An odd pair of distant worlds - one rocky like Earth and another gassy like Neptune - have been found doing the closest dance of any planetary pair ever discovered, US scientists said on Thursday.
The duo are orbiting their star about 1200 light years from Earth, and were discovered with NASA's Kepler space telescope, which launched in 2009 in search of Earth-like planets orbiting stars similar to our Sun.
Their star is probably a lot like our Sun but several billion years older, and the planets are much closer to it, said the study in Science Express, the online edition of the journal Science.
The rocky planet, Kepler-36b, is about 1.5 times the size of Earth and orbits about every two weeks at an average distance of less than 18 million kilometres from its star.
The Earth's average distance from the Sun is about 150 million kilometres.
The bigger, outer planet nicknamed Kepler-36c is almost four times as big as Earth and is known as a "hot Neptune" because its atmosphere is mainly made up of hydrogen and helium and it may have a rocky core.
Kepler-36c orbits once each 16 days at a distance of 19 million kilometres.
They share nearly the same orbital plane and never collide, skimming one another at a distance of 1.9 million kilometres on their closest approach, or five times the distance of the Earth to the Moon.
"These are the closest two planets to one another that have ever been found," said Eric Agol, a University of Washington associate professor of astronomy and co-lead author of the paper.
"The bigger planet is pushing the smaller planet around more, so the smaller planet is harder to find."
People would not be able to stand on the rocky planet - it is so close to the hot star that its surface might be like molten lava.
But if they could, they would likely see the giant gas planet about three times the size of the Moon looming in the skies.
"These two worlds are having close encounters," said Josh Carter, a Hubble fellow at the Harvard-Smithsonian Center for Astrophysics.