Stars emit powerful flares that can be deadly for any burgeoning life on nearby planets. Images from spacecraft that monitor the Sun show these flares in glorious, horrifying detail. But the flares from the Sun are mere nuisances compared to some stars. Some stars produce catastrophic superflares, which can be tens of thousands of times more energetic than the Sun’s. That much energy can sterilize a planet’s surface.
But new research shows that a certain amount of flaring activity on the Sun could’ve been beneficial. It could’ve kick-started life on Earth.
Life can’t get started without building blocks. The building blocks of life include amino acids like glycine and leucine. Somehow, those acids were assembled out of more basic chemicals in Earth’s early history, probably back during the Hadean eon. For that to happen, there had to be a source of energy.
Back in 1952, an American chemist named Stanley Miller gained recognition by showing that lightning could’ve acted as the energy source. In the well-known Miller-Urey experiment, Miller put water, ammonia, nitrogen, and methane into a sealed, sterile flask and applied an electric arc to simulate lightning. The groundbreaking experiment produced over 20 amino acids. (There are hundreds of them in nature, but only 22 are in the genetic code.)
This led to an overall acceptance in the scientific community that life can originate from chemicals and energy under the right conditions. It led to a host of similar experiments. Though, of course, none of them actually produced life.
It takes energy to form those molecules, and for a long time, scientists have pointed to lightning as the energy source that created amino acids.
The Miller-Urey experiment was a breakthrough, and when it was published, the results reverberated throughout the scientific community. “That was a big revelation,” said Vladimir Airapetian, a stellar astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “From the basic components of early Earth’s atmosphere, you can synthesize these complex organic molecules.”
But as time marched on, our understanding of Earth’s early atmosphere evolved. The modern scientific consensus is that methane and ammonia were far less abundant in the Hadean atmosphere. Instead, carbon monoxide and molecular nitrogen were the dominant chemicals, and they take more energy to break down than methane and ammonia. So the lightning hypothesis became less convincing.
Scientists wondered what other sources could’ve provided the energetic kickstart life on Earth needed to get going. Some suggested meteor impacts, while others cited ultraviolet energy from the Sun.
But data from the Kepler mission, even though its job was to hunt for exoplanets, has suggested another energy source. Airapetian is the co-author of a new study based on Kepler observations that says solar flares from the Sun could’ve provided the energy.
The paper is “Formation of Amino Acids and Carboxylic Acids in Weakly Reducing Planetary Atmospheres by Solar Energetic Particles from the Young Sun.” It was published in the journal Life, and the lead author is Kensei Kobayashi. Kobayashi is a professor of chemistry at Yokohama National University.
The Kepler spacecraft monitored over half a million stars and looked for exoplanets transiting in front of them. As it did so, it acquired an enormous amount of data on the stars themselves. These stars represented all of the stages in the lifecycle of stars. All that data suggested that our own Sun was much dimmer in its past than it is now, up to 30% dimmer during Earth’s first 100 million years. In 2016, Airapetian published a study presenting that result.
But luminosity wasn’t the only thing different about the Sun back then. It also produced powerful flares more frequently than it does now. In our age, the Sun emits powerful flares about once per century, depending on how powerful flares are defined. But younger stars emit more frequent powerful flares, once every few days, according to some studies. So that means that the Hadean Earth would’ve been regularly bombarded with energetic particles travelling near the speed of light. This is a virtual shower of energy that went on for a long time. Could it have provided the energy life needed to get going? Airapetian wanted to find out.
“As soon as I published that <2016> paper, the team from the Yokohama National University from Japan contacted me,” Airapetian said.
Professor Kobayashi has been working on Earth’s prebiotic chemistry for decades. He was interested in what role galactic cosmic rays (GCRs) may have played. GCRs come from outside the Solar System, primarily from elsewhere in the Milky Way. Studying the role GCRs play requires a particle accelerator, not a piece of equipment that everyone has easy access to. But Kobayashi had been fortunate.
“Most investigators ignore galactic cosmic rays because they require specialized equipment, like particle accelerators,” Kobayashi said. “I was fortunate enough to have access to several of them near our facilities.” With some preparation, Kobayashi’s experimental setup could put Airapetian’s ideas to the test.
