From Max K. Wallis and N. Chandra Wickramasinghe:

Re: Report: cells “from space” have unusual makeup (Sept. 8): The Princeton group’s study (Astrophysical J. Lett. ) ignores the solution we have found for the delivery problem – delivering solar system meteorites and life-bearing dust to other planetary systems and vice versa. The delivery problem is important for lithopanspermia (transfer of life within rocks) because meteorites hurtling at typically tens km per second into another system have an insignificant chance to be caught by direct impact or planets’ gravitational kicks.

The Princeton authors attempt to solve the problem when the solar system was part of a stellar cluster, before it separated from its birth cloud, in its first 10 million years or so. The terrestrial planets had hardly condensed and, even if seeded with life from the start, species and genetic evolution would not have progressed. Any seeds it generated would be hardly different from the starting forms already distributed within the cloud from which its neighbouring systems were condensing.

The solution we found (Interstellar transfer of planetary microbiota [here] Mon. Not. Roy. Astron. Soc. 348, 52-61) works for evolved planetary systems that send major impact debris like the Martian meteorites at hypervelocities through neighbouring systems, particularly to systems in their planet-forming phase that contain large amounts of solid grains. Impacts with mm-sized grains can then sputter off 10 000 times their mass in fragments or, on cm-sized debris, fragment it explosively.

The new protoplanets readily capture of some of this life bearing material, containing viable spores, without the losses such as experienced by the Martian meteorites when landing on Earth. Delivery of seeds of evolved terrestrial life to extra-solar planets at their formation stage is the key problem for panspermia. The Princeton group’s mechanism does not solve this problem.

Max K. Wallis
N. Chandra Wickramasinghe
Cardiff Centre for Astrobiology, Cardiff University, Wales UK