As seen in Forbes Science | December 31, 2018
On the cusp of yet another trip around the Sun, I found myself wondering how many alien civilizations within this sector of our Milky Way might have already come and gone? That is, bitten the dust due to wholly natural climate change long after their planet had sung its last Auld Lang Syne?
As stars age, their luminosity increases over billions of years, causing their habitable zones — the zones in which a terrestrial planet can have liquid water at its surface — to expand outward. Thus, extraterrestrial intelligence that happened to evolve on planets at the inner edges of their solar systems’ habitable zones might initially blossom into technological civilizations. But as their star’s luminosity increases over time, their planet would suffer a hellish, climatological holocaust.
Why is this important?
In the next decade, we’ll be peering at spectra and sometimes images of extrasolar Venus-type planets that may or may not have harbored technological civilizations for a few thousand years at least, Stephen Kane, a planetary astrophysicist at the University of California — Riverside, told me.
Would a putative alien civilization on a nearby, habitable exoVenus even be aware of such coming catastrophe?
Not necessarily, says Kane.
We are very fortunate to have a Venus in our solar system; essentially a planet the same size and mass as Earth, but at a much more advanced stage of planetary and atmospheric evolution than our own, says Kane.
“If we didn’t have our Venus, some of my colleagues have said that then an advanced runaway greenhouse would have been extremely difficult to predict,” said Kane.
That’s because, as he points out, the physics to make such predictions are very advanced and, in fact, even today’s best atmosphere models have trouble accounting for Venus’ current state.
Even if an advanced civilization understood how stars evolve and that this evolution would have an adverse effect on their planet’s oceans, says Kane, they might not understand the full implications of how environmentally-hostile their planet would become.
For exoplanets, the current detection methods are more sensitive to planets close to their host stars , meaning that we’re better at discovering Venus analogs than Earth analogs, says Kane. We, thus, expect to be discovering and characterizing many runaway greenhouse planets in the coming years, he says.
But some of these could have previously been habitable, says Kane.
Estimates range from 400 million to one billion for the duration of clement conditions on Venus.
Four hundred million years, or perhaps even one billion years of clement Venusian conditions are much too little for the evolution of a high-tech intelligent species, Dirk Schulze-Makuch, an astrobiologist at Germany’s Technical University Berlin, told me.
For Earth, intelligent life took roughly 4 billion years to evolve, says Kane. But he notes that there is a large range of stellar ages for the exoVenus host stars he and colleagues have discovered. And they lie mostly in the one to ten billion-year range. So, Kane thinks that many of these Venus-like planets will be more than 4 billion years old and could well have evolved intelligent life on them at some point.
ExoVenuses that were paired with lower mass stars would have had more time to play with before their parent stars robbed them of a habitable zone. That’s because as Kane points out, high-mass stars evolve very quickly and simply do not spend enough time on the hydrogen-burning main sequence. Thus, an exoVenus’ time in the habitable zone when orbiting a high-mass star should be relatively short-lived.
Venus has what is known as a “stagnant lid” surface, meaning that it’s not broken up into plates like earth’s surface but instead is mostly a solid surface ruptured by areas of volcanism, Kane explains. This means that Venus underwent a catastrophic resurfacing event about a billion years ago. This likely largely erased any record of its surface conditions.
“We can gain some insight from evidence in the atmosphere, particularly from isotopes that trace the presence of possible liquid water on the surface, said Kane. “But it is difficult to know for certain.”
This uncertainty, he says, has led to a lot of speculation regarding our own Venus’ past habitability, with some researchers contending that the planet may have had a temperate surface as recently as a billion years ago.
But the Sun’s increased luminosity soon caught up with our sister planet.
Would alien civilizations living on their own Venus-type planets be helpless in staving off this inevitable climate change? Or could they geo-engineer their way out of an eventual runaway greenhouse?
If so, Kane says it would need to be a coordinated planet-wide effort. He says this would entail not just extracting carbon dioxide (CO2) from a Venus-like atmosphere, but also storing it in a way that is sustainable.
Earth’s own thermostat is sensitive to changes in temperature and CO2 levels and adjusts itself accordingly over million-year time scales, Kane explains. If geoengineering is used to compensate for breaking that delicate balance, he says then that civilization will have to take over to maintain the planet’s thermostatic cycle.
Any disruption to their geoengineering efforts would immediately cause the planet to return to its journey towards a runaway greenhouse. So, they would see it coming, Kane says. But he says by that stage it’s probably much easier to leave the planet than manually take over its geological processes.
Kane’s hope is that once things get desperate, such threatened alien civilizations would be able to move to another suitable terrestrial planet a bit farther out from their parent star.
I’m a science journalist and author of “Distant Wanderers: the Search for Planets Beyond the Solar System” who writes about over-the-horizon technology, primarily astronomy and space science. I’m a former Hong Kong bureau chief for Aviation Week & Space Technology magaz…