Thanks to the Kepler Space Telescope, we now know the answer to a longstanding question in astronomy: how common are planetary systems around stars? Quite common, it turns out. In the relatively small patch of sky that Kepler studied, most of the stars had planets orbiting them. Scientists now believe that there are more planets than stars in our Milky Way galaxy.
But before the Kepler Space Telescope could reveal the great abundance of planets around other stars, scientists had to design a detection system that could do the job. This was no easy task.
I recently visited the NASA Ames Research Center in California, where I got the opportunity to talk to the two scientists who designed Kepler and proved that it would work. They filled me in on just how difficult it was to take this spacecraft from the drawing board to the launch pad.
The idea behind the Kepler Space Telescope is relatively simple. When a planet passes in between a distant star and the Earth (an occurrence called a “transit”), the amount of light that reaches us is less than when the light is unobstructed. If you can detect that change in light, then you can “hunt” planets – you can even use the difference in light to calculate the size of the planet and its distance from its star. This is what astronomers call the transit method of planetary detection.
What makes this simple idea not so simple is that the change in light the planet produces is incredibly small. This means that detecting a transit requires very precise instrumentation capable of very accurate measurements.
When astronomer Bill Borucki first dreamed up the Kepler Space Telescope in the early 1980s, he imagined that if you used the transit method to observe several thousand stars at once from ground-based telescopes, you could begin to detect large, Jupiter-sized planets in large slow orbits. Earth-size planets – especially those in “habitable zones” where liquid water might exist to support life – would be much more difficult to detect. To find an Earth-size planet, Borucki argued, you have to put a telescope in space.
Building, launching, and operating a space mission is not cheap. Before NASA was willing to fund the Kepler Space Telescope, the agency wanted proof that the instrument Borucki had in mind could make the sensitive measurements required to detect planets in the space environment. Borucki’s light detection system would have to work even when the telescope wobbled as it encountered radiation or electromagnetic disturbances.
How do you prove a space mission will work without actually sending your equipment into space? Borucki and his collaborator, Fred Witteborn, built a test demonstration in their lab at NASA Ames. The test bed they built had thick walls of insulation to keep the equipment inside the demonstration independent from the outside environment. Even the fans placed on the outside of the test bed for cooling (keeping the test at a constant temperature) had to be mounted on a frame that didn’t touch the insulated walls so no vibrations would be introduced.
Inside the test bed, the demonstration equipment included a light source, diffused evenly, which backlit a plate filled with small holes that simulated stars. Across some of the holes the two scientists placed small wires. These would provide the simulated transits. When electricity was run through these wires they heated up and expanded, blocking out some of the light coming through the hole.
Above the simulated stars were optics similar to what would be inside the finished telescope and a CCD (a charge coupled device, like in a digital camera) that would be the light detector. The CCD was attached to a computer that allowed them to collect their simulated data.
Borucki and Witteborn spent countless hours in their laboratory, testing the equipment and proving that even when the telescope wobbled, even when the computer simulated a gamma ray burst, they could detect the small changes in light that indicated a transit.
NASA was happy with the results of the Kepler Technology Demonstration. In 2009, Borucki finally got to see his telescope launched into space – more than 20 years after he first proposed putting a transit-detecting telescope in space. Thanks to his determination our view of the galaxy has been forever altered.
Learn more about how NASA scientists are hunting down exoplanets with the Kepler telescope at the 2017 Smithsonian Ingenuity Festival. On Thursday, November 30, Natalie Batalha, the lead scientist on the Kepler mission, will join the Museum for a day of space exploration. Follow the conversation online with #SmithsonianIngenuity.