Imagine a museum containing every type of life in the universe. What strange things world such a museum hold? What is possible under the laws of nature?
To have any hope of finding any alien life, we have to know what to look for. But where do we begin? How do we narrow down a seemingly infinite set of possibilities?
There's one thing we know for sure nature will have to play by her own rules. No matter how strange alien life might be, is going to be limited by the same physical and chemical laws that we are.
6 CO2 + 6 H2O + LIGHT ---> C6H12O6 + 6 O2
On top of this, each alien environment will further limit what kind of life forms can evolve there. despite these natural boundaries, the possibilities are staggering to imagine. Trillions of planets, each a unique cauldron of chemicals, undergoing their own complex evolution. To guide our thinking, this museum of alien life will be divided into two exhibits.
1. Life as we know it, home to beings with bio-chemistries like ours.
2. And Life as we don't know it, home to beings that challenge our concept of life itself.
Before we venture too far into the unknown, we have to ask ourselves. What if alien life is more like us than we think?
EXHIBIT I- LIFE AS WE KNOW IT-
If there's one feature that unites us with the other specimens in this museum, it's CARBON (atomic weight 12.011). Carbon is ubiquitous, it's one of the most common elements in the universe, and it's very good at forming at large stable molecules. Carbon has the rare ability to form four-way bounds with other elements and bind to itself in long stable chains enabling the formation of huge complex molecules. This versatility makes carbon the centerpiece in the molecular machinery of life. And the same carbon compounds that we use have been found far from Earth, clinging to meteorites ( GLYCINE) to floating in far off clouds of cosmic dust. The building blocks of life, drifting like snow through the universe. And if alien life has selected other carbon compounds for the biochemistry, they will have plenty to choose from. Scientists recently identified over a million possible alternatives to DNA all carbon-based. If we ever discover other carbon-based life forms, we will be fundamentally related. They will be our cosmic brother. But would they look anything like us?
If they hail from Earth-like planets, we could share even more in common, that just our biochemistry. On Earth, certain features like eyesight, echolocation, and flight have evolved multiple times, independently in different species. This process of convergent evolution could extend to alien planets like Earth, where creatures share similar environmental pressures. It's no guarantee, but there could be certain universalities of life, the greatest hits of evolution on repeat across the universe. Each feature would be a tune with its local environment. Dimly lit planets would produce huge eyes to suck in extra light, like nocturnal mammals. The existence of other human-like organisms seems unlikely given the long convoluted chain of events that produced us. But we can't rule it out. If just one in every 100 trillion Earth-like planets produced a human-like form there could still be thousands of creatures lie us out there. But in reality, we are more likely to find something lower on the food chain.
Convergent evolution is also rampant in plant life and C4 photosynthesis has arisen independently over 40 times would alien plants look like ours or something else entirely?
On Earth, plants appear green because they absorb the other wavelengths in the Sun's light spectrum. But stars come in many colors, alien plants would evolve different pigments to adapt to their sun's unique spectrum. Plants feeding off hotter stars could appear redder, by absorbing their energy-rich bluer light. Around dim Red Dwarfs stars, vegetation could appear black, adopted to absorb all visible wavelengths of light. Earth itself, may have once appeared purple, due to a pigment called retinal, that was an early-precursor to chlorophyll. Some think that retinal's molecular simplicity could make it a more universal pigment. If so, we may find that purple is life's favorite color. But the color of alien vegetation is more than just a curiosity, it's chemical information that could be seen from light-years away. Earth plants leave a signature bump, in the light reflected off our planet. Finding a similar signal from another world could point the way to alien vegetation. Perhaps this will be our first glimpse at alien life, a vibrant hue cast by a distant world.
But the biggest influence on life won't be its host star; it will be its home planet.
What happens when you change the day-length, the tilt, the shape of the orbit, the gravity of a planet?
Planets with long, elliptical orbits world see drastic seasons. There could be worlds did that appear dead for thousands of years, then suddenly spring to life. Most of the rocky planets discovered so far have been massive "Super-Earth".
GJ357D - Distance:~ 31 Light Years. Mass:~7X Earth. Temperature:~ -53 degree C
How would life evolve in these worlds? In the seas, gravity may not matter much at all. A high-gravity planet isn't high-gravity all over, If you're in the sea, and that's where all life starts, there's very nearly no gravity, cause you're much the density as the stuff around you. It's when the animals come out on land, that they feel the gravity. High g-force would necessitate large bones and muscle mass, in complex life on land. They would also demand a more robust circulatory system. And plant life could be stunted by the energy cost of carrying nutrients under stronger gravity. Low-gravity planets would more easily lose their atmospheres to space, and lack of magnetic field to protect from cosmic rays. But the smaller world could be home to secret oases; huge cave systems that provide hide-outs for life. With steadier temperatures and protection from cosmic rays, life could thrive underground on planets with deadly surfaces.
The smallest possible habitable planets are estimated at 2.5% of Earth's mass. If surface life does evolve on these worlds, it could be a sight to behold. Plantlife could grow to tower heights, able to carry nutrients higher, at lesser gravity. And without the need for bulky skeletons and muscle mass, animals could have body types, that boggle the mind.
Despite our eager imagination, large complex lifeforms are probably a cosmic rarity. Here on Earth, it took three billion years for evolution to produce complex plant and animal life. Simple organisms are hardier more adaptable and more widespread. The largest collection in the museum of alien life would likely be the "Hall of Microbes". Yet finding even the tiniest alien microbe would be a profound discovery. And bite-sized life could leave a big footprint. Like stromatolites on Earth, layers of microbes could build up into huge rock mounds overtimes. Leaving behind eery structures. And big enough numbers some alien bacteria could leave a distinct biosignature, by exhaling gases that wouldn't coexist naturally: like oxygen and methane.
There are ways to make oxygen without life; There are ways to make methane without life. But to have them in the atmosphere together? Is almost impossible unless you've got biology making those gases at the surface. And it would have an imprint on the planet's spectrum of colors.
Next-generation space telescopes could find a signal like this, on a world not far from home. The closest sun-like star with an Earth-like exoplanet in the habitable zone is probably only 20 light-years away and can be seen with a naked eye. But there may be an even easier target to aim for than tiny Earth-like planets.
We will discuss this in the second part of this blog.
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