Panspermia is the conjecture that life can propagate from one planet to another. One version of panspermia involves Solar System bodies grazing the Earth's atmosphere, picking up microbes, and being ejected from the Solar System. A 2019 study suggested that between 2 and 70 long-period comets and interstellar objects were expected to have undergone such a process over the lifetime of the Solar System, with the potential for such objects to be gravitationally captured by other star systems.Since then the Desert Fireball Network has reported the detection of a roughly 30 cm object that object on 7 July 2017 that reached a minimum altitude of about 58:5 km in the Earth's atmosphere during a 90 second grazing event, and was subsequently diverted from an Apollo-type orbit into a Jupiter-Family 'comet; orbit, making it likely to be ejected from the Solar System during a future gravitational encounter with Jupiter. This detection represents a new class of objects that can pick up life in the Earth's atmosphere before being ejected from the Solar System: rocky, inner-Solar System bodies that are scattered into Jupiter-crossing orbits after their grazing interactions with Earth and are subsequently ejected. These objects are important, since based on their characteristically higher densities relative to icy, outer-Solar System bodies, they can survive passes through the Earth's atmosphere at signi cantly smaller sizes.
In a paper published on the arXiv database at Cornell University Library on 7 January 2020, Amir Siraj and Abraham Loeb of the Department of Astronomy, Harvard University, consider the likelihood of life-bearing Solar System bodies being captured by exoplanetary systems.
Siraj and Loeb estimate that a 30 cm object grazing the atmosphere for 90 seconds at a speed of 15:5 km per second should collect 104 microbial colonies during its trip through the atmosphere, and that the collected microbes can likely survive the accelerations associated with transfer onto the grazing object. Furthermore, they estimate that polyextremophilic Bacteria such as Deinococcus radiodurans could be expected to survive for arounf 100 000 years on the surface of an extraterrestrial object with minimal radiation shielding. For the sake of convenience, they assume that such objects would be likely to capture one colony of polyextremophile micro-organisms.
The Desert Fireball Network uses about 50 cameras to cover 2 500 000 km² of the sky, or 0.05 of the Earth's surface. While the Desert Fireball Network only had 4 cameras in 2007, Siraj and Loeb conservatively estimate the relevant observation parameters for the 2017 detection to be the result of a survey that lasted 10 years and covered 0.05 of the Earth's surface. This leads to a rate estimate of 20 such 30 cm objects per year penetrating the Earth's atmosphere and eventually likely being ejected from the Solar System. Over 3 billion years this translates to about 60 billion ejected objects.
Triangulated luminous atmospheric trajectory for the 7 July 2017 event, as seen over Western Australia and South Australia. The triangulation method used involves fitting the line-of-sight observations directly to the meteoroid’s dynamic equations of motion, thereby dropping any straight-line assumptions. The event lasted 90 seconds, initially hitting the atmosphere at 4.6° and covering over 1300 km through the atmosphere. The white rays indicate the line-of-sight measurements from each Desert Fireball Network observatory, whereas the black path marks the triangulated trajectory based on the observations of the fireball. Shober et al. (2020).
Breakup is not considered to be a signi cant factor in Siraj and Loeb's analysis for typical rocky compositions, although they note that the 7 July 2017 meteor experienced a breakup event, which may be indicative of a composite makeup. Siraj and Loeb assume that an object with a diameter of 30 cm and a density of 3500 kg per m³ travelling at a velocity of 15.5 km per second will decelerate by 4 km per second over a 90 second journey through the Earth's upper atmosphere. They calculate that an object much smaller than that would be slowed signi cantly by friction, although this estimate depends on the shape and mean density, and therefore do not consider objects smaller than 30 cm.
Siraj and Loeb next consider the possibility of such a object reaching another stellar system. As tight stellar binary systems have the largest capture cross sections, they consider only such systems, and estimate the capture cross section of a stellar binary system of two solar-mass stars (such as Alpha Centauri A & B, although as the stars are moving relative to one-another as they orbit the galaxy, it should not be assumed that the stars currently closest to us have been so throughout the history of the planet) as a function of relative speed of the system and the object, assuming that the orbital speed of the bound orbit is near zero (which is likely to be the case for an ejected object relative to the movement of the stars).
The local number density of solar-type stars is estimated to be 0.016 per cubic parsec, of which about 20% are in equal-mass binary systems with a separation of less than 10 AU (10 times the distance between the Earth and the Sun). Given their assumption that ejected objects mirror the Sun's speed relative to the local standard of rest, Siraj and Loeb estimate that each object encounters a binary system every 500 000 years.
Siraj and Loeb therefore estimate that about 200 life-bearing Solar System bodies have been captured by exoplanetary systems over thelifetime of the Solar System while there still may have been living microbes on the objects.
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