Astronomers have found evidence suggesting that two infant planets are forming around a distant young star in our Milky Way galaxy.  

Located roughly 400 light-years from Earth, the young star, known as HD 163296, is approximately 5 million years old and about twice the mass of our sun. The two newborn planets orbiting HD 163296 are not yet fully formed, and each is about the size of Saturn, the second-largest planet in our solar system

Thousands of planets have been found around other stars in our galaxy. However, "nearly every one of these exoplanets has been discovered in orbit around a mature star with a fully evolved planetary system," officials said in a statement from the National Radio Astronomy Observatory. Therefore, the recent findings may reveal new clues about planet formation, the scientists said. [10 Exoplanets That Could Host Alien Life]

Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, astronomers examined the distribution of both the dust and the carbon monoxide gas components of the protoplanetary disk surrounding HD 163296. Observations of the star's disk revealed evidence of two infant planets forming there, officials said. 

The gas and dust of the protoplanetary disk surrounding a young star named HD 163296, in which two new planets are forming.
The gas and dust of the protoplanetary disk surrounding a young star named HD 163296, in which two new planets are forming.
Credit: ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)

After stars are born, clouds of leftover gas and dust condense into rings and form what is called a protoplanetary disk. Over time, the material in the disk clumps together under the pull of gravity to form new planets. In the dust and gas in the protoplanetary disk surrounding HD 163296, astronomers found three distinct gaps. 

The first gap is approximately 60 astronomical units (AU) from the central star. (One AU is the distance from the sun to the Earth — about 93 million miles, or 150 million kilometers — and 60 AU is about twice the distance from our sun to Neptune.) The other two gaps are located roughly 100 AU and 160 AU from the central star. At this distance, the farthest two gaps would extend well beyond our solar system's Kuiper Belt — the distant swath of icy bodies beyond Neptune, the outermost planet in our solar system. 

"ALMA has shown us amazing images and never-before-seen views of the rings and gaps around young stars that could be the hallmarks of planet formation," Andrea Isella, lead author of the study and an astronomer at Rice University in Houston, said in the statement. "However, since we were only looking at the dust in the disks with sufficient detail, we couldn't be sure what created these features."

The ALMA radio telescope can detect faint millimeter wavelengths of light emitted by gas molecules. ALMA data showed that the farther two gaps in HD 163296's protoplanetary disk had a considerable decrease in carbon monoxide, compared with the surrounding dust. This decrease suggests that planets are forming in the outer regions of the planetary system, officials said. 

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On the other hand, there was little or no difference in carbon-monoxide concentration observed in the gap nearest to the star, which indicates that it was created by something other than planet formation, Isella said.

"Dust and gas behave very differently around young stars," Isella said in the statement. "We know, for example, that there are certain chemical and physical process[es] that can produce ringed structures in the dust like the ones we have seen previously. We certainly believe these structures could be the work of a nascent (young) planet plowing through the dust, but we simply can't rule out other possible explanations. Our new observations provide intriguing evidence that planets are indeed forming around this one young star."

These are not the first signs of infant planets around young, distant stars. Astronomers have found similar evidence while studying other young stars, such as HL Tauri and TW Hydrae. In these studies, ALMA observations "revealed striking gaps and prominent ring structures in the stars' dusty (protoplanetary) disks," officials said. 

"These features may be the tantalizing first signs that planets are being born," they said. "Remarkably, these signs appeared around much younger stars than astronomers thought possible, suggesting that planet formation can begin soon after the formation of a protoplanetary disk." 

The new findings will be published in the journal Physical Review Letters. 

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