THE DAILY ASTRONOMER
Thursday, December  1, 2022
On "Habitable" Zones

“Habitable zone” is a particularly strange term which denotes a region around a star in which conditions would be conducive to the development of life. This zone is generally predicated on the notion that water must be as essential to extraterrestrial life forms as it is to Earth-life. (We might discover that this presumption is incorrect.) Also, a habitable zone should have sufficient heat energy to facilitate the myriad biochemical processes required to sustain and evolve life without being so hot as to prevent these reactions due to molecular disassociation.

The habitable zone, as currently defined, depends entirely on the star. The habitable zones around comparatively cool M-type dwarfs would be smaller and closer to the star than those around hotter stars, such as F and G type. (Note: The Sun is a G2 V…G-type dwarf.)

The diagram above, compliments of NASA, shows the habitable zones of M-type dwarfs* (top row); K-type (middle row) and G-type (bottom row.) Note the relative sizes and distances of the three different habitable zones. The next image, also from NASA, shows the habitable zones of even hotter blue stars.

Naturally, the habitable zones around hotter and more luminous blue stars are much farther away than those around G, K or M type stars.

THE PROBLEMS WITH “HABITABLE ZONE.”

However, the word “habitable zone” can be quite misleading. For one, the term, itself, implies that life exists on the worlds within such zones. That is not necessarily true. The image below, courtesy of Mythic Scribes, shows that Earth is not the only planet to revolve within the Sun’s habitable zone. Mars is within this zone, as well. Even a wider zone, dubbed “the optimistic zone,” includes Venus, which is a hell world with a global wide surface temperature exceeding 900 degrees F

SIZE MATTERS

We know that a life-sustaining world must necessarily have a substantial atmosphere. Astrobiologists are confident in their assertion that all life, no matter where it exists, must utilize some form of respiration. One will never find life thriving in an essentially airless world such as Mercury. A research paper published in the Astrophysical Journal (August 2019) entitled “Atmospheric Evolution on Low-gravity Waterworlds” (Constantin W. Arnscheidt1,2,3, Robin D. Wordsworth1,4, and Feng Ding) places a lower limit on habitable planet mass of 2.7% Earth’s mass (for comparison: the moon’s mass is 1.2% of Earth; Mercury’s is 5.5%) Any planet with a lower mass will not exert a strong enough gravitational pull to retain an atmosphere. However, planets more massive than this limit, but still much less massive than Earth could be suitable for life because as planets decrease in size, their atmospheres expand outward. These larger atmospheres increase both the absorption and radiation of heat, which could help to stabilize the atmosphere over extremely long time periods.

RADIATION AND TIDAL LOCKING
M-type dwarfs represent the majority of active stars -those which produce energy through core thermonuclear fusion reactions-. These stars tend to spin more quickly and will often produce dangerously powerful solar flares that could strip away the atmosphere of planets within the habitable zone. Although that could pose a true problem, a recent study conducted by a team from Leibniz Institute for Astrophysics Potsdam -using data collected by the Transiting Exoplanet Survey Satellite- found through the study of red dwarf light curves that these flares tend to erupt toward the star’s polar regions (north and south of 55–60 degrees). Pole oriented flares wouldn’t pose the same threat to planetary atmospheres as those that erupt out of the equatorial regions.

The other problem pertains to tidal locking. While the percentages are a matter of considerable dispute, the orbital mechanics between a red dwarf and a planet close enough to be inside its habitable zone are such that eventual tidal locking is highly probable, When a planet is tidally locked, the planet’s rotational period will often -but not always**- equal the revolutionary period so that one side of the planet is always directed toward the star. Such a planet would hardly be hospitable to life due to the vast temperature differential. Some have claimed that life would thrive within the more temperate “twilight zones.” Although that would leave precious little area in which life could develop.

NOT ENOUGH TIME
The more luminous the star, the shorter the lifespan. Yes, there is a “habitable zone” around hot, highly luminous O-type stars. However, these stars tend to evolve off the main sequence within tens of millions of years or less. For instance, a star that is 10 times more massive than the Sun will only “burn” hydrogen for about 20 million years before evolving into the giant stage. Based on our own example, life didn’t even begin to develop on Earth until approximately 800 million years after its formation. While this value is also disputed, it would seem highly unlikely that life could develop on a planet around a hot, highly luminous star.

Habitability involves a wide variety of factors -many of which I didn’t mention- not just temperature.

*Yes, the plural is “dwarfs,” not “dwarves.” Years ago I enjoyed an utterly pointless twenty-minute argument with a Tolkien-adoring student about the appropriate plural for dwarf. I assured him that the plural of stellar “dwarf” is the admittedly inelegant “dwarfs.” He remained frustratingly obstinate about the matter, even after I assured him that I was not trying to blaspheme the divine JRR, who I, too, greatly admire. He literally ended the discussion with the challenge, “Would you dare to have this argument with Gandalf?” To this question I had no response.

**Sometimes the locking will produce a spin-orbit resonance, such as Mercury’s 3:2 spin-orbit resonance.