Consisting primarily of sand and broken dreams, the fictional desert planet of Tatooine appears in five of George Lucas’ six “Star Wars” films, re-released this weekend on Blu-Ray. The planet is a barren wasteland orbiting two suns 43,000 light-years from civilization and populated by kleptomaniac midgets in robes and salty bartenders that look like the Elephant Man. Tatooine also happens to be the childhood home of famed one-armed moisture farmer Luke Skywalker.
In a research article published in Science Magazine on Sept. 16, astronomers working with the Kepler spacecraft revealed that the $600 million space observatory, launched in 2009 to find Earth-like planets orbiting other stars, had detected a planet that, like Tatooine, orbits two suns. But before you fire up the X-wing you built to scale in your parents’ garage out of an old Dodge Stratus and the disassembled Bowflex machine you found in the garbage behind Academy, know that the only actual similarities between this planet and the Tatooine of fiction are those two suns.
Rather than being a terrestrial Earth-like planet composed mostly of silicate rocks and metals like Lucas’ Tatooine, the planet known as Kepler-16b is a gas giant comparable to Saturn in mass and size.
Gas giants are planets at least 10 times the size of Earth that are primarily composed of gasses, most commonly hydrogen and helium. As such, they have no solid surfaces and very low densities. Saturn for instance is 750 times the size of the Earth, but because the gasses of Saturn take up so much space compared to its total amount of mass, the planet is less dense than water. If you could find a pool big enough, Saturn would float in it.
Kepler comes across these gas giants often, but they are generally insignificant to the mission design. The only thing that makes Kepler-16b stand out against the happenstance discoveries of other gas giants like it is that it orbits its two parent stars (Kepler-16A and Kepler-16B) at half the distance of what was previously thought to be the inner limit for planet formation in a stable binary star system.
It turns out that multiple star systems like Kepler-16 and the suns of Tatooine aren’t even anything special. The most recent data release from the Kepler team counts 2,225 eclipsing binaries observed over a three-month period.
“There’s a tongue-in-cheek statement that three out of every two stars is in a binary system,” said astronomy professor J. Craig Wheeler. “Which means if you go out there and look where you thought you had a star, you actually have two or three — it’s pretty common. The sun is sort of an exception to the rule.”
The real challenge and what is ultimately the mission goal of Kepler, is to find a planet the size of Earth orbiting a star the size of our sun at the distance necessary to maintain liquid water on its surface. Such a planet would likely harbor complex life. To find such a planet among the 100 × 10^22 stars in the visible universe would be nothing short of impossible — there are fewer grains of sand on all the beaches in all the world. Instead, the Kepler spacecraft attempts to find these planets by staring continuously at one particular area of the Milky Way between the constellations of Cygnus and Lyra. Every six seconds, the spacecraft measures the brightness of 170,000 stars for disturbances in a star’s observed brightness, indicative of a stellar event known as an “astronomical transit.”
“What we’re doing is we’re looking for those fortuitous cases where we are looking edge-on to a planetary system, and then, once per orbit, if a planet crosses between us and a star, it will block out part of the light of the star, and that’s called a transit. And we can measure that very, very precisely from space,” said Department of Astronomy senior researcher Bill Cochran, who is also a member of the Kepler team.
A planet meeting the criteria of Kepler’s mission would take one year to orbit its parent star. It is possible that Kepler has already found such an object, but the design of the project requires the observation of three transits to verify the detection of a planet.
“The mission has only been going on for two and a half years,” Cochran said. “You see something at a one-year orbit, you basically need three years to see three transits, and that’s why we’re not at the point yet at being able to find an Earth [at the same distance from its Sun as ours].”
Printed on September 22, 2011 as: A new planet, a new hope