Friday night, I discovered the YouTube-based public outreach and education channel called Cool Worlds. It is maintained by the Cool Worlds Lab at Columbia University, which in turn is run under the leadership of Prof. David Kipping.
A Columbia University Assistant Professor of Astronomy, Kipping's research areas involve extrasolar planets, moons and rings; stellar rotation, granulation & limb darkening; and astro-statistics.
His department webpage (linked above) states:
David received his B.A. and M.Sc. in Natural Sciences at Cambridge University, UK followed by a Ph.D. from University College London, UK. Before joining Columbia University, David spent time at Harvard as a postdoc through the Sagan and Menzel fellowships.
The Cool Words channel contains multiple video essays (each about 25 minutes in duration) that are at once detailed and sophisticated and yet very understandable and engaging for the informed and interested public (yes, let's caveat that sort of public).
Prof. Kipping has a nice presentation style -- eruditely British but minus any arrogance or condescension for his viewers.
Specifically, I discovered the Cool Worlds YouTube channel through the following video essay:
How Many "Earth-Like" Planets Are There Really?
This video essay delves into the Drake Equation and why so many researchers come up with such radically different estimates for "Earth-like" planets.
The Drake Equation as conceived by astronomer Frank Drake (who turns 90 this coming Thursday) in 1961:
N in the above equation is the number of communicative extraterrestrial civilizations (i.e., we can pick up their intentional or unintentional electromagnetic broadcast signals) in the Milky Way Galaxy. However, as we shall see below, N also denotes the number of inhabited worlds in any aggregation of planets in any galaxy, galaxy cluster, galactic supercluster, local universe, or even the entire Observable Universe. (You could even theoretically use it or number of inhabited Universes in the self-replicating Multiverse.)
N = R* x fp x ηe x fl x fi x fc x L
This video essay considers the terms fp -- that is, "f subscript p", to refer to the fraction of stars with planetary systems -- and ηe -- "η subscript e" where "η" is lower case Greek letter (although lower case "n" is sometimes used), and refers to number of habitable planets per planetary system.
These two terms are multiplied together into η🜨 ("eta earth") to refer to the number of habitable planets per star, or more properly, the frequency of Earth-like plants per star.
Size-based categories of planets size a frequency chart
Regarding the above image, I'm not sure on what the frequency values are based (perhaps its all from the now-retired Kepler Space Telescope).
The point, though, is that in considering a planetary taxonomy, a habitable planet refers, as based upon size and mass, to the two categories of "Earth" and "Super-Earth" planets and combined with the distance of the planet from its parent star. The Earths and Super-Earths are rocky planets to include the ocean worlds (since they have a rocky core).
Exoplanetary taxonomy: Size (relative to Earth) versus orbital period (in Earth days)
Not included in the "family" of habitable planets is the category of Mini-Neptunes, which are a type of gas dwarf that contains a rocky core and a dense gaseous envelope of volatiles. Larger and more massive than this type are the ice giants such as our Solar System's Uranus and Neptune. (The above taxonomy chart includes, somewhat unclearly, "ocean worlds" with ice giants. I say unclearly since, to me, ocean worlds are Earths or Super-Earths covered in liquid oceans -- potentially tens to hundreds of miles deep.)
Also not included as in the habitable planet classification are the larger planetary categories known as the gas giants -- such as our Jupiter and Saturn -- as well two planetary types not represented in our Solar System, namely, the cold gas giants and the "Hot Jupiters."
Then there is also how far away a planet is from its star -- and for a habitable planet, it refers to those that orbit within the star's circumstellar habitable zone (CHZ), a.k.a., the Goldilocks Zone. But that parameter itself is uncertain as it depends on the star's size and temperature and associated output. What's more, there is disagreement among astronomers are the appropriate width of a CHZ. Refer to the chart above by Chester Harman. For large version of it, see here.
Although we have the taxonomies described above, different astronomers use different assumptions about size, mass, and CHZ. As a result, as shown in the above image of a dozen different studies between 2011 and 2019, resulting η🜨 (eta earth) values range over a factor of 100, which is really not useful.
A way around that is to define a new quantity called gamma, denoted by upper case Greek letter gamma, Γ. In this case, Γ🜨 ("gamma earth") is defined as the rate of planets per logarithmic unit of radius per logarithmic unit of period.
