Toxic “Habitable” Worlds Could Be Havens for Alien Microbes

Don’t forget your spacesuit: Complex lifeforms, such as humans, would not survive on many of the worlds we thought would be interstellar tropical getaways


Worlds like Earth may be even rarer than we thought.

We live on a planet that provides the perfect balance of ingredients to support a vast ecosystem. This amazing world orbits the Sun at just the right distance where water can exist in a liquid state—a substance that, as we all know, is an essential component for our biology to function. Earth is also an oddball in our solar system, being the only planet where these vast oceans of liquid water persist on its surface, all enshrouded in a thick atmosphere that provides the stage for a complex global interplay of chemical and biological cycles that, before we industrialized humans came along, has supported billions of years of uninterrupted evolution and biological diversity.

Humans, being the proud intelligent beings that we profess to be, are stress-testing this delicate balance by pumping an unending supply of carbon dioxide into the atmosphere. Being a potent greenhouse gas, we’re currently living through a new epoch in our planet’s biological history where an exponential increase in CO2 is being closely followed by an increase in global average temperatures. We are, in effect, altering Earth’s habitability. Well done, humans!

While this trend is a clear threat to the sustainability of our biosphere, spare a thought for other “habitable” worlds that may appear to have all the right stuff for complex lifeforms to evolve, but toxic levels of the very chemicals that keep these worlds habitable has curtailed the possibility of complex life from gaining a foothold.

Welcome to the Not-So-Habitable Zone

Habitable zone exoplanets are the Gold Standard for exoplanet-hunters and astrobiologists alike. Finding a distant alien world within this zone—a region surrounding any star where it’s not too hot and not too cold for water to exist on its surface, a region also known as the “Goldilocks Zone” for obvious reasons—spawns a host of questions that our most advanced telescopes in space and on the ground try to answer: Is that exoplanet Earth-sized? Does it have an atmosphere? What kind of star is it orbiting? Does its system possess a Jupiter-like gas giant? These questions are all trying to help us understand whether that world has the Earthly qualities that could support hypothetical extraterrestrial life.

(Of course, there’s the debate as to whether all life in the universe is Earth-life-like, but as we’re the only biological examples that we know of in the entire galaxy, it’s the best place to start when pondering what biological similarities extraterrestrial life may have to us.)

The habitable zone for exoplanets is a little more complicated than simply the distance at which they orbit their host stars, however. Greenhouse gases, such as carbon dioxide, can extend the area of a star’s habitable zone. For example: If an atmosphere-less planet orbits beyond the outermost edge of its habitable zone, the water it has on its surface will remain in a solid, frozen state. Now, give that planet an atmosphere laced with greenhouse gases and its surface may become warm enough to maintain the water in a liquid state, thereby boosting its habitable potential.

But how much is too much of a good thing? And how might this determination impact our hunt for truly habitable worlds beyond our own?

In a new study published in the Astrophysical Journal, researchers have taken another look at the much-coveted habitable zone exoplanets to find that, while some of the atmospheric gases are essential to maintain a temperature balance, should there be too much of the stuff keeping some of those worlds at a habitable temperature, their toxicity could curtail any lifeforms more complex than a single-celled microbe from evolving.

“This is the first time the physiological limits of life on Earth have been considered to predict the distribution of complex life elsewhere in the universe,” said Timothy Lyons, of the University of California, Riverside, and director of the Alternative Earths Astrobiology Center.

“Imagine a ‘habitable zone for complex life’ defined as a safe zone where it would be plausible to support rich ecosystems like we find on Earth today,” he said in a statement. “Our results indicate that complex ecosystems like ours cannot exist in most regions of the habitable zone as traditionally defined.”

