Every once and a while, a discovery is made that throws previous theories into a cocked hat and sends scientists scurrying back to the blackboard to try and explain the new information and tell us what it means.
NASA’s Kepler Telescope has made such a discovery and our theories of how planets form will never be the same.
Coming less than a year after the announcement of the first circumbinary planet, Kepler-16b, NASA’s Kepler mission has discovered multiple transiting planets orbiting two suns for the first time. This system, known as a circumbinary planetary system, is 4,900 light-years from Earth in the constellation Cygnus.
This discovery proves that more than one planet can form and persist in the stressful realm of a binary star and demonstrates the diversity of planetary systems in our galaxy.
Astronomers detected two planets in the Kepler-47 system, a pair of orbiting stars that eclipse each other every 7.5 days from our vantage point on Earth. One star is similar to the sun in size, but only 84 percent as bright. The second star is diminutive, measuring only one-third the size of the sun and less than 1 percent as bright.
“In contrast to a single planet orbiting a single star, the planet in a circumbinary system must transit a ‘moving target.’ As a consequence, time intervals between the transits and their durations can vary substantially, sometimes short, other times long,” said Jerome Orosz, associate professor of astronomy at San Diego State University and lead author of the paper. “The intervals were the telltale sign these planets are in circumbinary orbits.”
The inner planet, Kepler-47b, orbits the pair of stars in less than 50 days. While it cannot be directly viewed, it is thought to be a sweltering world, where the destruction of methane in its super-heated atmosphere might lead to a thick haze that could blanket the planet. At three times the radius of Earth, Kepler-47b is the smallest known transiting circumbinary planet.
The outer planet, Kepler-47c, orbits its host pair every 303 days, placing it in the so-called “habitable zone,” the region in a planetary system where liquid water might exist on the surface of a planet. While not a world hospitable for life, Kepler-47c is thought to be a gaseous giant slightly larger than Neptune, where an atmosphere of thick bright water-vapor clouds might exist.
The reason that this discovery is causing such excitement in the planetary science community is that Kepler 47-B and 47-C defy current theories that tell us how planets form. The standard model for stars like our sun is that a ring of gas and dust called an accretion disk orbits the young star and inside that ring, grains of dust are attracted to each other, gradually forming larger and larger bodies until planet size balls emerge.
But a circumbinary system was thought to be too unstable an environment for the dust to eventually form into planets. The likelihood that one of the stars would act like a bowling ball, careening through the accretion disk scattering any nascent planetary bodies thus not allowing the planets to coalesce makes the Kepler discovery something of a mystery:
“The presence of a full-fledged circumbinary planetary system orbiting Kepler-47 is an amazing discovery,” said Greg Laughlin, professor of Astrophysics and Planetary Science at the University of California in Santa Cruz. “These planets are very difficult to form using the currently accepted paradigm, and I believe that theorists, myself included, will be going back to the drawing board to try to improve our understanding of how planets are assembled in dusty circumbinary disks.”
The Kepler Telescope has already revolutionized our notions of extra-solar planets. Kepler was “specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.” It’s a remarkable scientific instrument:
The Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope — 105 square degrees, which is comparable to the area of your hand held at arm’s length. It needs that large a field in order to observe the necessary large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for the life of the mission—3.5 or more years.
The telescope is able to determine the size and orbit of a planet by observing its transit across the face of the star, measuring the infinitesimally small decrease in brightness — likened to being able to measure the brightness of a firefly flying in front of a spotlight from 2,000 miles away.
So far, Kepler has confirmed the existence of 116 new planets in 67 systems with thousands of candidate bodies still being looked at. Several planets have been confirmed to orbit in the habitable zone — where water can exist as liquid. Kepler-22b is a little more than twice the size of earth and represents the best candidate body found so far where life could exist.
The Kepler Telescope is expected to be in operation through 2015.