Recently, a year after a potentially fatal setback, a rejuvenated Kepler telescope discovered the first exoplanet (a planet found outside the solar system) of its new mission. Kepler was originally launched in 2009 with the mission to find exoplanets. In 2013, after an extremely successful run, Kepler suffered a catastrophic failure in its guidance mechanisms. Engineers jury-rigged a solution using the pressure of sunlight, and now, back on the job, it started posting results almost immediately. But exactly what is it looking for?
Finding a planet outside the solar system is more difficult than it sounds. The distances are so vast, and, relatively speaking, planets are so small, that most of the universe is too distant for conventional light telescopes. The search is also closely tied to the search for extraterrestrial life, and indeed humanity’s sense of self, described in a paper by Geoffrey Macy in the Proceedings of the American Philosophical Society.
In the 1940s, astronomers determined that our sun is pretty average as stars go. If the sun is average, there must be many more sun-like stars, so why couldn’t some of these stars have Earth-like planets as well? It’s logical enough. Searching for life directly was bound to be tricky, but how about searching for planets that can sustain life?
At these distances, astronomers searching for exoplanets have to look for indirect evidence of a planer’s existence rather than the planets themselves, and the early search method was roundabout. First, it was realized that large planets such as Jupiter induced slight changes in the sun’s orbit when they passed by. A movement in the orbit of a star means a slight shift in the star’s light, caused by Doppler Shift. To understand Doppler Shift, think of the change in pitch of a passing police siren as it comes closer and then fades into the distance. The sound waves change in length relative to you as the position of the siren changes, changing the sound. That is Doppler Shift, and it happens to light as well—the wavelengths of starlight shift measurably in the presence of large planets. Searching for Doppler Shifts is painstaking—Macy says in some cases shifts had to be measured over 20 years— but effective. The method has discovered goliath exoplanets down to those only 3 or 4 times Earth’s mass.
New techniques were needed to detect Earth-sized planets. The Kepler telescope was first, scanning distant stars for the faint dimming that occurs when a planet passes between a star and its observer. Kepler first hit paydirt in 2010, finding Kepler-22b, a planet slightly larger than Earth orbiting its star at a suitable distance to support life as we know it. This technique requires very precise aiming, which is why the breakdown in the Kepler telescope was such a disaster.
As it turns out, Kepler-22b can’t possibly support life, but with a renewed mission, a repaired Kepler continues the search. In addition to planets, the rejuvenated Kepler telescope searches for supernovas, colliding galaxies, and any other phenomena of interest in its area of the sky. The discovery of the new planet is an encouraging first step.