Dance of the Planets Gets Intimate

Two giant planets circling a dying star about 223 light-years away sweep past one another closer than any other planetary pair, demonstrating orbital mechanics that break the bounds of what scientists thought possible.

The planets, which are about the size of Jupiter, likely formed 2.5 billion to 3 billion years ago from of disk of dust and gas circling a massive newborn star, now known as HD200964.

Typically, gravity ends up balancing planet pairs so that the inner world completes two orbits for every one made by its outlying sibling, among other configurations. HD200964’s inner Jupiter is making four orbits for every three completed by its partner.

The closer the planets, the trickier the balancing act due to the planets’ increasingly powerful gravitational influences on each other.

“The tighter you get the planets, the more fine-tuned their steps have to be or they’re going to force each other out,” astronomer John Johnson, with the California Institute of Technology, told Discovery News.

The synchronicity at HD200964 is particularly exquisite. An orbital dance brings the two giant planets as close as about 33 million miles to one another. In our solar system, Jupiter and its nearest neighbor Saturn are never closer than 10 times that distance. The planetary pair orbiting HD200964 is separated by a distance similar to the divide between Earth and Mars.

“They’re an island of stability in a sea of instability,” said University of Florida’s Eric Ford. “In the case of HD200964, it is particularly dramatic because it’s a pretty small island.”

Astronomers have been keeping a close watch on HD200964 for about five years, teasing out details from weird wobbles in its light waves, due to the planets’ gravitational tugs. They then run the data in computer models. The star is among 450 similar targets being scanned for planetary systems.

“One of the things we’d like to understand is how planet formation is impacted by the type of star,” Ford told Discovery News.

HD200964 is not the only star in the study that has closely orbiting planets. A pair of planets circling 24 Sextanis, located 244 light-years away, pass as close as about 70 million miles from one another.

Johnson estimates that about 20 or even 25 percent of massive stars like HD2000964 and 24 Sextanis have large, Jupiter-class planets.

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