Starship Kepler—seeking out new worlds
By Amir Alexander
Science Writer, The Planetary Society
Birdlike creatures fly over a beach with dramatic rock formations, greeting the dawn of an alien sun on a world far away.
Imagine if you will the following dialogue— Stardate 3173.4, Bridge Communications Log, aboard the Federation Starship Enterprise
Spock: Captain, our sensors have detected a system of 5 planets around a Sun-like star at coordinates 19 10 48, +42 20 19, distance 950 light years. Three of the planets appear to be gas-coated giants, but the other 2 are solid rock and no more than 103% the diameter of Earth. Should we investigate further?
Kirk: Thank you, Mr Spock. We need to find out more about conditions on those planets. Lt Uhura, please set course to coordinates 19 10 48, +42 20 19. Mr Scott, direct all available power to the engines—Warp 5!"
Traveling at warp speed, Starship Enterprise would probably cover the 950 light years separating Earth from the star in a matter of a few hours or days, and be on the spot to study the terrestrial planets at close range. Scientists of the 21st century are not so lucky.
It would take even the fastest of today's spacecraft millions of years to reach that neighborhood, so scientists have to do their work at long range. Even so, astronomers do have their own spacecraft with a mission not too different from Capt Kirk's famous starship. It is called Kepler and—just like the Enterprise—its mission is "to seek out new worlds" in the depths of space.
The dream of finding worlds like our own orbiting faraway stars goes back to the dawn of modern astronomy. Armed with the newly invented telescope, astronomers looked at the skies and saw thousands upon thousands of new stars where only darkness or the glow of the Milky Way could be seen before. New planets were also discovered around the Sun—first Uranus (1781) and then Neptune (1846). The discovery of worlds orbiting faraway suns seemed just around the corner.
Red shift, blue shift
Launch! Kepler heads to space atop a Delta II rocket on Mar 6, 2009.
But decades passed, then a century, and still no planet had been found outside our own solar system.
The reason for this failure was clear—planets are small and dark, and located right next to the stars, lost in the glare of their brilliant mother suns.
In effect, they are well-nigh invisible when viewed from Earth, and most astronomers gave up the hopeless hunt, concentrating instead on more promising areas of research.
A few persisted, however. Despite the challenges, they believed that the question of whether other worlds like our own are out there was too big to ignore.
And since direct observation proved impossible, they looked for more roundabout methods of detecting faraway planets.
Depending on their specialty and experience, astronomers tried a range of approaches, but one of the most promising turned out to be the spectroscopic or radial velocity (RV) method. When a star is orbited by a planet, it is not only the planet but also the star that moves in a small circle.
The RV method looks for those slight movements of the star, and to detect them it measures the star's light spectrum—the precise range of colors, visible and invisible, that emanate from it. When a star moves toward Earth its spectrum, as viewed by us, changes slightly, becoming just a tiny bit "bluer" than it otherwise would be. Conversely, when it moves away from Earth its spectrum becomes slightly "redder" than it would be otherwise.
These shifts are the result of the Doppler effect, which increases or decreases wave frequencies depending on whether an object is moving toward us or away from us. Now if, while measuring a star's spectrum, astronomers notice a slight "blue shift" followed repeatedly and at regular intervals by a slight "red shift," it means that the star is rocking back and forth—almost certainly pulled back and forth by an orbiting planet.
For decades, while on TV screens Enterprise's 23rd-century sensors reached out to innumerable "new worlds and new civilizations," 20th-century astronomers tried in vain to detect the spectroscopic signal of even a single faraway planet.
Sadly, their 20th-century instruments were not sensitive enough to detect the minute spectrum shifts caused by a planet in orbit. But in Oct 1995, as the 2nd (Christian) millennium was drawing to a close, astronomers finally broke through.
Night descends and the lights turn on in an alien city, tucked between steep canyon walls on a planet many light years away. Note the outlines of nearby moons or planets hovering in the evening sky.
A team led by Michel Mayor of the Geneva Observatory announced that it had detected a planet orbiting the nearby star 55 Cancri.
And with that announcement it was as if the dam had burst—over the next decade dozens, and then hundreds, of new planets were detected. The field of planet-hunting, long a backwater of astronomy populated by a few dedicated fanatics, suddenly became one of the hottest fields of research in all of the natural sciences.
For a decade after the detection of the first exoplanet, practically all new planet detections were made by the RV method. But, even though it was the only detection method that actually worked, it had some serious limitations.
For one thing it was most effective for nearby stars, but beyond a distance of a few dozen light years its effectiveness dropped dramatically. In astronomical terms this means that the method is good for finding planets in our immediate neighborhood but not beyond.
For another, the RV method gives no information about a planet's size but only of its mass. And even the mass estimate is suspect, since it depends on the unknown angle between the planet's orbital plane and the line of sight from Earth.
Finally, there's this. Radial velocity is particularly good at detecting giant planets orbiting very fast and very close to their star. As a result, the first faraway planets ever discovered were a very strange lot.
Most of them were enormous gas balls dozens of times the mass of Earth, similar to our own Jupiter but scorchingly hot, and completing each orbit in a matter of days. They were, to be sure, planets, but not at all the kind that the crew of Enterprise liked to visit. To find more familiar planets—small, rocky, of moderate temperature and full of water and (perhaps) life—a different method was required.