U N I VE RS E (Continued fiviii page 22) 



state, something must be liberating it 

 as fast as it's being consumed. Here 

 on Earth, the hberation is traceable 

 to life. Photosynthesis, carried out 

 by plants and many bacteria, creates 

 free oxygen in the oceans and in 

 the atmosphere. Free oxygen, m 

 turn, enables the existence of oxy- 

 gen-metabolizing creatures, includ- 

 ing us and practically every other 

 creature in the animal kingdom. 



We earthlings already know the sig- 

 nificance of Earth s distinctive chemical 

 fingerprints. But distant aliens who 

 come upon us will have to interpret their 

 findings and test their assumptions. Must 

 the periodic appearance of sodium be 

 technogenic? Free oxygen is surely bio- 

 genic. How about methane? It, too, is 

 chemically unstable, and yes, some of it 

 is anthropogenic. But methane is also 

 produced by bacteria, cows, permafrost, 

 soils, termites, wetlands, and other liv- 

 ing and nonliving agents. In fact, at this 

 very moment, astrobiologists are argu- 

 ing about the exact origin of trace 

 amounts of methane on Mars and the 

 copious quantities ot methane detected 

 on Saturn's moon Titan, where cows 

 and termites surely do not dwell. 



If the aliens decide that Earth's chem- 

 ical features are sure evidence for life, 

 maybe they'll wonder if the life is in- 

 telligent. Presumably the aliens com- 

 municate with one another, and per- 

 haps they'll presume that other intelli- 

 gent life-forms communicate too. 

 Maybe that's when they'll decide to 

 eavesdrop on Earth with their radio 

 telescopes to see what part of the elec- 

 tromagnetic spectrum its inhabitants 

 have mastered. So, whether the aliens 

 explore with chemistry or with radio 

 waves, they might come to the same 

 conclusion: a planet where there's ad- 

 vanced technology must be populated 

 with intelligent life-forms, who may 

 occupy themselves discovering how 

 the universe works and how to apply 

 its laws for personal or public gain. 



Beginning in 1*^95, my planet- 

 hunting colleagues got busy. 

 Since then, they've discovered more 



Earth (pale dot near center of light ray at 

 right) as photographed in 1990 by Voyager 1, 

 from a distance of 4 billion miles 



than 150 exoplanets, and there's plenty 

 more where they came from. After all, 

 the known universe harbors some 100 

 billion galaxies, each with some 100 

 billion stars. 



The search for lite drives the search 

 for exoplanets, some of which probably 

 look like Earth — not in detail, of course, 

 but in overall properties. Those are the 

 planets our descendants might want to 

 visit someday, by choice or by necessi- 

 ty. So far, though, nearly all the exo- 

 planets detected by the planet hunters 

 are much larger than Earth. Most are at 

 least as massive as Jupiter, which is more 

 than 300 times Earth's mass. Neverthe- 

 less, as astrophysicists design hardware 

 that can detect smaller and smaller jig- 

 gles of a host star, the ability to find 

 punier and punier planets will grow. 



In spite of the 150-planet tally, plan- 

 et hunting by earthlings is still in its 

 horse-and-buggy stage, and only the 

 most basic questions can be answered: 

 Is this thing a planet? How massive is 

 it? How long does it take to orbit its 

 host star? No one knows for sure what 

 aU those exoplanets are made of, and 

 only a couple of them eclipse their host 

 stars, permitting cosmochemists to do 

 their thing. 



But abstract measurements of chem- 

 ical properties do not feed the imagi- 

 nations of either poets or scientists. On- 

 ly through images that capture surface 



detail do our minds transform exo- 

 planets into "worlds." Those orbs 

 have to occupy more than just a few 

 pixels in the family portrait to qual- 

 ify, and a magazine reader should 

 not need a caption to find the 

 planet in the photo. We have to 

 do better than the pale blue dot. 



Only then will we be able to 

 conjure what a faraway planet looks 

 like when seen from the edge of its 

 own star system — or perhaps from the 

 planet's surface itself. For that, we will 

 need spaceborne telescopes with stu- 

 pendous light-gathering power. 



Nope. We're not there yet. But per- 

 haps the aliens are. 



Asnophysicist Neil DEGiLisSE Tyson is tlie 

 director of tiie Haydcii Planetarium at tlic Amer- 

 ican Muscnm of IS'atnral History. His Natur- 

 al History essay "In tlie Beginning" (Septem- 

 ber 2003) won tlie 2005 Science Writing 

 Award from the American Institute of Physics. 

 An anthology of his Natural History essays 

 will be pnblished in 2006 by IV.W Norton. 



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