The Known Habitable Zones & One Remarkable Planet

I constantly tell my Facebook and Twitter followers to beware of the claims of popular science web articles. Let me give you an example. Last July, Yahoo News ran an article titled "Astronomers Say We Could Be Just 10 Years Away from Finding Alien Life."1 But what one of the authors of the research paper, Julien de Wit, actually said was, "In five to ten years we will be able to say if they [two of the extrasolar planets they were studying] are habitable."2 By habitable, de Wit and his coauthors meant whether or not either of the two planets possessed properties that would allow liquid water to exist on part of the planet's surface for at least part of the planet's history.

A Trillion Alien Civilizations?

Another example that had the internet buzzing for many weeks involved several articles and posts claiming that "at least a trillion alien civilizations have almost certainly existed in the universe."3 These claims are founded on four presuppositions:

1. The density and kinds of planets throughout our galaxy and all other galaxies in the universe are roughly the same as what we observe in the vicinity of our solar system.

2. About 20 percent of all planets are habitable.

3. Life inevitably will arise on all habitable planets.

4. The probability of a technologically advanced civilization arising from simple life forms is better than one chance in 10 billion.

Origins of Life (my book coauthored with biochemist Fazale Rana) demonstrates that, from a naturalistic perspective, assumption #3 is certainly incorrect. The probability of life originating naturalistically on a habitable planet is mathematically indistinguishable from zero.4 Since zero times any other factor or set of factors equals zero, then, from a naturalistic perspective, the number of civilizations besides our own that exist in the universe is zero.

The Trouble with the 20 Percent Assumption

There is also much I could say about why assumptions #1 and #4 are deeply flawed, but I will focus on #2 and a new discovery that establishes that far, far less than 20 percent of all planets are habitable.

Assumption #2 only takes into account the "water habitable zone," which is the range of distances from a planet's host star within which—and only within which—it would conceivably be possible for liquid water to exist at some place on the planet's surface at some time during the planet's history. And, indeed, about 20 percent of the more than 3,000 planets discovered so far fall within this water habitable zone.

The percentage drops precipitously, however, if one does not allow the greenhouse effect of the planet's atmosphere to take on a value that perfectly compensates for the host star's brightness. And the percentage takes another steep drop if one desires the planet to retain liquid water on more than 10 percent of its surface for more than a billion years. (Liquid water must be present on a planet's surface for at least 3.5 billion years for there to be even the remotest possibility of the planet sustaining advanced life.)

Eight Habitable Zones

Moreover, in addition to the water habitable zone, there are seven other known habitable zones. I listed and briefly described these zones in "Earth in the Zone," an article I wrote two years ago for Salvo 30 (Fall 2014). For those who missed the article or need a refresher, here again is a list of the eight zones:

1. Water habitable zone

2. Ultraviolet habitable zone

3. Photosynthetic habitable zone

4. Ozone habitable zone

5. Planetary rotation rate habitable zone

6. Planetary obliquity habitable zone

7. Tidal habitable zone

8. Astrosphere habitable zone

In chapter 7 of my just-released book, Improbable Planet, I provide a detailed, thoroughly documented explanation of all eight of these habitable zones.5

Typically, these zones do not overlap. For example, the distance a planet must be from its host star so that it receives enough ultraviolet radiation to enable the synthesis of many life-essential compounds, but not so much as to kill living things, is rarely the same distance that a planet must be from its host star for liquid water to possibly exist on its surface. For 97 percent of all stars, the liquid water habitable zone does not overlap the ultraviolet habitable zone.

A planet is a true candidate for habitability only if it simultaneously resides in all eight habitable zones. So far, the only known planet that dwells in all eight is Earth.

Venus's Desiccating Factor

Now, a ninth habitable zone has been discovered—

9. Electric wind habitable zone.6

This zone was discovered thanks to an electron spectrometer on board the European Space Agency's spacecraft Venus Express. This instrument measured the electric potential in Venus's atmosphere. The result was surprising. At 10 volts, Venus's atmospheric electric field proved to be far stronger than what any astronomer had expected.

The high voltage drives an electric wind in Venus's atmosphere that is powerful enough to drive all heavy ions (atomic nuclei stripped of their electrons) in Venus's ionosphere into interplanetary space. These heavy ions include oxygen ions that once belonged to water molecules.

Previously, astronomers had presumed that the solar wind was responsible for drying out Venus. This new discovery shows that Venus's electric field is the dominant desiccating factor.

But the Sun still plays a role, though less directly. The Venus Express research team determined that Venus's proximity to the Sun explains its strong atmospheric electric field. Venus receives twice as much ultraviolet radiation as does Earth. All this ultraviolet radiation results in a high density of free electrons and ions in Venus's atmosphere, which generates a strong electric field above the planet's surface.7

Confirmation that Venus's proximity to the Sun explains its strong atmospheric electric field comes from failed attempts to detect strong atmospheric electric fields either on Earth or on Mars. Instruments establish that both Earth and Mars possess atmospheric electric fields weaker than 2 volts.8

It's Distance, Not Atmosphere

The discovery of a strong atmospheric electric field on Venus has serious implications for the possible habitability of exoplanets (planets beyond our solar system). The discovery implies that any planet that has an atmosphere thicker than one percent of Earth's and that is closer to its star than about 90 percent of Earth's distance from the Sun will very likely possess an atmospheric electric field strong enough to completely dry out the planet.

It has been known for some time that planets with atmospheres thinner than one percent of Earth's will be unable to prevent deadly radiation, which emanates both from their host stars and from cosmic rays, from penetrating to their surfaces. These planets will also have a water problem. With such thin atmospheres, the boiling point of water will not be greater than the freezing point, implying that liquid water will be absent from their surfaces. For these two reasons, planets with atmospheres thinner than one percent of Earth's cannot be classified as habitable.

What the discovery of a strong electric field on Venus means, with respect to habitability, is that, for planets closer to their host stars than 90 percent of Earth's distance from the Sun, it does not matter how thick their atmospheres are. If the atmospheres are thinner than one percent of Earth's, they will dry out, because they will not be able to hold liquid water. But if the atmospheres are thicker than one percent of Earth's, they also will dry out, because the strong electric field in their atmospheres will desiccate them.

This set of conclusions has enormous implications. The vast majority (over 95 percent) of exoplanets currently classified as "habitable" are closer to their stars than 90 percent of Earth's distance from the Sun. Thus, they no longer can be classified as habitable. To put it another way, for the vast majority of stars, the water habitable zone does not overlap the electric wind habitable zone.

Nine Zones Required

The discovery of the electric wind habitable zone means that for a planet to be a viable candidate for possibly sustaining life, it must simultaneously reside in nine different habitable zones. So far, astronomers have measured the characteristics of 3,484 planets.9 Only one of all these 3,484 planets resides in all nine known habitable zones. That one is Earth.

Given the fine-tuning inherent in each of the nine known habitable zones, and given the extreme improbability that even three of them will overlap, let alone all nine, it seems that nothing less than the supernatural handiwork of a Creator will suffice to explain how a planet could meet all these known conditions for habitability. •

is an astrophysicist and the founder and president of the science-faith think tank Reasons to Believe (RTB).

This article originally appeared in Salvo, Issue #39, Winter 2016 Copyright © 2019 Salvo |