Snowballs remind us of the presence of water on Earth—a feature that highlights our planet’s habitability. However, most of the so-called habitable planets in the universe are tidally locked, a problem, it turns out, for the accumulation of snow and ice.
Astronomers typically identify a planet as habitable if it happens to lie at a distance from its host star where the planet’s surface temperature might fall within the range where water could be in a liquid state. However, most of the stars in the universe are much dimmer than the Sun. This lower stellar luminosity means that for the planets orbiting these stars to possibly possess liquid water on their surfaces, they must orbit their host stars more closely than Earth orbits the Sun.
Planets orbiting their host stars more closely than Earth orbits the Sun will experience a substantially stronger gravitational force from the host star on the side that faces its host star compared to the opposite side. The difference gradually forces one side of the planet to always face its host star, an effect known as tidal locking. The closer a planet is to its host star, the more rapidly it becomes tidally locked.
The Earth will become tidally locked to the Sun in about 40 billion years. However, for most of the universe’s planets that have a possibility of possessing liquid water on their surfaces, the time for them to become tidally locked is less than a billion years.
Another Big Problem for Tidally Locked Planets
On August 28 I posted a blog1 where I described two recently published studies that establish additional reasons why it is highly unlikely that any tidally locked planet would ever possess life. Now, three astronomers from the University of Toronto and the University of Chicago have published yet another reason.2 That reason does not necessarily eliminate the possibility of microbes existing on a planet for a relatively short time period, but it does rule out any kind of advanced life or life that remains on a planet for more than about a billion years.
The three astronomers first point out that stars dimmer than the Sun are redder than the Sun. At red and near-infrared wavelengths, the difference between the amount of starlight reflected by ice or snow on a planet’s surface compared to liquid water or exposed land is much less than it is at yellow, green, or blue wavelengths. The team then performed calculations that showed for tidally locked planets the top-of-the-atmosphere ice/ocean albedo (albedo refers to the amount of light reflected away) contrast is even smaller. The three then performed additional calculations that established tidally locked planets—even those possessing extensive surface liquid water and functioning enduring silicate weathering—will “not be able to exist in a snowball state for an extended period of time.”3 A snowball state is where all or most of a planet’s surface becomes covered in ice. The researchers finish their paper with the following conclusion, “We [astronomers] will not find habitable tidally locked exoplanets with an active carbon cycle in a snowball state.”4 No long-lasting life is possible on a planet without a carbon cycle.
Silicate weathering requires a planet with both surface oceans and surface continents where abundant rain falls upon the continents. It is silicate weathering that draws down carbon dioxide from a planet’s atmosphere to a sufficient degree to cool the planet enough for ice to form in spite of the planet’s host star getting progressively brighter as it continues to fuse hydrogen into helium in its nuclear furnace.
Why Snowball States Are Critical for Advanced Life
In my book Improbable Planet, I described how Earth experienced several snowball events.5 I also explained how each of these snowball events transformed the chemistry of Earth’s atmosphere and oceans so as to make possible the introduction of more advanced forms of life. In particular, snowball events resulted in more oxygen being pumped into Earth’s atmosphere and oceans. They also impacted Earth’s geochemistry. Without the greatly enhanced atmospheric and oceanic oxygen and the transformed geochemistry, plants and animals could never have existed on Earth, nor could microbes have persisted for more than about a billion and a half years.
Earth’s snowball events never covered all of Earth’s surface with ice. However, they did cover between 70–90% of Earth’s surface. For comparison, the greatest ice coverage during the ice age cycle that has persisted for the past 2.59 million years was 23%.
Life as advanced as birds, mammals, and human beings would not be possible unless Earth had a highly fine-tuned and perfectly timed set of snowball events. The research achieved by the three astronomers shows that snowball events are not the norm for liquid-water-possessing planets. Furthermore, the kind of snowball events essential for birds, mammals, and humans to possibly exist likely only occurred once in the universe’s history.
Original article: Thank God for Snowballs