Signs Of Comets Detected Around A Sun-Like Star

Artist's impression of the dust ring around HD 181327. Amanda Smith, University of Cambridge

Carbon monoxide has been observed around the star HD 181327, signaling the presence of comets. Although not the first evidence of extra-solar comets, the discovery supports the theory that other solar systems share common features with our own, and could help us learn about the early stages of our own system's development.

HD 181327 is 30 percent more massive than the Sun, but is likely to develop along a broadly similar path. However, at just 23 million years old it is as yet far from a mature star at the center of an established system. At a relatively close 160 light-years away, it gives us a ringside view of early stellar development.

"Young systems such as this one are very active, with comets and asteroids slamming into each other and into planets," said Sebastián Marino, a PhD student at Cambridge University in a statement. "The system has a similar ice composition to our own, so it's a good one to study in order to learn what our Solar System looked like early in its existence."

HD 181327 has probably already formed planets, but their orbits would be almost face-on to Earth, rather than the side-on alignment that might provide an opportunity for detection when they cross the face of the star. However, this same face-on orientation is ideal for us to observe gas clouds and rings of dust around the star, which is what Marino has done, using the Atacama Large Millimeter Array (ALMA) in Chile.

False color version of ALMA image of HD181327 with the boundaries of the Kuiper Belt added as lines for scale. Amanda Smith, University of Cambridge

Gas and dust get replenished as comets and asteroids respectively collide, leaving an (astronomically speaking) short window in which we can detect them before collision frequency falls. Even so, finding the telltale gasses was not easy.

"This is the lowest gas concentration ever detected in a belt of asteroids and comets – we're really pushing ALMA to its limits," Marino said

Luca Matrà, co-author of a forthcoming paper in Monthly Notices of the Royal Astronomical Society and fellow Cambridge student, added, "The amount of gas we detected is analogous to a 200-kilometer [124 miles] diameter ice ball, which is impressive considering how far away the star is."

Similar rings of cometary debris have been observed before, but only around much larger stars. Since these giants burn out too quickly to ever support life, they represent poor models for the development of systems like our own.

Possibly reflecting HD 181327's greater mass, the gas ring is further out than the Kuiper Belt, around 200 Astronomical Units (AU) from the star. As seen by ALMA, the dust ring is 23.2 AU wide with an average orbit 86 AU from the star. The Hubble Space Telescope previously detected dust slightly further out. The authors attribute the difference to the different wavelengths at which the telescopes operate and radiation pressure pushing smaller particles, which radiate at longer wavelengths, further from the star.

Asymmetry in the gas could be the result of interaction with the Interstellar Medium or a planet-sized body trailing dust particles behind it.

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