The two planets circling Kepler-36, a sun-like star in its senior years, are as different as Earth and Neptune. But unlike the hundreds of millions of miles that separate our solar system's rocky worlds from its gas giants, Kepler-36's brood come as close as 1.2 million miles (1.9 million kilometers, or 0.01 AU) from one another -- about five times the distance between Earth and the moon.
"When they're at their closest, it presents a spectacular view in the sky," astronomer Josh Carter, with the Harvard-Smithsonian Center for Astrophysics, told Discovery News.
The planet nearest to the parent star, designated Kepler-36b, is a solid body roughly 1.5 times the size of Earth, but more than four times heavier.
Its fraternal twin, Kepler-36c, is three times larger and twice as heavy. But the outer planet is shrouded in gas, similar to Neptune. Its little sister either lost its atmosphere or never had one.
Both scenarios are puzzling.
In our solar system, the division of rocky bodies inward and gas giants beyond is believed to be the result of temperature differences in the original disk of gas and dust from which the sun and planets formed.
The discovery of so-called "hot Jupiters" -- gas planets that circle very close to their parent stars -- spurred new theories that planets migrate into different orbits after they are born.
Systems like Kepler-36, however, will prompt scientists to fine-tune planet-formation theories, Carter said.
"These two planets are quite different. There's only a 10 percent difference in their distance (to the parent star), but their densities differ by a factor of eight," Carter said.
"I think it's very exciting," added astronomer Andrew Youdin, also with the Harvard-Smithsonian Center for Astrophysics. "It raises all kinds of questions about how planets get their atmosphere. Maybe one didn't get much and another further out did. Maybe one formed quicker.
The Kepler-36 discovery is a good example about how scientists piece together individual details to form a bigger picture of what is going on, said Steve Howell, deputy project scientist with NASA's Kepler space telescope.
"If you want to understand, you have to dig into a system. The weirder they are, the more scientifically interesting they are," Howell told Discovery News.
Carter's research, which is based on observations by the Kepler telescope, appears in this week's Science.
© 2012 Discovery Channel