Astronomers have discovered two neighboring planets - one a bigger version of Earth - orbiting closer to each other than any planets discovered before.
The team, from the University of Washington and Harvard University, used data from NASA's Kepler space telescope, which measures dips in the brightness of more than 150,000 stars, to search for transiting planets.
Kepler-36b is a rocky world measuring 1.5 times the radius and 4.5 times the mass of Earth, while Kepler-36c is a Neptune-sized giant composed of liquid or gas.
The two planets orbit a star that's slightly hotter and about two billion years older than our sun, 1,200 light-years away. Kepler-36b orbits every 13.8 days, and Kepler-36c every 16.2 days.
Since Kepler-36c is much larger than the moon, it makes for a spectacular view from its neighbor.
At their closest, the two planets come within about 1.2 million miles of each other - only five times the Earth-moon distance and about 20 times closer to one another than any two planets in our solar system. But the timing of their orbits means they'll never collide.
"These are the closest two planets to one another that have ever been found," says UW astronomy professor Eric Agol. "The bigger planet is pushing the smaller planet around more, so the smaller planet was harder to find."
The fact that the two planets are so close to each other and exhibit specific orbital patterns has made it possible to make fairly precise estimates of each planet's characteristics, based on their gravitational effects on each other and the resulting variations in the orbits.
The team believes the smaller planet is 30 percent iron, less than one percent atmospheric hydrogen and helium and probably no more than 15 percent water. The larger planet, on the other hand, likely has a rocky core surrounded by a substantial amount of atmospheric hydrogen and helium.
The planets' densities differ by a factor of eight but their orbits differ by only 10 percent - making the differences in composition difficult to explain using current models of planet formation. Within our solar system, rocky planets reside close to the sun, with the gas giants much more distant.
Although Kepler-36 is the first planetary system found to experience such close encounters, it won't be the last.
"We're wondering how many more like this are out there," says Agol.