How do we compare to other life forms in the Universe?

Media Release

So far, searches for ET have come up empty-handed. No signals have been detected. But UNSW astronomer Dr Charles Lineweaver has come up with a new way to tell us something about extraterrestrials. By cleverly combining observations of extrasolar planets and the rate of star formation in the Universe, he finds that compared to other Earth-like planets in the Universe, our Earth is extremely young.

In a paper recently submitted to the journal "Icarus" Dr Lineweaver reports that "three quarters of the Earth-like planets in the Universe are older than the Earth and their average age is 1.8 (plus or minus 0.9) billion years older than the Earth."

One part of Dr Lineweaver's argument runs as follows: Immediately after the big bang, 13 billion years ago, the Universe was made of hydrogen and helium. There was no carbon, oxygen, iron, silicon; therefore no Earth-like planets could form around the first stars. However, in a strong burst of star formation which lasted a few billion years, these ingredients were produced in abundance by stars, and thereafter, the formation of Earth-like planets became possible. But there is a catch. Too many of these ingredients seems to be a bad thing for Earth formation. The 50 or so huge extrasolar planets detected so far are found preferentially around stars rich in these ingredients. And these huge planets are in orbits that would destroy any Earth-like planets. Dr Lineweaver calls this a Goldilocks effect: "with too few ingredients earths are unable to form, with too many, giant planets destroy any earths trying to form".

Although the analysis is about terrestrial planets, not the life on them, Dr Lineweaver concludes that "If life forms readily on Earth-like planets -- as suggested by the rapid appearance of life on Earth -- this analysis gives us an age distribution for life on such planets and a rare clue about how we compare to other life which may inhabit the Universe. "The "rare clue" is this: most of the life forms in the Universe have had two billion years longer to evolve than we have. To put this time span in perspective, two billion years ago our ancestors were amoebas.

Dr Lineweaver's article "An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect" is available online at http://xxx.adelaide.edu.au/abs/astro-ph/0012399.
The January 13th issue of New Scientist describes this research in an article by Marcus Chown.

• Dr Charlse Lineweaver    (02-9385-5168) School of Physics UNSW
• Rory McGuire        (02-9385-5751 or 0413 930 728) Faculty of Science and Technology