Free Astronomy Magazine January-February 2019

41 JANUARY-FEBRUARY 2019 ASTROBIOLOGY A s this graphic shows, the TRAPPIST-1 sys- tem is so compact that it can be eas- ily contained in- side the orbit of Mercury. The green areas high- light the habitable zones of the two systems. Below, the TRAPPIST-1 system seen from its outermost planet, TRAPPIST- 1h. [NASA] seem much more likely that a meteoroid with a vital load can fall right on a hos- pitable planet after an interstellar journey of hundreds of thousands or millions of years. Even the most basic colonies of bac- teria and the simplest spores we know can- not remain viable for such long periods of time. Moreover, in order for meteoroids to be able to transport these organisms, it is necessary that the planet hosting those forms of life suffers heavy- enough asteroid impacts to throw rock fragments beyond the escape velocity of the planet’s gravity well. This pre- supposes the existence of at least an asteroid belt and a pe- riod of intense bombardment triggered by planetary migra- tion. We do not know within how many planetary systems this has happened, so we can- not even remotely estimate the average number of meteoroids expelled by a typical planetary system - this also because we know almost nothing about the architecture and evolutionary history of extrasolar systems. lions of members within our galaxy. If panspermia were a consolidated reality, the universe could be teeming with life and our most distant ancestors could have been ex- traterrestrials − two non-negligible conse- quences. Nevertheless, there are many arguments against panspermia. Interstellar space is so vast that astronomers calculated a collision between two single stars has never happened in our galaxy; it does not