Story and photo by Mali Main
UW News Lab

At the tip of the swan’s wing in the Cygnus constellation, two moonless planets orbit an ancient, dying sun, humbling our own geocentric assumptions about the universe.

A study of this two-planet solar system, Kepler-36, was published last month in the journal Science. The research was led by Edmonds resident Eric Agol, an astronomy professor at the University of Washington, and Joshua Carter, Hubble Fellow with the Harvard-Smithsonian Center for Astrophysics.

“I always wondered how special Earth was compared to other solar systems,” Agol said. “Now we are at the point where we can start to answer the question — how unique is the Earth? Right now we can only explore the local neighborhoods.”

Agol has lived in Edmonds since 2005. It’s where he spends most of his free time, with his wife and two children, ages 6 and 3.

“It’s hard to do sky-gazing in Edmonds, because of the clouds,” Agol said with a laugh, adding that he does talk to his kids about the stars. His 6-year-old even has a college-level astronomy textbook.

On a recent weekday, the mid-morning light filled Agol’s office, tucked away on the third floor of the Physics Building at the UW Seattle campus. Unlike his son, Agol didn’t appreciate the intrigue of astronomy until later in life. “I didn’t get my first telescope until after my Ph.D.,” he said.

Growing up in Walnut Creek, Calif., he and his identical twin brother focused more on math than science. But while earning his doctorate at UC Santa Barbara, he began to apply physics to the study of black holes. His curiosity led him to the field of astrophysics.

The Discovery
Agol has been working on Kepler Satellite ideas for years. But he didn’t have access to proprietary data until last summer, when he worked with Carter at the Center for Astrophysics.

The Kepler Satellite, part of NASA’s mission to search the galaxy for Earth-like planets, measures changes in the luminosity of stars. “When two planets pass in front of the star, the star dims. Like an inverted pulse,” Agol explained.

Bill Chaplin, an astrophysicist at the University of Birmingham in England, worked with Agol and Carter on the data. He studied Kepler-36a, the sun-like host star. Sound passing through a star causes oscillations, he wrote in an email. “They resonate like musical instruments,” Chaplin continued, “allowing us to measure their size, age and mass.

“Kepler-36a shows beautiful oscillations, so the most exciting part was knowing that we had the tools to provide Eric and Josh with the accurate data on the host star that they needed, to get the most out of their fantastic analysis of the discovered planets,” Chaplin wrote.

The team identified one planet orbiting the star, but suspected there were more. Traditional planet-finding techniques wouldn’t have worked, said  Agol. They are based on the orbiting behavior of planets in the Earth’s solar system. They knew Kepler-36 was different. But they wouldn’t know how unusual it was until they found that second planet.

Agol suggested an algorithm he’d found through a Google search, in a paper written by a Russian mathematician. “The algorithm went through all the data on the star to identify at which point in time it gets dimmer,” he said. “Then the algorithm stitches those periods of time together and gets all the pulses to line up.”

The Planets
The Kepler-36 solar system is just above the plane of the Milky Way, about 1,200 light years from Earth.

Its sun is hotter and less metallic than ours and about 2 billion years older. Also, said Agol, it’s dying.

“We know that it is starting to expand and become more luminous. But it won’t engulf the other planets for 1 to 2 billion years,” he said.

The Kepler-36 planets are not within what NASA refers to as “a habitable zone of their star.” The temperature on each planet is about 1,200 degrees Fahrenheit. They are so close to their sun that one year in that solar system is about two weeks long.

The astronomers were fascinated by the planetary blueprint. Every 97 days the planets come within about 1 million miles of the other, 20 times closer than Earth gets to Venus. And unlike neighboring planets in our solar system, these two do not share physical features.

“The two planets have orbits so close to each other, but with very different densities,” Travis Metcalfe wrote in an email. The research scientist at the Space Science Institute in Boulder, Colo., also worked with Agol and Carter.

“This poses a serious challenge to astronomer’s current understanding of how planets form and evolve over time,” Metcalfe said.

Planet C, eight times the size of Earth, was found first. Its small rocky core is cloaked in a dense fog-like atmosphere of helium and hydrogen.

The smaller of the two is Planet B. What astronomers call a “hot super-Earth,” it is mostly composed of iron-embedded rock, but is nearly 1.5 times bigger and five times the mass of our planet. It is only 15 percent water.

“It’s probably too hot to have oceans. It’s quite possible it’s hot enough to melt aluminum even, but maybe there are metallic oceans,” Agol mused.

The astronomer said the Kepler-36 solar system opens up new possibilities about the universe. “It’s telling us that our solar system is not the rule, it is just one planetary system. There are a whole slew of different types of planets. It makes you wonder what other planets are out there?  It excites my imagination; I hope it does for others as well.”

Mali Main is a student in the University of Washington Department of Communication News Laboratory.