At Bad Astronomy:

Are we aliens? This question has come up due to a new result from studying meteorites, and is getting a lot of web-chatter. I figure I’d better get on this sooner rather than later!

First, the science. Then the chatter. Finally, the caution flag. :-)

The Science:

A fragment of the Murchison meteoriteSome meteorites have been found to contain some relatively complicated organic compounds, including molecules that are components of amino acids, the building blocks of life. For example, the Murchison meteorite, which fell on Australia in 1969, has been found to contain purines and pyrimidines, which are crucial to a large number of biological molecules like DNA, RNA, and ATP (adenosine triphosophate, a chemical our cells use for fuel).

Now, you have to be careful. A meteorite might have had these molecules in it before it slammed into the Earth, or it may have absorbed them from the ground after impact. One way to tell the difference is to look at isotopes of the elements. An element like carbon usually has 6 protons and 6 neutrons in its nucleus, but an isotope is when the number of neutrons is different. Carbon 13, as an example, has 6 protons and 7 neutrons in the nucleus (the number of protons determines the chemical properties, so carbon 13 is still like carbon, though a tad heavier).

The ratio of isotopes in a sample can be different for objects in space versus on Earth. Various process can change the ratio, so that’s a good way to find out if these molecules are native to a meteorite, or if it was contaminated after it fell.

OK, that’s the primer. Now the good part: scientists studying the Murchison meteorite have determined that the purines and pyrimidines — specifically, uracil and xanthine — have a non-terrestrial origin. In other words, the molecules in this meteorite, so crucial for life, were actually formed in outer space and fell to Earth.

That is very, very cool.

So this means that the some of the basic building blocks of life formed out in space, and came to Earth via meteorites and, presumably, comets.

Note that "some of". That’s important. Because…