Toxic Limits

Carbon dioxide is an essential component of our ecosystem, particularly as it’s a greenhouse gas. Acting like an insulator, CO2 absorbs energy from the Sun and heats our atmosphere. When in balance, it stops too much energy from being radiated back out into space, thereby preventing our planet from being turned into a snowball. Levels of CO2 have ebbed and flowed throughout the biological history of our planet and it has always been a minor component of atmospheric gases, but its greenhouse effect (i.e. the atmospheric heating effect) is extremely potent and the human-driven 400+ppm levels are causing dramatic climate changes that modern biological systems haven’t experienced for millions of years. That said, the CO2 levels required to keep some “habitable” exoplanets in a warm enough state would need to be a lot more concentrated than the current terrestrial levels, potentially making their atmospheres toxic.

“To sustain liquid water at the outer edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today,” said lead author Edward Schwieterman, of the NASA Astrobiology Institute. “That’s far beyond the levels known to be toxic to human and animal life on Earth.”

In the blue zone: some of the known exoplanets that fall within the habitable zones of their stars may have an overabundance of CO (yellow/brown), at a level that is toxic to human life. Likewise, the more CO2 (from blue to white) will become toxic at a certain point. The sweet-spot is where Earth sits, with Kepler 442b (if it has a habitable atmosphere) coming in second [Schwieterman et al., 2019. Link to paper]

From their computer simulations, to keep CO2 at acceptable non-toxic levels, while maintaining planetary habitability, the researchers realized that for simple animal life to survive, the habitable zone will shrink to no more than half of the traditional habitable zone. For more complex lifeforms—like humans—to survive, that zone will shrink even more, to less than one third. In other words, to strike the right balance between keeping a hypothetical planet warm enough, but not succumbing to CO2 toxicity, the more complex the lifeform, the more compact the habitable zone.

This issue doesn’t stop with CO2. Carbon monoxide (CO) doesn’t exist at toxic levels in Earth’s atmosphere as our hot and bright Sun drives chemical reactions that remove dangerous levels of the molecule. But for exoplanets orbiting cooler stars that emit lower levels of ultraviolet radiation, such as those that orbit red dwarf stars (re: Proxima Centauri and TRAPPIST-1), dangerous levels of this gas can accumulate. Interestingly, though CO is a very well-known toxic gas that prevents animal blood from carrying oxygen around the body, it is harmless to microbes on Earth. So it may be that habitable zone exoplanets orbiting red dwarfs could be a microbial heaven, but an asphyxiation hell for more complex lifeforms that have cardiovascular systems.

While it could be argued that life finds a way—extraterrestrial organisms may have evolved into more complex states after adapting to their environments, thereby circumventing the problems complex terrestrial life has with CO2 and CO—if we are to find a truly “Earth-like” habitable world that could support human biology, these factors need to be considered before declaring an exoplanet habitable. And, besides, we might want to make the interstellar journey to one of these alien destinations in the distant future; it would be nice to chill on an extraterrestrial beach without having to wear a spacesuit.

“Our discoveries provide one way to decide which of these myriad planets we should observe in more detail,” said Christopher Reinhard, of the Georgia Institute of Technology and co-leader of the Alternative Earths team. “We could identify otherwise habitable planets with carbon dioxide or carbon monoxide levels that are likely too high to support complex life.”

Earth: Unique, Precious

Like many astronomical and astrobiological studies, our ongoing quest to explore strange, new (and habitable) worlds has inevitably led back to our home and the relationship we have with our delicate ecosystem.

“I think showing how rare and special our planet is only enhances the case for protecting it,” Schwieterman said. “As far as we know, Earth is the only planet in the universe that can sustain human life.”

So, before we test the breaking point of our atmosphere’s sustainability, perhaps we should consider our own existential habitability before its too late to repair the damage of carbon dioxide emissions. That’s the only way that we, as complex (and allegedly intelligent) lifeforms, can continue to ask the biggest questions of our rich and mysterious universe.

