Why some planets spin backward

WASHINGTON Fri May 13, 2011 8:55am EDT

This false-color composite image, released by NASA September 23, 2010, is constructed from data obtained by NASA's Cassini spacecraft, shows the glow of auroras streaking out about 1,000 kilometers (600 miles) from the cloud tops of Saturn's south polar region REUTERS/NASA/JPL/University of Arizona/University of Leicester/Handout

This false-color composite image, released by NASA September 23, 2010, is constructed from data obtained by NASA's Cassini spacecraft, shows the glow of auroras streaking out about 1,000 kilometers (600 miles) from the cloud tops of Saturn's south polar region

Credit: Reuters/NASA/JPL/University of Arizona/University of Leicester/Handout

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WASHINGTON (Reuters) - Some planets are just flipping backward.

Of the more than 500 planets detected around stars besides our Sun, the vast majority appear to spin the same way the star does, scientists reported on Wednesday in the journal Nature.

But some of these extrasolar planets spin in the opposite direction of the stars they orbit, astronomers found. These strange, backward-spinning planets are usually gassy giants called hot Jupiters, not rocky orbs like Earth.

Besides their backwards twirling, which the astronomers call flipped orbits, these big planets huddle close to their stars, unlike Jupiter, which is about 483 million miles (778 million km) from the Sun, more than five times as distant from the Sun as Earth.

"That's really weird, and it's even weirder because the planet is so close to the star," Frederic Rasio of Northwestern University said in a statement.

"How can one be spinning one way and the other orbiting exactly the other way? It's crazy. It so obviously violates our most basic picture of planet and star formation."

Astronomers have long theorized that big gas planets form further away from their stars, while Earth-like rocks are born closer in.

But just because a Jupiter-like planet forms in the planetary boondocks doesn't mean it stays there, Rasio and his colleagues reported.

When planetary systems contain more than one planet, in addition to a star, each planet has its own gravitational force, causing the planets to interact and eventually pulling the gas giants close to the star and even reversing its orbit, the scientists found.

This process is known as gravitational perturbation, or an exchange of angular momentum.

Astronomers have been detecting extrasolar planets since 1995, but have seen only a handful. The others are inferred by the gravitational pull they exert on the stars they orbit, creating a starry wobble that indicates one or more planets present but unseen in the planetary system.

The National Science Foundation supported this research.

(Editing by Todd Eastham)

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Comments (6)
APE-CAN wrote:
Has anyone thought that there is nothing weird at all about a small percentage of planets spinning in the opposite direction? Massive gravitational pulls from various wandering celestial objects travelling perpendicular to the plane of a solar system can “grab” or smash into a planet and flip it 180 degrees. We don’t need a complex computer program to envision that scenario.

May 14, 2011 4:12pm EDT  --  Report as abuse
poppyrich wrote:
So…why do some planets spin backwards? The title implies that this question was answered at some point in the research that has been quoted here. Why pretend to have an answer when you don’t?

May 14, 2011 7:41pm EDT  --  Report as abuse
nfarnold21 wrote:
New planet have been being discovered when a telescope is fixed on a star for an extended period of time. As the planet passes in front of the star the luminosity decreases. This is the only way that we will be able to discover new planets until the Webb telescope is launched in 2012. That being said the only way that we can judge the spin of a planet is by using red light and blue light shift which can not be judged in this fashion. Also these gas giants would not be able to spin in retro-grade motion due to their density. Planets only acquire a retro-grade motion if that have been hit by a large object that has altered it’s path, a planets with a large rocky core or frozen surface are the only candidates for this.

May 15, 2011 2:46pm EDT  --  Report as abuse
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