An image of how Kepler-36c might appear from the surface of the smaller nearby… (David Aguilar / Harvard-Smithsonian…)
NASA's orbiting Kepler telescope has discovered an unlikely pair of planets orbiting a distant star. One is small and rocky, the other is large and gaseous. What makes them unusual is that both are orbiting close to their parent star and they routinely come very close to each other, within only 1.2 million miles.
"Here we have a pair of planets in nearby orbits but with very different densities," said astronomer Steve Kawaler of Iowa State University, one of the co-authors of the report appearing in the journal Science. "How they both got there and survived is a mystery."
Kepler is designed to detect planets circling other stars by observing variations in the stars' brightness as their planets pass between the star and Earth. Astronomers using it have so far identified 72 confirmed planets and have several hundred more possibilities. The new planetary pair was identified circling a star called Kepler-36, which is about 1,200 light-years from Earth in the constellation Cygnus. The star was already known to have one planet circling it.
A team headed by Joshua Carter of the Harvard-Smithsonian Center for Astrophysics was examining such systems looking for examples with multiple planets. Astronomer Eric Agol of the University of Washington suggested that the team use a different algorithm to analyze the subtle changes in brightness that are detected by Kepler, and the Kepler-36 pair popped up immediately.
"We found this one on a quick first look," Carter said. "We're now combing through the Kepler data to try to find more."
The smaller of the new planets is called Kepler-36b and is rocky like Earth, but with a diameter about 1.5 times greater and a mass 4.5 times that of Earth. The researchers estimate the planet is 30% iron, less than 1% atmospheric hydrogen and helium, and no more than 15% water. Kepler-36c, which is either gaseous like Jupiter or watery, is 8.1 times as massive as Earth and has a radius 3.7 times larger. It probably has a rocky core surrounded by a substantial amount of atmospheric hydrogen and helium.
But what makes them unusual is their orbits. Kepler-36b orbits its star every 14 days at an average distance of less than 11 million miles. Kepler-36c orbits the star every 16 days at a distance of 12 million miles. Every 97 days, on average, the planets come within 1.2 million miles of each other, about five times the distance from Earth to the moon. The smaller Kepler-36b would appear to be about the size of Earth's moon when viewed from Kepler-36c's surface, but the opposite view would be much more spectacular.
Normally, gaseous planets like Kepler-36c can form only farther away from their stars, where temperatures are cool enough for the gases to condense. They sometimes migrate closer to the mother star but, in doing so, they tend to sweep away smaller planets. Why that has not happened at Kepler-36 is a mystery.
The star itself has about the same mass as our own sun, but is only a quarter as dense. It is also slightly hotter and contains slightly less metal. The team concluded that the star is a few billion years older than our sun and no longer burns hydrogen at its core, so it has entered a sub-giant stage in which its radius is 60% larger than the sun's.
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