The Search for Other Worlds

In 1988, astronomers found the first extrasolar planet. Today, the number of bodies found that orbit stars other than our Sun is close to 2,000. They are a source of speculation about other Earth-like planets and inform astrophysicists' body of knowledge about the formation of star systems.

Astrophysicist and assistant professor of physics Joshua Pepper has been chasing exoplanets since his graduate school years at Ohio State University. While studying a black hole problem, he helped formulate a plan for a NASA mission to find exoplanets using the transit method, which examines an apparent periodic dimness in a star due to a planet passing in front of it. Pepper realized that he could design and build a telescope dedicated to finding exoplanets.

Pepper's initial project has evolved into the Kilodegree Extremely Little Telescope (KELT) survey. It uses two robotic programmable telescopes, one in Arizona and the other in South Africa. The survey has confirmed 15 exoplanets using the transit method.

Lehigh, Vanderbilt University and Ohio State run the KELT project together, and the project's low-resolution telescopes are dwarfed by other telescopes that have apertures of several meters in order to stare at tiny sections of the sky at high resolution. The wide-angle KELT view of the universe, by contrast, comes from a mere 4.5-centimeter aperture with a high-quality digital camera and lens assembly that captures the light of 100,000 stars with each exposure.

"The goal of KELT is to discover more planets that are orbiting the brightest stars we can see. In essence, those give us the very rare, very valuable planets," Pepper says. Planets orbiting bright stars are easier to characterize with follow-up observations that provide more information about mass, size, density, orbit, radiation patterns and atmospheric composition.

A sophisticated software program helps narrow down the 3 to 5 million stars observed over the whole survey to roughly 5,000 that bear further scrutiny. When enough data has been gathered that can be attributed to transiting planets, the most likely exoplanet candidates are selected, and members of a secondary cohort of researchers perform their own observations and analyses.

The ability to confirm an exoplanet's existence hinges on the precise measurement of its host star's brightness. The typical operation of the KELT scopes involves observing a single star several times a night for about five years. In the process, astronomers gather at least 8,000 brightness data points. A complete graph reveals a line representing that star's brightness profile, including any periodic dips in brightness.

Once exoplanet status has been determined, astronomers can use the host star's change in brightness, or transit depth, to calculate several figures that paint a fuller portrait of the planet. They can measure its radius and the extent of its atmosphere. The gravitational wobble the planet causes in the star helps them figure out the planet's mass and then the density. Using transmission spectroscopy to examine the transit depth leads to information about the atmospheric composition.

How many of the 1,984 planets found so far by KELT and other telescopes could be habitable? The answer is probably still far in the future, Pepper says. For now, astronomers are still trying to decide what defines an Earth-like planet.

The first KELT discovery, KELT-1b—a brown dwarf located in the constellation Andromeda—is too cool to be a star and too hot to be a planet. Fewer than a dozen brown dwarfs have been found that transit a star, and KELT-1b has the brightest host star of any transiting brown dwarf ever found. A bit larger than Jupiter in size, it is nevertheless about 30 times Jupiter's mass, making it a highly unusual specimen.

"Clearly, this brown dwarf doesn't fit the theoretical models, so the models need to be adjusted. That's a game changer," Pepper says. Astronomers theorize that KELT-1b's inflated size is due to extremely intense radiation from its host star.

Pepper is now looking for targets for the upcoming NASA Transiting Exoplanet Survey Satellite (TESS), slated to launch in 2017. TESS will survey the entire sky for 200,000 of the brightest stars, in the hope of finding small, rocky planets. In the first two years, the orbiting telescope will sweep 90 percent of the sky and continue until it has covered the entire sky visible from anywhere around Earth.

"There are still a ton of questions out there. We're still trying to figure out if other solar systems are like ours, if planets like the Earth are common or not," Pepper says. "The potential of this line of research is absolutely enormous."

Joshua Pepper received his Ph.D. from Ohio State University and completed his postdoctoral studies at Vanderbilt University. His research involves the discovery of extrasolar planets with particular focus on the KELT project.

Story by Manasee Wagh

Illustration by Hvass & Hannibal