MadSci Network: Astronomy

Re: Would we recognize 'extrasolar' meteorites if they landed here on earth?

Date: Sun Nov 7 11:48:24 2004
Posted By: Todd Whitcombe, Associate Professor, Chemistry
Area of science: Astronomy
ID: 1099514731.As

What an interesting question! My immediate answer would be "no, there 
isn't a unique fingerprint to our solar system" but that would be missing 
some of the detail because, in fact, there is a way to distinguish 
extrasolar material.

The elements present in our solar system are the same elements that are 
present - as best we can tell - throughout the Universe. That is, 
astronomical observation has been able to detect all 92 naturally 
occuring elements in the emission lines from objects throughout the 
galaxy and beyond. And we have been able to observe the spectroscopic 
signatures of the same sorts of chemical compounds in the starlight of 
both distant stars and other galaxies as we find on Earth. In other words,
it appears that both the building blocks of matter and the chemical 
compounds formed are invariant throughout all of visible space. Chemistry 
and the construction of compounds seems to be the same wherever we look.

This is why I would say that we are not "special" or unique. There 
doesn't appear to be anything that is peculiar to our solar system and 
that would allow us to say that something was "extrasolar" 
versus "homegrown". Our solar system appears to be like any other in the 

But having said this, not all meteorites are identical - meteorites being 
the pieces of asteroids or comets that would actually land here. While 
radiometric dating on these objects typically place their origins at 
around 4.5 billion years ago - which would be consistent with the age of 
the solar system and thus, appear to rule out extrasolar origins - there 
are other signs or indications that indeed some of the material that 
fills up space may be extrasolar (or maybe "pre-solar").

Any rock (or grain of dust) that passed between solar systems - between 
two neighbouring stars - would have to spend an enormously long period of 
time in the deep space. The length of time would ensure that the atoms 
present have been exposed to extreme levels of solar radiation. That is, 
they would have been exposed to the detrius in the solar wind for a long 
period of time. This might result in the capture protons or neutrons by 
some of the atoms, thereby resulting in unusual isotopic distributions 
relative to the rocks that we have found floating within our solar 
system. That is, they would have an isotopic signature that would be 
different from that typically found amongst material in the solar system.

While we haven't found any "asteroids or comets" (or meteorites) with an 
isotopic pattern that would mark them as "extrasolar", the presolar 
grains of stardust that we have collected do show some remarkable 
variations in isotopic ratios. It would appear that a very small but 
definite percentage of the dust that permeates our solar system pre-dates 
the formation of the sun and planets and is "extrasolar" in origin 
insofar as the Sun is a second generation star formed from the left over 
dust and gas of previous stars. They would appear to be part of the 
interstellar medium from which our Solar System condensed long ago. 
However, having hung around the Solar System for the past 4.6 billion 
years, are they really "extrasolar"? 

As to how much is presently arriving from other stellar systems, there 
isn't a real good way to measure this. That is, how much of this stardust 
is of recent origin and not part of the cloud from which our solar system 
condensed is still a question mark. But it would seem likely that some of 
these grains must be from "out of town" and not just from our cosmic 
beginnings. The isotopic signatures found in dust grains does suggest 
that some of the stardust that permeates the solar system must not have 
originated here.

So, does the local material have a unique fingerprint distinctive from 
the material around, say, Polaris? To the best of our knowledge, no, in 
the sense that the same isotopic distributions and the same sorts of 
chemical compounds present in our solar system have been observed around 
other systems. The elemental composition and the chemical compounds 
present here and there would appear to be the same. The compounds within 
different systems would appear to be the same.

Is it then possible to see a fingerprint that would indicate that some of 
the material in the solar system is from "out of town"? Yes. Invariably 
the trip between systems will have resulted in transformations of the 
elements or the chemical compositions due the the prolonged journey 
between the stars. Such changes are detectable but will likely require 
that one look for material in outer space rather than on the surface of 
this planet. Space missions, like NASA's "Stardust", have collected data 
to be returned to Earth for analysis and will tell us a great deal about 
the composition of matter in outer space. Hopefully, some of the isotopic 
signatures will tell us more about pre-solar and extra-solar material - 
and might even allow us to distinguish between the two forms of stardust.

Hope this answers your question.

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