American astronomer Carl Sagan 

 called it. And that's generous. Without 

 the help of a picture caption, you might 

 not even find it. 



What would happen if some big- 

 brained aliens from the great beyond 

 scanned the skies with their naturally 

 superb visual organs, further aided by 

 alien-state-of-the-art optical acces- 

 sories? What visible features of planet 

 Earth might they detect? 



Blueness woixld be first and foremost. 

 Water covers more than two-thirds ot 

 Earth s surface; the Pacific Ocean alone 

 makes up an entire side of the planet. 

 Any beings with enough equipment 

 and expertise to detect our planet s col- 

 or would surely infer the presence of 

 water, the third most abundant mole- 

 cule in the universe. 



If the resolution of their equip 

 nient were high enough, the aliens 

 would see more than just a pale 

 blue dot. They would see intri- 

 cate coastlines, too, strongly sug- i 

 gesting that the water is liquid. I 

 And smart aliens would surely I 

 know that if a planet has Liquid ' 

 water, the planet's temperature 

 and atmospheric pressure fall 

 within a well-determined range. 



Earth's distinctive polar ice caps, 

 which grow and shrink from the sea- 

 sonal temperature variations, could 

 also be seen optically. So could our 

 planet's twenty-four-hour rotation, 

 because recognizable landmasses ro- 

 tate into view at predictable intervals. 

 The aliens would also see major 

 weather systems come and go; with 

 careful stuciy, they could readily dis- 

 tinguish features related to clouds in 

 the atmosphere from features related 

 to the surface of Earth itself. 



Time for a reality check: We live less 

 than a dozen light-years from the 

 nearest known exoplanet — that is, a 

 planet orbiting a star otlicr than the Sun. 

 Most exoplanets lie more than a hun- 

 dred light-years away. Earth's brightness 

 is less than one-billionth that of the Sun, 

 and our planet's proximity to the Sun 

 would make it extremely hard for any- 

 body to see Earth directly with an op- 



tical telescope. So if ahens have found 

 us, they are likely searching in wave- 

 lengths other than visible light — or else 

 their engineers are adapting some oth- 

 er strategy altogether. 



Maybe they're doing what our own 

 planet hunters typically do: monitor 

 stars to see if they jiggle at regvilar in- 

 tervals. A star's periodic jiggle betrays 

 the existence of an orbiting planet that 

 may otherwise be too dim to see di- 

 rectly. The planet and the host star both 

 revolve around their common center 

 of mass. The more massive the planet, 

 the larger the star's orbit must be, and 



Earth and the Moon as photographed from 

 Mars by the Mars Orbiter Camera, 2003 



the more measurable the jiggle when 

 you analyze the star's light. Unfortu- 

 nately for planet-hunting aliens, Earth 

 is puny, and so the Sun barely budges, 

 further challenging alien engineers. 



Radio waves might work, though. 

 Maybe our eavesdropping aliens have 

 something like the Arecibo Observatory 

 in Puerto Rico, home ot Earth's largest 

 single-dish radio telescope — which you 

 might have seen in the early location 

 shots in the 1997 movie Contact, based 

 on a novel by Carl Sagan. It they do, and 

 if they tune to the right frequencies, 

 they'll certainly notice Earth, one of die 

 loudest radio stnirces in the skv. C'on- 



sider everything we've got that gener- 

 ates radio waves: not only radio itself, 

 but also broadcast television, mobile 

 phones, microwave ovens, garage-door 

 openers, car-door unlockers, commer- 

 cial radar, military radar, and communi- 

 cations satellites. We're just blazing — 

 spectacular evidence that something 

 unusual is going on here, because in 

 their natural state, small rocky planets 

 emit hardly any radio waves at all. 



So if those alien eavesciroppers turn 

 their own version of a radio tele- 

 scope in our direction, they might in- 

 fer that our planet hosts technology. 

 One complication, though: other in- 

 terpretations are possible. Maybe they 

 wouldn't be able to distinguish Earth's 

 signal from those of the larger plan- 

 ets in our solar system, all of 

 which are sizable sources of ra- 

 dio waves. Maybe they would 

 think we're a new kind of odd, 

 radio-intensive planet. Maybe 

 they wouldn't be able to dis- 

 tinguish Earth's radio emis- 

 sions from those of the Sun, 

 forcing them to conclude that 

 the Sun is a new kind of odd, 

 radio-intensive star. 

 Astrophysicists right here on 

 Earth, at the University of Cam- 

 bridge in England, were similarly 

 stumped back in 1967. While survey- 

 ing the skies with a radio telescope for 

 any source of strong radio waves, An- 

 thony Hewish and his team discoverecl 

 something extremely odd: an object 

 pulsing at precise, repeating intervals of 

 slightly more than a second. Jocelyn 

 Bell, a graduate student of Hewish's at 

 the time, was the first to notice it. 



Soon Bell's colleagues established 

 that the pulses came from a great dis- 

 tance. The thought that the signal was 

 technological — another culture beam- 

 ing evidence of its activities across 

 space — was irresistible. As Bell re- 

 counts, "We had no prcnif that it was 

 an entirely natural radio emission. . . . 

 Here was I trying to get a Ph.! ). out of 

 a new technique, and some silly lot ot 

 little green men had to choose my aer- 

 ial and my trequency to coninumuate 



February 2006 nmvr.w misioio 2 