The video uses an analogy for gamma earth involving number of gas stations per mile in a habitable zone of specified size, as shown above. As I understand, by using logarithmic scale for radius and for orbital period of planet about its parent star, it effectively sidesteps the issue of a fixed CHZ size.
Most exoplanets are detected using the transit method -- with the resulting (tiny) dip in starlight allowing for a calculation of the planet's size. The frequency of transits lets astronomers determine a planet's orbital period and hence distance from its parent star. However, stellar rotations -- and hence any planetary systems and associated planes of the ecliptic -- are randomly oriented with respect to each other including to our own Solar System.
The resulting steradian angle adjustment means that it is necessary to adjust (increase) the number of suspected planets by a factor of ~100.
But even doing that, we are still really don't know how many habitable planets per planetary system exist -- and, in fact, the Kepler mission only found 1 exoplanet (Kepler 452-b) that is orbiting in the habitable zone of a Sun-like star.
The point of this is put in perspective certain breathless media accounts -- from the New York Times, no less -- declaring that the Kepler spacecraft found evidence for as many as 40 billion "habitable Earth-sized planets in the Milky Way Galaxy," and yes, I enthusiastically wrote about that here.
And, of course, we still know nothing involving the last four Drake Equation terms. And that brings me to the second, even more thought-provoking is the following video-essay by Prof. Kipping.
Could We Be Alone?
Description: There are trillions upon trillions of stars and worlds in our Universe. Faced with such large numbers, it's tempting to conclude that there must surely be other life out there, somewhere. But is this right? Could the probability of life beginning be a number so small that we are alone? A video essay by Professor David Kipping.
Clarification: At the opening of this video, Prof. Kipping states that, as far as we know, there are approximately 70 sextillion stars (that's 70 x 10^21 stars) in the Observable Universe, which is to say 70 billion trillion stars. A working assumption -- based upon what we've found so far within a few hundred light years of Earth -- is about 5 to 10 planets per star. But just assuming just one habitable planet per star (i.e., a planet that meets the upfront baseline conditions involving size, mass, and CHZ position for habitability), this also results in an estimated 70 sextillion habitable planets in the Observable Universe.
Or as he poetically states: "The unfathomable scale of this celestial ocean transcends human comprehension."
It is on that mind boggling number -- which is said to be greater than the number grains of sand on all the seaside beaches of Earth -- that folks typically assert that there HAS to be intelligent life on other planets. As he frames that question:
"How could it be that amongst this unimaginable number of opportunities for life to get going, it only happened once??"
Prof. Kipping makes a compelling case on why it COULD be the case that Earth represents an incredibly rare instance of "abiogenesis"-- biological life (as we know it) emerging out of nothing but the right basic organic ingredients -- in the Cosmos to the point that it COULD be the ONLY such example in our (Observable) Universe.
To clarify, he isn't saying this IS the case or that it is even LIKELY the case, but rather, based upon the information we have so far and how the statistics of it could work out, this COULD be the case.
The analogy he uses for spontaneous abiogenesis on our planet is that of successfully picking (getting through) a lock that our planet did so quickly -- with life arising within a few hundred million years of Earth's formation precisely because life "got through the lock" but that doing so (getting through the lock) represents a statistical outlier case.
Is it on the order of one in a billion, meaning we live in a Universe teeming with life to include many advanced alien civilizations? Is it one in a trillion, so that life arises about once per galaxy? Is it one in a quadrillion, meaning life occurs basically once per galactic cluster?
Or is it some arbitrarily infinitesimal probability, e.g., one in an octillion, so that Earth is the only instance of life in the Observable Universe?
That is, we happen to be the case where it happened. The as-yet-unanswerable question is what are the odds of getting through the lock?
Consider N habitable worlds in a galaxy. Prof. Kipping argues that the case of exactly one inhabited world (also denoted by N) in each galaxy, i.e., N = 1 per N is, in and of itself, highly unlikely.
Instead, statistically, it is much more likely that the probability of abiogenesis and hence inhabited worlds, N, in a galaxy is much greater than 1 or much less than 1. The former represents a crowded galaxy (and, by inference, Universe) and the latter represents mostly empty galaxies. For the latter case, it is schematically shown as follows:
In this case, the result is that in any collection of N galaxies are mostly empty. In our own Local Group, we would almost certainly live in the only galaxy containing an inhabited world, namely, our own Earth.
