(Sorry for the delays! Fuller updates are on the way!)
Can life, or at least meteorites, travel between the stars? If so, how would we know where they had come from? Paul Wiegert addressed this question in his PSERF talk on the 21st of January, 2011
This week's PSURF Talk (a link will be posted here once it is online) dealt with a topic of eminent importance to us Astrobiologists - the concept of panspermia. What is panspermia, you might ask? Well, basically it describes a theory wherein life does not need to originate separately one each body where it is found. Life could originate at one place, at one time, in conditions that are truly extraordinary and then propagate to other worlds and other situations. If you believe the theory, then there is no need to be bounded by the conditions of the early Earth when considering life's abiotic origins. Instead, we can also consider whether life got an easier start on Mars, or inside a differentiated asteroid (this was discussed extensively at the York University Physics and Astronomy Journal Club last week; last week's CATP seminar, which I will post soon!) or even within Giant Molecular Clouds.
Of course, wherever life formed, it needs some way to get from there to here. This is where Paul Wiegert's research comes in. Paul is part of the meteor group at UWO in Physics and Astronomy. Together, this group operates a network of ground stations which examine a particular volume of sky in Southwestern Ontario for meteors. From the triangulation that the network makes possible, it is possible to back out the trajectory of the incoming particles, their speeds, and if they are big enough (fireballs) where any meteorites might land.
Would we expect to find any "bugs" hitching a ride on these meteorites? It's not as crazy an idea as you might think. Computer simulations show that impacts into terrestrial bodies, such as Mars, can loft rocks large enough to fall to Earth as meteories without subjecting the rocks to too much shock. The shock is still considerable, however, and if you were standing on the rock, you would become something like raspberry jam coating the escaping meteorite, however, the E-coli in your bowel can withstand Gigapascals of pressure and would not even notice the jolt. Of course, in addition to the inital shock, those bugs would need to deal with long residence times in space (perhaps millions of years) which entails irradiation, vacuum and very low temperatures. While it's not known whether this is a survivable by any organism, we know that each of these characteristics are individually survivable by extremophiles here on Earth.
Cross-talk of this kind between planets is actually quite common. Estimates suggest that almost 500 kg of Mars-origin material falls to Earth each and every year, and we have many examples in our meteorite collections. These are typically referred to as SNC meteorites and we know that they are from Mars because of the composition of the gas bubbles in a glassy-phase known as Maskelynite.
What Paul wonders about is that if cross-talk of this kind can exist between the planets, then why not between different stars? The journey to another star, expected to take on the order of a few hundred thousand years for material ejected from a nearby solar system, is not large compared to the typical in-space residence times of meteorites. Furthermore, there are 400 billion other systems within our own milky way, many of which are likely to be spewing out meteoritic material.
However, while the flux of material early in the solar system's formation in a star cluster was large (for instance, it is now estimated that up to 90% of Oort cloud comets did not form from the solar nebula), the flux of material has fallen off considerably. A 2003 paper by Melosh et al estimates a less than 1% chance that an extrasolar meteorite has fallen somewhere on the surface of the Earth over its entire history.
The reason for this is that space is unfathomably big. For the cross-talk between the Earth and Mars, you're talking about a volume of space perhaps 1-AU wide and a fraction of an AU thick, so of order a few cubic AU. But even if the nearest star, Proxima Centuri, was emitting meteorites, at our distance they would be spread over a sphere with a radius of 265,000 AU. This gives a probability of a meteorite in the space between Earth and Mars of close to 1 in 10 thousand trillion times (10^16) less than a meteorite originating from Mars*. This is just for Proxima Centuri. As we head out to further stars the chances get even smaller.
However, while you need about a cubic meter of material at the top of the atmosphere to produce a 10-cm-sized meteorite at the surface, the smaller a particle is the more of them there are. While these smaller particles tend to burn up in our atmosphere, a few have been detected by cosmic dust detectors** on distant spacecraft, including Galileo, during its mission around Jupiter. How do we know that they are extrasolar in origin? Direction and velocity: that is, any dust grain that is travelling so fast that it cannot be bound to our solar system and that has not originated in the direction of any large interacting mass (such as Jupiter or the Sun) which could have boosted its speed, cannot be something that has formed "locally."
Space-borne dust detectors are not the only way to determine the direction and velocity of a particle. You can use the Earth's Atmosphere as a detector. By looking at the glowing streaks of meteors that burn up high in the atmosphere, it is possible to back out where they came from and how fast they are travelling (and eventually, spectroscopy could be used to determine their composition, as those who have done the high-school experiment of putting different compounds in a bunsen burner knows). It turns out that anything moving more than about 42 km/sec is a good candidate for this. This is the work that Paul does with a wide-baseline pair of telescopes which observe the whole sky from two locations in southern Ontario. As of yet it seems things are a bit like Kepler: lots of candidates, but much work remains to be done to determine whether they have, in fact, observed the first interstellar voyagers to have visited the Earth.
* Melosh's paper deals with more subtleties then I have allowed here. The Sun's gravitational cross-section means that more meteorites would find their way into the volume of the solar system than another equally size volume of space that did not contain a large body. Even so, the statistics don't look good.
** Paul suggested that, eventually, a large dust detector could gather enough particles to act as a kind of interstellar dust "telescope" or mapper. However, the size of such a detector required, maybe 10-m across - the size of the main mirror on Keck, is currently beyond our ability to manufacture.