Our Sun is a yellow dwarf star, shiny but comparatively short-lived. Red dwarfs are said to be the most common and longest-lived type of star in our galaxy. They are long-lived because they do not emit much radiance. How large are their habitable zones? A key problem turns on how much light they emit that is useful for life.

That was quite a topic of discussion earlier this year at Centauri Dreams (dedicated to “Imagining and Planning Interstellar Exploration”). Astronomer and mathematician Henry Cordova offers some thoughts:

All else being equal, the radiant flux received by the planet must then be directly proportional to the luminosity of the star, and inversely proportional to the square of the planet’s distance from it. In other words, if one star is a hundred times more luminous than another, a planet orbiting the fainter star must be ten times closer to its primary in order to receive the same flux. The same reasoning can be applied to both the inner and outer edges of the star’s habitable zone. Regardless of how we define the HZ, it will become much narrower as the luminosity of the primary decreases. And the narrower an HZ is, the less likely there is a planet there.

Consider our own Sun’s habitable zone. Although there is some controversy about its dimensions, let us for the purposes of this argument say that it is limited by the orbits of Mars and Venus.The two planets have semi-major orbital axes of roughly 1.5 and 0.7 AU, respectively. These two figures mark the limits of Sol’s HZ, and their difference gives an HZ width of 0.8 AU, plenty of room to squeeze Earth in.

If our Sun were a hundred times less luminous, the HZ boundaries would be at 0.15 and 0.07 AU, which translates to an HZ width of only 0.08 AU! Clearly, the HZs of faint stars can be very narrow. The chances of a planet forming there, or migrating in, are substantially reduced.

Paul Gilster, “The Red Dwarf Habitable Zone Dilemma” at Centauri Dreams (January 7, 2021)

Cordova refers us to a 2012 paper on the subject (paywalled) and to a 2012 discussion thereof at Centauri Dreams.

While some planets have been detected within the assumed habitable zones of red dwarfs, Cordova doubts that they are good candidates for life. A number of readers disagree, including one who writes, “Yes, the absolute width of the HZ is narrower with M-dwarfs, but so is the size of the disk the planets form in. For any system of n planets, they will space themselves out and the probability that any will be in the HZ should remain nearly constant. I do understand that there might only be room for 1 planet in the HZ, while wider HZs could accommodate more than 1 planet, but how important is that?”

Incidentally, Cordova has also been a critic of Search for Extraterrestrial Intelligence (SETI)’s current strategy, based on the assumption that the development of intelligence is normal in the universe. But is it?

SETI requires reasonably complex and active life forms as participants–on both sides. We must expect that candidate species share an oxygen metabolism and a multicellular architecture comparable to vertebrates in sophistication. We know that intelligent primates, and possibly intelligent cetaceans, appeared on Earth only a half-billion years after the first appearance of multicellular creatures, a relatively short time by astronomical standards. But we should not conclude that intelligence was inevitable, simply awaiting a mammalian nervous system of sufficient capacity to support it. Mammals have been around for as long as the reptiles and have spent most of this time without developing any technical abilities. The dinosaurs and birds never chose intelligence as a strategy, and the cetaceans prove that even intelligence does not necessarily lead to tool-making. Primate cooperative technical intelligence, aided by language, arose only a heartbeat ago in the cosmic time scale. Its late arrival and explosive growth suggests it was not inevitable, and probably accidental. We cannot even conclude from this history that intelligence is a survival trait; it is possible that its very advantage to a species is destabilizing to the biosphere as a whole. I shall omit at this point the usual cautionary remarks about nuclear war and environmental pollution… Neither should we forget that highly advanced civilizations need not necessarily be technological, even if they start out that way.

Paul Gilster, “A SETI Reality Check” at Centauri Dreams (July 3, 2020)

Some hypotheses about why we don’t yet contact ET touch on these very issues, including:

3.Did the smart machines destroy the aliens who invented them? That’s the Berserker hypothesis. A smart deadly weapon could well decide to do without its inventor and, lacking moral guidance, destroy everything in sight. Extinction of a highly advanced civilization by its own lethal technology may be more likely than extinction by natural disaster. They could control nature.

5.Maybe there are just very few aliens out there… The Rare Earth hypothesis offers science-based reasons that life in the universe is rare. Even if life is rare in the universe, Earth may be uniquely suited to space exploration, as the Privileged Planet hypothesis suggests.

11 Is intelligent life in the universe living in interior oceans of planets and moons? The Ocean Planets Hypothesis is that intelligent beings may flourish in the interior oceans of the moons of gas giant planets — or within exoplanets — but they are trapped there. If intelligent life forms are trapped in the interior oceans of rocky moons and planets, Earth is a special planet—much better suited to space exploration.

Here are some other hypotheses as to why we don’t see ET.

The only star we know of whose system certainly hosts life is our Sun, a yellow dwarf or “main sequence” star. Its lifespan is considered to be 10 billion years and it is halfway through that. In a billion years or so it will become a white dwarf, exhausting its nuclear fuel. Only a small proportion of the stars in our galaxy are yellow dwarfs like the sun. But that is still billions of stars, so even if Cordova proves right about red dwarfs, that is not a reason for those who reasonably believe that we just can’t be all alone in the galaxy to give up the hunt.

You may also wish to read: Is intelligence common or rare in the universe? A recent analysis says that life is common but intelligence not so much. Let’s explore the reasoning.