to allow SETI astronomers time to analyze the immense amount of data this program collected.  IMAGE: [log in to unmask]

We "listen,"** because we assume that aliens would prefer to communicate through electromagnetic radiation transmissions as opposed to establishing personal contact, the separation distances between stars being so vast. Remember that if the Sun were the size of a golf ball the closest star, Proxima Centauri (4.2 light years away) would be the size of a larger red marble at a distance of 733 miles! Unlike any spacecraft which would be constrained by relativistic limits, EM waves travel at light speed and, moreover, don't have to be housed, fed, and entertained on holodecks.

Although Frank Drake's most significant contribution to extraterrestrial science will be the equation that bears his name. One can see this equation in the top image. We review it below.

 

N      =  number of advanced civilizations in the Milky Way Galaxy

This is the quantity we're trying to determine with all these values.   As we shall see, N can vary widely from person to person, depending on their assumptions about some of the factors within the equation.       We know that the minimum value of N is 1 because of Earth.  Of course, some might insist that based on human behavior the minimum could be zero.   Well, bah humbug to that bunch.

R   = rate of star formation in the galaxy

According to recent estimates, about three solar masses of material are converted into stars each year.   That material is not necessarily transformed into three stars.    Remember that most stars are less massive than the Sun.    This matter could be turned into as many as seven or eight stars.      Or, perhaps, just one star more massive than the Sun.      Also, statistics is a tricky business.     Some years will experience more star formation than others.    We also remember that star formation requires millions of years.   By star formation, we refer to the point at which the star becomes active by thermonuclear fusion reactions.    

fp = fraction of stars with planets

Based on what astronomers have discovered so far, we can assume that many stars have planets.      Analysis of the Kepler Space Telescope findings shows that one in six stars has an Earth-sized planet in a tight orbit.  About twenty percent have a "Super Earth" in orbit around them.  (A Super Earth is one that is more massive than Earth but considerably less massive than gas giant planets.)      

ne  =  number of planets within a star's "ecoshell" or habitable zone

Astronomers estimate that 40 billion worlds within the Milky Way Galaxy are likely revolving within the habitable zones of their parent stars.   Eleven billion of these might be orbiting Sun-like (G type) stars.    These estimates are all based on the Kepler Space Telescope findings.   Data collected from future missions might induce astronomers to modify these estimates to some extent.

fl  = fraction of those planets on which life develops

Now we've careened headlong into the realm of speculation.    Unfortunately, the data point available in our own solar system.  As we learned earlier this week, Venus, Earth and Mars are all within the Sun's habitable zones.   However, Earth alone contains life.  That gives us a 1:2 ratio of life-bearing worlds to non-life bearing worlds in the solar system.   Were that ratio to apply to other solar systems, we could assert that life proliferates throughout the galaxy.    Our estimate becomes even more optimistic if we find conclusive evidence that life actually did develop on Mars early in its history.  That discovery would support the notion that life arises quickly when conditions are conducive to its inception.   Of course, it would also suggest that life can be readily snuffed out, as well.   

fi  =  fraction of living species that develops intelligence

Now we're truly limited.      Based on our solar system, the rate of intelligent life developing on a life-bearing world is one hundred percent. A beautiful 1/1 fraction.   Approximately 3.7 billion years ago, the first traces of primordial life developed on the young, toxic Earth.  Now, we're in the age of space stations and 5 G networks.   This fraction will remain wholly unknown, unless, of course, some alien emissaries visit Earth and present us with a comprehensive list of life-bearing worlds and intelligent life-bearing worlds.    On the other hand, if we accept that microbial life did arise on Mars before it was eradicated, then this fraction becomes 1/2.       Both fractions, however, are predicated only on our own solar system and therefore might not be indicative of the galaxy in general. 

fc = fraction releasing detectable signals into space

Well, we know we did, although we needed 3.7 billion years to develop the capability.         During that immense amount of time, Earth sustained asteroid bombardments and snowball Earth epochs, to name just two of the perils life has confronted over the course of Earth's natural history.        The greatest mass extinction, the Permian-triassic event, occurred 250 million years ago.    That mass extinction devastated 96 percent of all ocean species and a majority of the other life forms, as well.     Earth life literally required millions of years to recover from that extinction event.       It is possible that life on other worlds was completely destroyed by some great extinction.    We just know that life on Earth survived.  

L  = length of time releasing detectable signals

Two ways to approach this last value.    First, radio was invented in the late 19th century.  By the 1940's humans developed atomic bomb technology.   Not much later the nuclear stockpile we amassed was so large it could have wiped out Earth life ten times over.    Humans were transmitting signals for slightly more than half a century before they were at risk of self-annihilation.          Secondly, many modern radio signals don't propagate like the original waves did.      Many of today's broadcasts are not broadcasting into space anymore.   Even if the outer space broadcasting ended altogether today, humans have been transmitting detectable signals for more than a century.       Again, Earth is our only data point and might not be representative of other life-bearing planets, if they exist in the first place.  

Unlike other daunting equations, this one doesn't separate the A students from the B students.  It does, however, divide the optimists from the pessimists.    While the latter assert that life is so rare as to be an anomaly, the former believe life abounds in both the galaxy and Universe.    In fact, in time, we might well find that Earth is but a studio apartment in a sprawling galactic megalopolis.    Only when we connect to other beings  will we truly come to acknowledge the Universe's staggeringly prodigious powers of creativity.      

When that time does arrive and our descendants become citizens of a bustling galactic empire, perhaps someone will tell one of our alien cousins about Frank Drake while saying, "he believed in you."  And, if Star Trek depictions are at all accurate, that cousin will raise a few eyebrows while stating, in a perfectly cultivated accent, "Of course.  How could he not?"

As we gaze into the boundless sky tonight and try to imagine the frenetic activity still hidden from us, we can think that perhaps Frank Drake's ghost will somebody enjoy the faint comfort of posthumous vindication.     For, in  life, he knew that pessimists never built any pyramids and will never know the secrets of the skies.

*Epsilon Eridani is certainly significant to Trekkies, as it is the parent star of Vulcan, home of the inimitable Mr. Spock. Unfortunately for Dr. Drake, the inception of Project Ozma predated the first broadcast of Star Trek by six years. Had he known that Vulcan had evolved well beyond crude radio transmission, he likely wouldn't have bothered to monitor it.

**Forgive us this poetic license. Astronomers aren't actually listening to radio signals, but, instead are striving to detect them. Unfortunately, as we equate radio waves with actual radios, the term "listening" is often used in reference to radio signals.

To subscribe or unsubscribe from the Daily Astronomer:

https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A=