Not Just a Satellite: NASA’s Orbiting Carbon Observatory Fails (Update)

The fairing of the Taurus XL rocket upper stage failed to separate correctly, in this morning's OCO launch (Vandenberg Air Force Base/NASA)
The fairing of the Taurus XL rocket upper stage failed to separate correctly in this morning's OCO launch (Vandenberg Air Force Base/NASA)

In the early hours of this morning at 1:55am PST, a carbon dioxide monitoring mission was launched from Vandenberg Air Force Base in California. NASA’s Orbiting Carbon Observatory (OCO) was being carried into a 700 km polar orbit by a Taurus XL rocket. Unfortunately, 12 minutes and 30 seconds into the flight, the rocket upper stage suffered an anomaly, and the fairing failed to separate. Although it appears the rocket attained the desired altitude The vehicle did not attain the desired altitude and the $270 million satellite was doomed, trapped inside the the nose cone. The upper stage fairing was protecting the OCO as it ascended through the atmosphere; once in space it should have separated, peeled off and dropped away. That didn’t happen.
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Oh No! Rocket Launches Are Bad for the Environment? We’d Better Stay at Home Then

A small environmental impact, Falcon 1 launches in September 2008 (SpaceX)
A small environmental impact, Falcon 1 launches in September 2008 (SpaceX)

For every article written about the amazing advances in space vehicle technology, there are two negative comments about the pointlessness of space exploration.What’s the point?“, “We have war, famine, poverty and human suffering around the world, why invest billions on space?“, “What’s space exploration ever done for me?“. However, today, after I wrote a pretty innocuous article about the awesome SpaceX Falcon 9 rocket being hoisted vertically on the launchpad at Cape Canaveral, I get a comment (anonymous, naturally) starting off with, “This launch and others like it should be halted indefinitely until it’s carbon footprint and environmental impact can be accounted for.” The commenter then goes into something about making an environmental assessment, levying SpaceX’s taxes and setting up a board of environmental scientists. Oh please.

On the one hand, I’m impressed by this person’s spirited stand against environmental damage, carbon emissions and global warming, but on the other, this is probably one of the most misplaced environmentalism attacks I have seen to date. There are extremists on both sides of the “green” debate, but the last thing we need is an attack against the only answer we have to fight climate change. And that answer comes in the form of a cigar shaped polluter, blasting into Earth orbit; whether you like it or not, it is a necessary (yet small) evil…
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Climate Change, More Human Than We Thought

The Aztecs caused it too! (Getty)
The Aztecs, affected atmospheric carbon levels? (Getty)

Global warming anyone? I ask as I don’t want to upset anybody. Forget it, I’m going to talk about it anyway.

Climate change is an important subject worthy of debate. But for a debate to develop into something constructive, all sides need to have some scientific merit. Clearly, if we listen to Leo DiCaprio, Al Gore and the world’s carbon-cutting politicians, we might be led to believe we are damaging the environment… hell, we might even be warming the whole planet through carbon emissions! So, strip the Hollywood glamour and political spin from the debate, does the global warming debate have any science linking human activity with increased global temperatures?

In a new study, focusing on Central and South America, scientists have uncovered possibly one of the earliest recorded cases of human-induced climate change, possibly amplifying (or even triggering) the Little Ice Age in Europe throughout the 16th century and beyond…
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CryoSat-2, a Satellite that Looks Like a Shed, Doing Science in the Freezer

Cryosat-2. Resembles something Da Vinci would design (ESA)
Cryosat-2. Resembles something Da Vinci would have designed (ESA)

ESA Cryosat-2 is set for launch in 2009 and it is the second attempt at getting the technology into orbit. Back in 2005, the original CryoSat was lost after a rocket malfunction caused it to fall short of the desired orbit, but much like the Phoenix Mars Lander story (i.e. it rose from the ashes of the lost Mars Polar Lander mission, recycled spare parts and reassembled the robot), Cryosat will fly once more. So what makes this mission so important? Well, it will carry out an essential three-year survey, measuring the thickness of global ice sheets.

But why am I really mentioning it? Like many ESA missions, the designs of their satellites and robots are so cool, and Cryosat-2 is no different. From some angles it looks like a sturdy intergalactic battleship, from others it looks like it was painstakingly designed by Da Vinci. Sometimes it even looks like a flying shed. In my books, that’s one interesting satellite. The science isn’t bad either…
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