Panspermia

Cosmic Dust Could Spread Life from World to World Across the Galaxy

Does life appear independently on different planets in the galaxy? Or does it spread from world to world? Or does it do both?

New research shows how life could spread via a basic, simple pathway: cosmic dust.

One thing scientists have learned in the past few decades is that life on Earth might have had an early start. The Earth is about 4.53 billion years old, and some evidence shows that simple life existed here at least 3.5 billion years ago. Some evidence suggests life was here even before that, only about 500 million years after Earth formed when it had cooled down. The life would have been extremely simple, but it may have been there.

But life may not have originated here. Researchers question if there was enough time for life to appear spontaneously in early Earth conditions.

New research examines the idea that cosmic dust could be responsible for spreading life throughout the galaxy by panspermia. Life arose elsewhere, and was delivered to the young Earth. This is not a new idea, but in this work, the author calculates how quickly it could happen.

Artist’s impression of Earth in the early Archean with a purplish hydrosphere and coastal regions. Even in this early period, life flourished and was gaining complexity. Credit: Oleg Kuznetsov

The research is titled “The Possibility of Panspermia in the Deep Cosmos by Means of the Planetary Dust Grains.” The sole author is Z.N. Osmanov, from the School of Physics at the Free University of Tbilisi in the country Georgia. The paper is in pre-print and hasn’t been published yet.

No matter how much we ponder and investigate the origins of life, we don’t know how it starts. We have an idea about the type of environment that could spawn it, but even that is an idea obscured by billions of years. “It is clear that the main problem is the origin of life or abiogenesis, the details of which are still unknown to us,” Osmanov writes.

But it started somehow. Leaving life’s original appearance aside, for now, Osmanov moves on to how it could spread.

“By obtaining the assumption that planetary dust particles can escape from the gravitational attraction of a planet, we consider the possibility for the dust grains to leave the star’s system by means of the radiation pressure,” Osmanov writes.

The idea that life itself could travel through space on comets and asteroids is familiar to many people. When these objects crash into planets, the thinking goes, hitchhiking life is delivered, and if there’s a niche it can exploit, it will. But how could simple dust accomplish the same thing?

This artistic conception illustrates large asteroids penetrating Earth’s early atmosphere. Credit: SwRI/Dan Durda/Simone Marchi

For dust to carry life, it must originate from a planet that hosts life. This can happen in specific circumstances. Research shows that dust particles from Earth in the planet’s high-altitude atmosphere can scatter against cosmic dust grains. A 2017 paper in the journal Astrobiology showed how hypervelocity space dust can interact with this Earth dust, creating powerful momentum flows. A small fraction of the planetary dust particles can be accelerated enough to escape the planet’s gravity.

Once free of its planet’s gravity, dust is then at the mercy of stellar radiation pressure. “If a similar scenario takes place in other systems, the planetary dust particles, being already free from the planet’s gravitational field, might escape from the star’s system by means of the radiation pressure and initial velocity, spreading life into the cosmos,” Osmanov explains.

Life would need to be very hardy to survive on a dust grain as it travels through interstellar space. It would have to avoid hazards like radiation and heat. If life itself couldn’t do it, maybe complex molecules that lead to life could. If we grant that it’s possible, the next question concerns how quickly it could spread.

“It has been shown that, during 5 billion years, the dust grains will reach 105 stellar systems, and by taking the Drake equation into account, it has been shown that the whole galaxy will be full of planetary dust particles,” Osmanov explains.

Osmanov points to other research into panspermia and how it could happen in our neighbourhood of the galaxy. “In particular, it has been pointed out that, by means of the solar radiation pressure, small dust grains containing live organisms can travel to the nearest solar system, Alpha Centauri, in nine thousand years,” Osmanov writes. Our powerful rockets, like the Space Launch System and the Falcon Heavy, would take over 100,000 years to make the journey.

Panspermia is the idea that life is spread throughout the galaxy, or even the Universe, by dust, asteroids, comets, and even minor planets. Credit: NASA/Jenny Mottor

It’s an intriguing idea. Osmanov calculates that a significant number of dust grains will survive interstellar space with life or complex molecules intact. But his thinking hits a bit of a speed bump at one point. He takes a bold step beyond our current knowledge when he writes, “On the other hand, it is natural to assume that the number of planets with at least primitive life should be enormous.” It might be a natural assumption, but there’s little evidence that it’s true. It’s conjecture, stimulating conjecture, but conjecture nonetheless.

Working with a statistical approach to the Drake Equation, Osmanov writes that the number of planets that developed life is on “the order of 3×107.

“This value is so huge that if dust particles can travel a distance of the order of several hundred light years, one can conclude that the MW, with a diameter of 100,000 light-years, should be full of complex molecules distributed throughout the whole galaxy,” Osmanov explains. “Even if we assume that life is destroyed during this time, the vast majority of complex molecules will remain intact.”

Standing beside the Milky Way. If Osmanov is correct, the entire galaxy is full of planetary dust. Credit: P. Horálek/ESO

It’s very interesting work. But the frustrating thing about this entire topic is that we still don’t know how life appears or how often it appears. So, all of our thought experiments and calculations, including Osmanov’s, have a stubborn nugget of the unknown at the centre.

If we’re fortunate to find solid evidence of life on Mars, for instance, then this type of research and the conversations it spawns will take on a new lustre. But for now, Osmanov’s work and similar work by other researchers leaves us in a funny spot: we can imagine and calculate how life can spread and how far and how fast.

Osmanov claims that the number of planets with primitive life is enormous. We don’t know that. Planets are extraordinarily complex, and there are a bewildering number of variables. Even if there are an enormous number of planets with primitive life, many of them will be more massive than Earth. Will dust particles carrying life or complex organic molecules be able to escape the gravitational grasp of super-Earths, for example?

This research shows how life, or at least its building blocks, could escape from planets and survive the interstellar journey to another world. If it’s true, and panspermia can account for life appearing on Earth so soon after it formed and cooled, then it changes our understanding of our origins and even the rest of the Universe.

But we don’t know how true it is, and we still don’t know how it starts.

Evan Gough

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