What drives us to send probes throughout the Solar System and rovers and landers to Mars? It’s not cheap, and it’s not easy. It’s because we live inside a big, natural puzzle, and we want to understand it.
That’s one reason. But the main reason for space exploration is to search for life beyond Earth. That our planet could be the only planet to host life is a disquieting thought.
Our search for life is focused on Mars and the Solar System’s icy ocean moons. Venus attracts some attention, even though it appears to be inhospitable. Despite its inhospitable nature, Venus is a terrestrial planet that resembles our own in size, mass, and bulk composition.
Venus and Earth are both in the habitable zone, though some say Venus only gets in on a technicality. Somehow, their climates diverged dramatically, with Earth remaining habitable and Venus suffering an extreme greenhouse effect.
So, Venus has something to tell us about how rocky planets that are similar in so many ways can be vastly different in others.
As our search for life, or at least habitability, extends to distant exoplanets around other suns, Venus has lessons for us. It can help us understand rocky planets in the habitable zones of other stars.
According to a presentation at the recent 2025 Lunar and Planetary Science Conference, an equation reminiscent of the Drake Equation can determine the probability that there’s extant life on Venus and what this can tell us about other worlds.
The presentation is titled “Probability of Planetary Life: The Venus Life Equation and Uknowns for Other Worlds.” The lead author is Diana Gentry, Director of Ames’s Aerobiology Laboratory at NASA’s Ames Research Center.
Just like the Drake Equation (DE), we can’t fill in all of the values. Instead, the DE and Venus Life Equation (VLE) are frameworks for thinking about life in the galaxy and on Venus, respectively.
The values in the equation aren’t static and can change over time, so the VLE gives us a framework for thinking about the likelihood of life in the past, the present, and the future.
“The fundamental goal of the VLE is to provide a scaffold for estimating the chance of life based on factors that can be constrained or quantified through observation, experiment, and modelling,” the authors write.
There are many questions about Venus’ history that lack satisfactory answers. Still, scientists have pieced some things together.
The scorching hot planet may have enjoyed a period of watery warmth. During that time, there would have been land-water interfaces that are important for life. This period coincided with Earth’s late Hadean and early Archean eons.
Since this is when life appeared on Earth, it’s firmly within the realm of possibility that life was able to arise on Venus.
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This leads us to a controversial idea: it’s possible that if Venus hosted simple life, it would be able to survive to this day in the planet’s clouds. At around 50 km altitude, conditions are surprisingly temperate, with the temperature and pressure similar to Earth’s.
Like the DE, the VLE is based on key parameters. While the DE uses eight parameters, the VLE uses three: Origination, Robustness, and Continuity. The VLE equation is L=OxRxC.
“The VLE’s terms are L, the likelihood of there being life at the time in question; O (origination), the chance of life arising and becoming established prior to the time in question; R (robustness), the potential size and diversity of the biosphere over time; and C (continuity), the chance that conditions amenable to life persisted spatially and temporally until the time in question,” the authors explain.
The equation is agnostic to the type of life and its scale, and all factors in the equation span from 0, which means there’s no chance, and 1, which indicates certainty.
There are different factors behind each variable. For origination, consideration is given to these factors:
- The likelihood of origin by abiogenesis
- The likelihood of origin by panspermia, informed by the likelihood of life elsewhere in the star system and the dynamics of interplanetary matter transport
- The chances of two or more separate geneses (e.g., both abiogenesis and panspermia occurring)
- The chance of breakout, or life expanding beyond its point(s) of origin to occupy the planet
Some of these are extremely difficult to quantify, like the chance of breakout. We know that life spread around Earth relatively quickly, but we don’t know much about the details.
Origination is the only factor in the VLE that doesn’t change over time. It’s either a 0 or a 1.
When it comes to R or Robustness, the authors consider a best-case scenario of a planet’s biomass over time. That depends on the availability of essential nutrients like CHNOPS, and on the availability of energy.
When considering Venus specifically, nutrients became less available when the land-water interfaces disappeared. Venus may also have enjoyed a period of plate tectonics which influences the availability of CHNOPS. Once that ended, it affected Robustness.
The functional diversity of life also affects R since the more niches life has adapted to, the greater its chances of survival when conditions change. “A low R value indicates a small or fragile biosphere more vulnerable to extinction from the threats captured in the final continuity term,” the authors write.
Carl Sagan described Earth as “a world positively rippling with life.” Earth appears to have a high R value, which helps explain why it persists to this day.
“Life on Earth has been widespread and diverse enough to persist through a number of mass extinction (bottleneck) events, including asteroid impacts and global glaciation – some of which occurred quite early in its history,” the authors explain.
The third factor, Continuity, also depends on several factors. These include the stability and lifetime of the star, the planet’s orbital stability, planetary geological stability, including things like continuous nutrient recycling, and the likelihood of major disruptive events like extended volcanism or large impacts.
Another is biogenic (life-caused) instability, for example when Earth’s Great Oxygenation Event changed the chemistry of the oceans and the atmosphere.
Scientists have a pretty good handle on some of these factors, like stellar lifetimes and planetary orbits. However, others, like biogenic instability, are difficult to constrain.
“A value of 0 for C indicates that there has been at least one total extinction event between the point in time of the origin event (including breakout) and the time being assessed,” the authors explain.
It doesn’t appear that Earth ever suffered a total extinction, but it’s possible. There’s no way for us to know if life has survived uninterrupted on our planet or if it was extinguished at one point early in Earth’s history and then reappeared.
The VLE suffers from the same handicap as the DE. We know of only one place where life has appeared: Earth. However, whether we like it or not, that’s our starting point, and the VLE is a framework for growing our understanding.
“Although we are currently limited by the n=1 problem of having Earth-based life as our only concrete example, we can nevertheless use our understanding of the genesis and evolution of life on Earth to establish a framework for identifying the unknowns and uncertainties of life on other worlds,” the authors write.
This article was originally published by Universe Today. Read the original article.
