Chemistry is helping us figure out how life got started on Earth and is giving us molecules to look for on other planets. In this episode of Reactions, we break down what “life” is and how likely we are to find it out in the cosmos.
There are over 100 proposed definitions of life.
Some of them say life needs cells, others focus on certain traits, like the ability to adapt to an environment, use energy, and to reproduce.
Mules, which are what you get when you cross a donkey and a horse — they have cells, they use energy, but they can’t reproduce. So are they not alive?
[Is there life on Maaaarrrr…].
My point here is what constitutes life is kind of complicated, and even more complicated when we’re trying to figure out what indicates life on other planets.
It’s probably not going to look like this.
I’m going to have so many Trekkies (Trekkers?) coming after me in the comments. Sorry.
NASA is the main US agency. That’s looking for life beyond Earth, and their definition of life is a self-sustaining chemical system capable of Darwinian evolution.
What does that mean?
Darwinian evolution says that organisms change over the course of generations in response to their environment, eventually giving rise to new species.
In the case of us humans, we evolved from other primates, which evolved from other mammals, and if you go back far enough, you’ll reach single-celled organisms.
The ability to evolve from something like that to a full-fledged human has to do with our genes.
And that’s where we get into the chemical part of NASA’s definition.
Life can evolve because we have this relatively stable genetic code that’s passed down from one generation to the next, changing ever so slightly over time.
We use a mix of DNA, RNA, and protein to maintain that self-sustaining chemical system, which means we’re able to do things like metabolize food and heal a wound and reproduce — passing on our DNA.
Will the life we find on other planets use genetic material like ours, or will it have an entirely different system that allows it to reproduce and evolve?
I don’t know how to shrug.
But what we’re learning on Earth is still giving us an idea of what chemical elements to look for on other planets. Carbon, nitrogen, hydrogen oxygen, sulfur, and phosphorus are all essential to make the molecules that allow us to be here right now — things like DNA, RNA, fats, and the amino acids that make up proteins.
Beyond their importance for life today, what’s incredible about these elements is that, with a little bit of help, they could have been all that was needed to get life started in the first place, around 4 billion years ago.
Back in 1953, chemists Stanley Miller and Harold Urey tested out the theory that early Earth was a hydrogen-rich environment, much like Jupiter’s. And that something happened that allowed simple compounds to react and create more complex ones.
They sealed up a mixture of gases and water inside a connected system of flasks. Then they heated them and zapped them with electricity, mimicking lightning. After a week they found amino acids in the water.
Okay, let’s not get ahead of ourselves.
Amino acids cannot reproduce and evolve on their own, but they are what make up proteins. So this was still a huge conceptual step toward something living.
Scientists think there’s a good chance that something similar is happening, or are already happened, on other planets.
We’ve found amino acids on meteorites that have fallen to Earth, and we’re collecting surface samples from asteroids and other bodies to look for traces of the chemistry of life.
In 2013, the Curiosity Rover detected organic compounds in Martian soil. So maybe, billions of years ago, something similar to what happened here on Earth happened on Mars.
Now the Perseverance is up there too it’ll be really cool to see how much more researchers might find.
The instruments onboard aren’t designed to detect really complex molecules, but a decade or so from now those samples will make it back to Earth to be analyzed by more advanced equipment, maybe by some of the kids that just watched the Perseverance landing.
It’s so cute.
But for far-off planets that we can’t get rovers to or get samples back from, scientists are using light to try and figure out what chemicals might be in their atmospheres and, based on that, predict if there could be life.
Here’s how that works: When a planet passes in front of its star, a tiny bit of light from that star travels through the planet’s atmosphere. Chemicals in the atmosphere dictate which wavelengths are absorbed and which ones are not.
By measuring these changes in the star’s light using a powerful telescope, researchers can predict what chemicals are present.
That’s how scientists found phosphine gas in Venus’s atmosphere back in the fall 2020, or at least how they thought they found phosphine.
Those findings have been hard to replicate and are still being debated, but when they published that paper, some scientists got really excited because on Earth phosphine gas is usually produced by bacteria, which are living.
So maybe there is a Venus form of bacteria that are producing phosphine, but maybe it’s coming from something totally unrelated and non-living.
Another reason the whole life on Venus thing seems unlikely is that Venus is incredibly dry. On Earth, the chemistry that keeps life going couldn’t happen without water.
I spoke with Dr. Nicholas Hud, who is working with molecules and environments believed to have been present on early Earth, to understand how molecules can combine and give rise to things that can reproduce and evolve.
And he’s found that molecules produced in ways similar to what Miller and Urey did can self-assemble in water and form structures that look something like RNA.
So it makes sense that scientists think that planets with water on them are most likely to support life. And for a planet to have flowing water. It needs to be at the right distance from its star.
If a planet is too close to its star, it’ll be so hot that any water will evaporate. If it’s too far from its star, it will be so cold that any water would freeze over.
The distance from its star that allows an orbiting planet to have flowing water is called the habitable zone or the Goldilocks range. It’s not too close, not too far, just right.
Earth is the only planet in our solar system considered within that range. But there are moons and what are considered dwarf planets like Pluto that could have oceans under their frozen surfaces, which could allow microorganisms and fish-like creatures to live there.
So maybe we’re not totally alone in our solar system.
Even if we are, I still have good news. Since 1992, over 4,000 exoplanets, which are planets outside of our solar system, had been found. And some of them seem kind of promising.
Within my lifetime, the discovery of exoplanets has had the greatest impact on my view of how likely it is that life exists elsewhere in the universe. We now know of thousands of other planets and some of them look like they could support life.
Looking for other planets that might be within their star’s habitable zone seems like our best shot at detecting life. At least for now. Dr. Hud is confident that it will happen.
As time goes on, we’re only going to understand more and more about the chemicals that are required to get life started. I also think that our ability to detect and focus on the chemical species most likely associated with life is going to continuously improve.
I don’t know if it will happen in my lifetime, but I feel very confident that human beings will eventually have the knowledge and the tools that will result in the detection of life on another planet in our galaxy.