[BRIEF NOTE] On the Kepler-20 system

[rfmcdonald]

I really like living in an era when the discovery of Earth-like planets hundreds of light years away is almost quotidian.

NASA's Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system. The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet's surface, but they are the smallest exoplanets ever confirmed around a star like our sun.

The discovery marks the next important milestone in the ultimate search for planets like Earth. The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth. Kepler-20f is slightly larger than Earth, measuring 1.03 times its radius. Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away in the constellation Lyra.

Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days. These short orbital periods mean very hot, inhospitable worlds. Kepler-20f, at 800 degrees Fahrenheit (427 degrees Celsius), is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit (760 degrees Celsius), would melt glass.

"The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature. "This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them."

The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days, respectively. All five planets have orbits lying roughly within Mercury's orbit in our solar system. The host star belongs to the same G-type class as our sun, although it is slightly smaller and cooler.

The Kepler-20 planetary system is unusual, however. Ars Technica observes that the arrangement of the system's planets is unexpected.

The big point of discussion that didn't make it into the paper, however, was the truly unexpected nature of Kepler-20's companions. As Harvard's David Charbonneau put it, "the architecture of that planetary system is crazy." With the new finds, we now have five planets that, as you move further from the host star, alternate between Neptune-sized and small, rocky bodies, with the furthest of the five orbiting closer than Mercury's distance from the Sun.

Our models of planet formation can account for rocky inner planets, or systems where gas giants have moved in close to a star and booted anything else out of its way, but there's nothing that can account for a collection of planets like this one.

Charbonneau said, "I really want to dare my fellow astronomers to explain how this system could have formed," and admitted there was a bit of self interest in his challenge. He's teaching planet formation at Harvard next year, and fully expects some freshman to ask him to explain Kepler-20's oddball assortment of planets.