Mankind's most ambitious mission to find habitable planets orbiting distant stars roared into the night sky from Cape Canaveral, Florida, Friday evening at 10:49 PM. Perched atop the Delta II rocket is a revolutionary observatory designed to detect earth-like planets thousands of light-years away. NASA's newest mission is appropriately named after one the most influential pioneers of modern planetary astronomy, Johannes Kepler.
Spurred by 16th century advances in global positioning technology critical to maritime explorers, small discrepancies began to appear in the measurements of stars and planets that could not be fully explained by the then prevailing geocentric model. A competing proposal with the sun at the center of the solar system slowly gained support and by 1550 a raging debate with profound theological and historical overtones was on to determine which, if either, was right.
Johannes Kepler (1571 - 1630), part astronomer, part astrologer, and full time brilliant mathematician, became deeply interested in the debate while teaching in Austria. As fate would have it, Kepler fell under the patronage of a wealthy benefactor named Tycho Brahe around the year 1600. At first glance, Brahe was a rock-and-rolling party animal, so rich he owned his own castle and island, more interested in throwing lavish legendary drunken bashes and slapping nubile servent girls on the arse than in making scientific history. But Brahe had another side, a serious one; he was a great thinker, and perhaps the most dedicated stargazer of his time. Before the first prototype telescopes were widely available, using only precision crafted, custom designed versions of the sextants, compasses, and protracters used on ships at sea, using only his naked eye, Tycho Brahe recorded volumes of data on the movement of planets relative to the fixed stars night after night, month after month, year after year. Until he fell deathly ill from a probable ruptured bladder after a particularly epic drinking party. Legend has it his dying words were "Ne frusta vixisse vidar" (May I not have lived in vain)". The wish would soon be granted.
Upon Brahe's death, Kepler poured over his former mentor's prolific records -- think of an excel spreadsheet with thousands of rows and columns, and no auto sorting! After several false starts, Kepler eventually derived an elegant solution in 1605 that unified the apparent motion of all planets -- including earth! -- under a heliocentric framework, and made testable predictions about the future position of a planet at any time. Within a few years, the first crude telescopes were pointing hungrily at the heavens, tracking the movement and activity of all known planets (Much to the dismay of entrenched religious and political interests in some cases), Kepler's Laws of Planetary Motion were confirmed, and the modern science of planetary astronomy was born.
For centuries scientists and writers speculated on the possibility that distant stars might have planets of their own, faithfully following the same ellipses and other rules Kepler described so well for our solar system. But it was only 14 years ago that the first exosolar planet was solidly confirmed orbiting a sun-like yellow-dwarf, known to stargazers as 51 Pegasus. Since then the list of exoplanets has exploded: 342 candidates now reside in the Extrasolar Planets Encyclopedia. Most were inferred by their gravitational effect on their primary star. As clever and difficult as it is, that method works best when the system they belong to is very different from our familiar solar system.
Planets circle stars, but as illustrated in the image to the left, the planet also swings its star just a bit. We can't see the planets, yet, but we can see the wobbling stars! The problem for finding small, earth-like worlds using this technique is it selects for relatively large planets that are tightly orbiting their comparatively small star. And so the list of exosolar planets to date is dominated by giant planets whizzing perilously close around modest stars in days or weeks. It's debatable if an earthlike world orbting in the habitable "Goldilocks" zone could survive in such an inhospitable system. Fortunately, there's another way, and that's where the Kepler Mission comes in.
Kepler is basically a horrendously accurate photometer married to a powerful wide view telescope that will trail the earth in deep space, undeterred by earth's shadow, designed to detect minute changes in stellar brightness as planets cross in front of a parent star. The accuracy of Kepler's photometer is so sensitive that it could detect a moth fluttering in front of a searchlight from hundreds of miles away. Only a tiny fraction of exosolar systems are likely to align in such a way that one or more planets conveniently eclipses its sun from our local perspective, but Kepler can look at one-hundred thousand stars at once!
The instrument should be able to detect the transit of smaller, more earthlike worlds along with their larger siblings, and hopefully produce a fair estimate on their size and mass. Future space and earth-based observatories will build on Kepler's results to obtain spectra, and maybe one day the first ever direct images, revealing the atmospheric and surface composition of some of these new planets.
Left: A hypothetical view from the surface of a recently discovered exosolar planet, here depicted as an ocean world, circling red dwarf Gleise 581. Center: The fictional homeworld of Stephen Baxter's QAX; a hot new world orbiting a young giant star. Right: Carbon planet. Click to enlarge.
What kind of objects might Kepler detect? Traditional gas and ice giants along with smaller, terrestrial planets we know from our own solar neighborhood are a solid bet. Kepler might sense the presence of super KBOs a dozen times the size of Pluto, or brown dwarfs on the precipice of starhood many times more massive than mighty Jupiter, some possibly accompanied by a secondary retinue of moons large enough to rate as full blown earth-like planets in their own right. But Kepler and subsequent research may confirm wholly new, exotic denizens: ocean or methane planets swaddled by blankets of water or compressed natural gas thousands of kilometers thick, carbon planets with mantles of graphite and solid diamond, or worlds stranger than any yet dreamed up by the most visionary scientists and authors. As an added benefit, the same design will allow Kepler to produce reams of data on minute fluctuations on distant stars, flares and sunspot activity, lurking in every niche of the Hertzsprung-Russell diagram.
At the risk of briefly letting our imagination soar free of formal science, future devices like Kepler could ostensibly infer other large scale structural anomalies; naturally occurring rings -- or artificial ringworlds -- stars with dense, well defined Oort Clouds -- or encased by Dyson swarms. Or for all we know, hollowed out, planet-sized objects familiar to any fan of Star Wars.
In short, if all goes well, our species is on the cusp of a golden age of planetary astronomy as revolutionary in scope as the one started when Johannes Kepler worked out the intricacies of our local planetary system four-hundred years ago. And if even the simplest bacterial life is, as some astrobiologists speculate and earth certainly demonstrates, inextricably intertwined with the surface and atmospheric chemistry of its native world after billions of year of evolutionary resonance, the dawn of astro-exobiology may not be far behind.