Exoplanet hunters struck gold earlier this year with the discovery of seven rocky bodies orbiting around dwarf star TRAPPIST-1, a find that both raised hopes, and provided a new target, for understanding if life exists elsewhere in the cosmos. Now, a new Harvard paper suggests this populous planetary system could also test the ease with which life can hop between planets, and perhaps even end uncertainty over our own status as true Earth-lings.
This artist’s impression displays TRAPPIST-1 and its planets reflected in a surface. Image credit: NASA / R. Hurt / T. Pyle.
For several decades, proponents of interplanetary exchange of life have imagined a broadly similar scenario — a meteorite or asteroid impacts a life-supporting planet throwing up rocks containing living or almost living stowaways.
Traveling across interplanetary space some of this material eventually impacts a neighbor planet where life, or its seeds, are successfully introduced.
Known as panspermia some suggest this contamination mechanism may be the most common method for life’s distribution across the Universe, and even raises the possibility that all life on Earth might have originated elsewhere.
However, the panspermia model is plagued by uncertainties. The intense heat generated by impacts could be a roadblock, as could the high concentration of cosmic and UV rays found across interplanetary space. Anything hitching a ride would need to survive a long time — several millions years between Earth and Mars.
All these uncertainties remain today despite advanced simulations and modeling.
Perhaps there is an easier way? Could we prove panspermia’s validity by finding evidence of it happening elsewhere?
“If the same biosignature gases are detected on planets in a single system, or if the spectral feature of vegetation occurs at the same wavelength, this could be a ‘smoking gun’,” says Manasvi Lingam from the Harvard-Smithsonian Center for Astrophysics, who believes such observations may fall within the capabilities of future planned telescopes like the Large UV/Optical/Infrared Surveyor (LUVOIR).
So where to look? Lingam thinks TRAPPIST-1 might not be a bad place to start.
The distance between the three planets confirmed to orbit within the system’s narrow habitable zone is 50 times less than that from Earth to Mars.
Lingam and his colleague Avi Loeb reasoned this should benefit panspermia’s chances both by increasing the amount of material exchanged, and reducing the travel time through dangerous interplanetary space.
“Even if one believes that the probability for life, as we know it, is small, the dice was rolled three times in the TRAPPIST-1 system leading to a higher chance of success,” says Loeb.
To get firmer answers, the two ‘did the Math’ – or at least some of it.
In a new paper published on the arXiv.org site, Lingam and Loeb use a simple model of the mechanics of the TRAPPIST-1 habitable zone to answer two questions: if debris is ejected from one planet, what is the probability it will be captured by a neighbor, and what would be the average travel time for this journey?
“These are two quantifiable mechanical factors that have significant biological implications,” says Lingam.
Their model suggests panspermia is several orders of magnitude more likely to occur in the TRAPPIST-1 system than the Earth-Mars system. In fact, they conclude that the more congested planetary orbits of planetary systems around most of these M-dwarf stars (the most common stars in our Galaxy), means the fraction of rock leaving one planet and hitting another could be as much as 1,000 times higher than between Earth and Mars.
For Loeb and Lingam, the close proximity of the TRAPPIST-1 planets was not just common to other M-dwarf star systems, but also reminiscent of an environment on the Earth, namely islands, which are subject to their own ‘immigration.’
Drawing on models of island biogeography and theoretical ecology, they suggest it might not be just the likelihood of panspermia that increases in around M-dwarfs but also the number of species potentially transferred, increasing biodiversity.
There are, however, limitations of this simple model.
Caleb Scharf, Director of Astrobiology at Colombia University, cautions against assuming the scale of impacts would be the same as we see in our solar system.
“In a system like TRAPPIST-1 where planets are so close-packed, there may not be a population of long-term asteroids or short period comets to provide the impacts needed to eject material and allow transfer between planets.”
The model also says nothing about the chances of life started in the first place, and is only able to quantify the fraction of rocks that would impact a particular planet, not the total number. Finally, it says nothing of the complex chemical biology that would underpin whether life would survive the impact of the travel journey.
Despite this, a confirmation of TRAPPIST-1’s relatively optimal conditions for panspermia could have significant implications in the future.
“If life is confirmed within this system but we find no evidence that panspermia has transferred it to another planet, it would be hard to envisage it happening in a far less suitable system like our own,” says Lingam, dealing a blow to theories of our own Martian origin.
Whilst any future discovery of panspermia around TRAPPIST-1, or another M-dwarf system, might seem like coming ‘after the Lord Mayor’s show,’ considering it would follow the realization that we are not alone in the Universe, it would still be revolutionary in its own right.
Vindicating this life spreading mechanism would fundamentally change our understanding of how life is distributed around the cosmos and change completely any debate around our own extra-terrestrial origins.
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Manasvi Lingam & Abraham Loeb. 2017. Enhanced interplanetary panspermia in the TRAPPIST-1 system. PNAS, submitted for publication; arXiv: 1703.00878
