Rocky Planets Might Inhabit Popular Nearby Star Cluster
Color composite image of the Pleiades star cluster and surrounding region produced by Inseok Song of the Spitzer Science Center. The image was created by combining B, R and I band images from individual second generation Digital Sky Survey images into blue, green and red image layers, respectively. The location of HD 23514 is shown by the yellow arrow.
Credit: Inseok Song/Digital Sky Survey, inset: Gemini Observatory/Lynette Cook

Evidence for rocky planets has been found in one of humanity's most beloved star clusters.

Astronomers have spotted a thick cloud of dust surrounding a star in the Pleiades, or "Seven Sisters," star cluster that they speculate is debris created from the collisions of rocky planet embryos or full-fledged planets.

"Our observations indicate that terrestrial planets similar to those in our solar system are probably quite common," said Benjamin Zuckerman, an astronomer at the University of California, Los Angeles, who was involved in the discovery.

The findings will be detailed in an upcoming issue of the Astrophysical Journal.

Using the Gemini Observatory and NASA's Spitzer Space Telescope, the researchers found that dust surrounding HD 23514, a star in the Pleiades that is slightly more massive and bright than our sun, is hundreds of thousands of times thicker than in our solar system.

While very young stars 10 million years old or younger are typically cloaked in thick dust clouds, HD 23514 is about 100 million years old, and its "primordial" dust should have long dissipated away. Thus, the dust that astronomers are seeing now is likely second-generation debris generated by the collision of large objects.

"In the process of creating rocky, terrestrial planets, some objects collide and grow into planets, while others shatter into dust," said study team member Inseok Song at Caltech. "We are seeing that dust."

The team calculates that terrestrial planets or planetary embryos in the Pleiades collided within the last few hundred thousand years and perhaps much more recently.

Particles created during the collisions could, in time, coalesce to form comets and asteroids or even new planets.

The Pleiades was well known throughout recorded human history. The ancient Greeks named it the Seven Sisters after its most visible hot blue stars; the Aztecs of Mexico and Central America based their calendar upon the Pleiades; and the cluster is mentioned in the Bible, when God asks Job, "Can you bind the beautiful Pleiades? Can you loose the cords of Orion?"

The Pleiades actually contains a total of about 1,400 stars. At 400 light years away in the constellation Taurus, it is one of the closest star clusters to Earth.

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