Lead author Kobayashi, co-author Airapetian, and their colleagues performed a series of experiments to test the idea that SEPs from solar flares could produce amino acids. They prepared mixtures of carbon dioxide, methane, nitrogen, and water in various ratios. They then applied three energy sources to the mixtures: spark discharges to simulate lightning, UV irradiation to simulate the Sun’s UV output, and proton irradiation to simulate particles from solar flares. They also varied the levels of methane in the mixture since it’s a critical chemical, and scientists are uncertain exactly how much was present.
This figure from the study shows the results of the spark experiment. Line (c) represents the solution with higher amounts of methane and clearly shows the production of amino acids. Image Credit: Kobayashi et al. 2023
In the lightning simulation, amino acids only formed when the chemical mixture contained at least 15% methane. The UV simulation produced no amino acids, even when the mixture was 50% methane. In the proton irradiation experiment to simulate SEPs, even mixtures with very low levels of methane produced amino acids. This is important because even though scientists are uncertain of the exact amount of methane in the early atmosphere, they think it was low.
This figure from the study shows results from the proton irradiation experiments. Lines (a) through (f) represent increasing ratios of methane in the mixture, from a low of 0 to a high of 0.5. Glycine, the simplest stable amino acid, is prominent. Image Credit: Kobayashi et al. 2023
“And even at 15% methane, the production rate of the amino acids by lightning is a million times less than by protons,” Airapetian said. But things were different when it came to carboxylic acids, another critical building block that is a precursor to amino acids. The experiments produced carboxylic acids in 0% methane mixtures by both proton irradiation and spark discharges.
For the team of researchers, these results are pretty clear. “Hence, we suggest that GCRs (Galactic Cosmic Rays) and SEP (Solar Energetic Particle) events from the young Sun represent the most effective energy sources for the prebiotic formation of biologically important organic compounds from weakly reducing atmospheres.”
Illustration of what the Sun may have been like 4 billion years ago from the surface of a barren planet. The young Sun was more magnetically active when it was younger and produced more frequent, powerful flares. Credit: NASA’s Goddard Space Flight Center/Conceptual Image Lab
They go even further in their analysis by pointing out that SEPs were far more prevalent than GCRs on early Earth. “Since the energy flux of space weather, which generated frequent SEPs from the young Sun in the first 600 million years after the birth of the solar system, was expected to be much greater than that of GCRs, we conclude that SEP-driven energetic protons are the most promising energy sources for the prebiotic production of bioorganic compounds in the atmosphere of the Hadean Earth.”
The authors say that their work is important when it comes to understanding early Earth and the eventual appearance of life. “Our study has important implications for the emergence of precursors of life in the early Hadean period of the Earth. This is consistent with recent studies suggesting that the basic conditions for the emergence of life were met as early as 4.4 billion years ago,” they write. At that time, they explain, the young Sun was a “particularly magnetically active young star,” and its frequent and powerful flares could’ve produced the SEP events that kickstarted life.
Other evidence also contra-indicates lightning as the energy source that kicked things off. Previous researchers like Stanley Miller of Miller-Urey fame supposed that lightning was as common on Hadean Earth as it is today. They envisioned warm ponds full of chemical mixtures being energized by lightning and creating amino acids and their precursors. But that image might not be real.
We’ve all seen the towering cumulonimbus clouds that foster thunder and lighting. They’re convection driven and typically climb as high as 39,000 feet. On rare occasions, they reach as high as 69,000 feet, maybe even higher. It takes a lot of heat energy for warm air to rise so high.
But the early Sun was dimmer by about 30% and didn’t heat the atmosphere as much. That means lightning may not have been common. “During cold conditions, you never have lightning, and early Earth was under a pretty faint Sun,” Airapetian said. “That’s not saying that it couldn’t have come from lightning, but lightning seems less likely now, and solar particles seem more likely.”
There may have been multiple sources for the building blocks of life on Earth. Research shows that some important chemicals can form in space and could have been delivered to young Earth by comets and other impactors. That possibility can’t be eliminated.
But for the authors, their results show that the amount of chemical building blocks created here on Earth could have played a greater role than what could’ve been delivered by impactors. “Our experimental results also suggest that endogenous production of amino acids on Earth via SEPs could have surpassed that of extraterrestrial delivery via impacts from comets and carbonaceous chondrites.” It also means it could happen on other planets.
While researchers are still uncertain about the exact functioning of solar flares, coronal mass ejections, and how everything works together to produce solar energetic particles, they know that Earth is directly in their path.
And while being directly in the path of powerful flares can be dangerous, it could also be what got everything started.