AsianScientist (Sep. 3, 2014) – A international team of scientists from Thailand, Japan and the United States have observed the aftermath of a planetary collision around a distant star for the first time, using the Spitzer Space Telescope’s infrared camera. This research was published in Science.
Planetary systems—like our own Solar System around the Sun—begin their lives as a cloud of dust and gas orbiting a star, which then clump into larger aggregates through adhesive and gravitational forces. In the final stage of planetary formation, these aggregates undergo massive collisions before forming a stable collection of planets orbiting a star. These collisions constantly release debris into the surroundings of the star, and the resulting “debris disks” can absorb the star’s energy output and re-radiate it as infrared radiation, which astronomers on Earth can then detect and analyse.
A team of researchers, including first author Meng Huan from the University of Arizona, studied the debris disk around NGC2547 ID8, a 35-million-year-old star about 2,000 light years from Earth. Along with optical observations of the star itself, performed by Thai astronomers Wiphu Rujopakarn and Saran Poshyachinda, the team measured the infrared radiation from the debris disk using the Spitzer space telescope.
After three months of collecting data, from May to mid-August 2012, the position of the star in the sky came too close to the Sun for the Spitzer telescope to be pointed at it and observations were paused. But when ID8 came back into view in January 2013 the debris disk was nearly 50 percent brighter, with the brightness rapidly decreasing over successive months.
“We were able to use computer models to explain the increased brightness as the result of a large amount of fine dust added to the debris disk,” Meng explained. “Furthermore, the brightness showed a consistent monthly rise and fall as it faded. This could be explained if the additional dust was concentrated into an elongated arc along its orbit around the star, such that the proportion of light facing the Earth from the dust cloud varied as the dust revolved around the star.”
There was only one plausible explanation: “There must have been a violent collision between large planetesimals, or planet-sized bodies, around ID8 during the pause in observations, and we were now seeing the aftermath,” Meng continued.
“Around ID8, the remains spattered from the collision had been ground by successive collisions into a dust cloud, which was then quickly blown away from the vicinity of the star by radiation pressure. The constant escape of dust explains why the additional brightness quickly faded away, and makes it very unlikely that the source could have been anything other than a recent collision.”
This study marks the first time that large variations in a debris disk have been observed over time scales as short as months, providing new dimensions to study the aftermath of a large impact in the planetary system of a star. In particular, observing a massive collision in the planetary system of distant stars places the formation of our own Solar System in a broader context, as meteorics and dynamical modelling indicate that a similar large impact involving the Earth formed the Moon more than 4.5 billion years ago.
“This would have occurred when the Sun was 30-100 million years old,” Meng said, “and ID8 is now 35 million years old, right in this age range.”
The study of planetary formation also goes beyond the Solar System, to address an important question about how often we should expect to find terrestrial planets around stars that resemble the Sun. According to Meng, only about one percent of solar-like stars in the age range of terrestrial planet formation show debris disks similar to that around ID8.
Traditional theories about debris disks predict that debris disks should persist for hundreds of millions of years after the collisions that cause them; if they are right, then giant collisions and thus terrestrial planet formation must also be very rare. However, more and more terrestrial planets are being discovered around distant stars, with NASA’s Kepler mission suggesting that at least a fifth of all solar-like stars have terrestrial planets orbiting them.
This study offers a possible resolution: if impact-produced debris disks decay on much shorter timescales, as observed around ID8, then their short lifespans explain why they are so rarely observed. As such, this study indirectly provides yet more evidence that Earth-like planets are common around stars like the Sun.
The article can be found at: Meng et al. (2014) Large Impacts Around a Solar-Analog Star in the Era of Terrestrial Planet Formation.
———-
Copyright: Asian Scientist Magazine; Photo: Spitzer Space Telescope.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.