Ditto to include the Virgo Supercluster, a.k.a., the Local Supercluster. In fact, the argument extends to the Observable Universe -- which brings us back to the possibility that we are the only example of life in it. What's more, you could use the same argument from a multiverse perspective -- most universes are actually devoid of life.
Schematic of the Virgo Supercluster (or Local Supercluster) of galaxies including our Local Group
I would point out that the Universe is still in its childhood, so to speak, so abiogenesis could occur elsewhere a billion years hence. Or maybe humans will someone colonize the Milky Way Galaxy -- and, its descendant species, beyond, although that would more likely be machine-based rather than biological.
That aside, Prof. Kipping features some nice interview quotes from the late Carl Sagan and late Richard Feynman. And his ending is quite moving -- assuming that we are Alone:
What a responsibility it is then to be alive. This one place, this one Earth, may be the diamond of the Universe. And so everyone of us here on Earth would be incredibly special. You, me, every person you bump into, every person you see. Every person you've loved or hated in your life. Everyone of the billions of people living here on this planet would be incredibly special, incredibly rare. The diamond of the Universe.
It was in November 2018 that Prof. Kipping and his grad student, Jingjing Chen, had published in Astrobiology the following paper (link to abstract): On the Rate of Abiogenesis from a Bayesian Informatics Perspective.
Just to be clear, this article is different from the June 2018 piece "Dissolving the Fermi Paradox" by Anders Sandberg, Eric Drexler, and Toby Ord that analyzed the Drake Equation in the context of the Fermi Paradox (i.e., "where is everybody?").
That piece concludes thusly, hubristically:
When we take account of realistic uncertainty, replacing point estimates by probability distributions that reflect current scientific understanding, we find no reason to be highly confident that the galaxy (or observable universe) contains other civilizations, and thus no longer find our observations in conflict with our prior probabilities.
We found qualitatively similar results through two different methods: using the authors’ assessments of current scientific knowledge bearing on key parameters, and using the divergent estimates of these parameters in the astrobiology literature as a proxy for current scientific uncertainty.
When we update this prior in light of the Fermi observation, we find a substantial probability that we are alone in our galaxy, and perhaps even in our observable universe (53% – 99.6% and 39% – 85% respectively). 'Where are they?' -- probably extremely far away, and quite possibly beyond the cosmological horizon and forever unreachable.
Bitch. That piece garnered a fair amount of media attention and some real pushback.
Let's just be clear here: Prof. Kipping is providing a coherent and logical argument for what COULD be the case for abiogenesis and hence life elsewhere in the Cosmos. He is NOT saying that is MUST be that because, as he freely admits, we simply don't know if we are alone or not.
And on that note, I think it's time to end this entry.
UPDATED 8:31 p.m. May 27, 2020
I must point out that Prof. Kipping just had published a paper on the very topic about abiogenesis, specifically, this paper in the Proceedings of the National Academy of Sciences (PNAS) [link embedded]: Columbia astronomer uses Bayesian statistics to shed light on the odds of life and intelligence emerging.
The PDF version is here.
First, let me say that I honestly don't know if it is just a coincidence or not that came across the Cool Worlds Lab YouTube website right at the time that this piece was published. It was late Friday night and I was surfing astronomy stuff (not necessarily "news") and came across the website.
Secondly, this study seems to counter, at least a little bit, what Prof. Kipping argues above.
Quoting the first sentence of the last paragraph of the Conclusions section: Overall, our work supports an optimistic outlook for future searches for biosignatures.
The "Significance" paragraph states:
Does life’s early emergence mean that it would reappear quickly if we were to rerun Earth’s clock?
If the timescale for intelligence evolution is very slow, then a quick start to life is actually necessary for our existence -- and thus does not necessarily mean it is a generally quick process. Employing objective Bayesianism and a uniform-rate process assumption, we use just the chronology of life’s appearance in the fossil record, that of ourselves, and Earth’s habitability window to infer the true underlying rates accounting for this subtle selection effect. Our results find betting odds of >3:1 that abiogenesis is indeed a rapid process versus a slow and rare scenario, but 3:2 odds that intelligence may be rare.
End of Update and of Entry.