In a study published in the latest issue of Science, astronomers led by graduate student Huan Meng, of the University of Arizona in Tucson, announced the discovery of remains of a mammoth planetary collision.
The team made its observations using NASA's Spitzer Space Telescope and several different ground-based instruments. The collision occurred between two planets orbiting a sunlike star called NGC-2547 ID8, which lies about 1,140 light-years from the earth. The star is a young one, with a system of planets still in the process of formation. The collision, which may have occurred as recently as two years ago, left a ring of dust and debris circling the star.
"This is the first detection of a planetary impact outside of our own solar system," said Meng. It provides a glimpse into the earliest era of the formation of our own solar system, since NGC-2547 ID8 is the same mass and size as our parent star, and, at just 35-million years old, the same age our sun was when its planets were accreting from the material that circled it.
"We think two big asteroids crashed into each other," said Meng, "creating a huge cloud of grains the size of very fine sand, which are now smashing themselves into smithereens and slowly leaking away from the star."
Previous observations of NGC-2547 ID8 had already recorded variations in the amount of dust around the star, hinting at possible ongoing asteroid collisions. In the hope of witnessing an even larger impact – which is a key step in the birth of a terrestrial planet – Meng and his team turned to the Spitzer space telescope to observe the star on a regular basis. "Spitzer," said co-author Kate Su, "is the best telescope for monitoring stars regularly and precisely for small changes in infrared light over months and even years." Beginning in May 2012, the telescope began watching the star, often on a daily basis.
The collision occurred sometime in the five months between observations that took place in 2012 and those that took place in 2013. Collisions between rocky bodies create a lot of dust, which in turn shine brightly in the infrared A dramatic increase in infrared light from NGC-2547 ID8 suggested the recent formation of a cloud of fine dust particles. Computer modeling work suggested that the increase—which gradually faded over the following year—was caused by the impact of two large, rocky objects, perhaps similar to the asteroid-like protoplanetary bodies in our own early solar system that eventually coalesced to form planets like the earth and Mars. "We are watching rocky planet formation happen right in front of us," said co-author George Rieke. "This is a unique chance to study this process in near real-time."
The collision would have created a gigantic mass of incandescent, vaporized rock. Out of this a thick cloud of silicate spherules condensed that were then ground into dust by collisions. "I can't imagine something else," Meng explains, "that could produce so much dust just within four months."
"The signal is fading," Su said, "as the cloud destroys itself by grinding its grains down so they escape from the star. Expelled from the system by radiation pressure from NGC-2547 ID8 (explaining the spike in the infrared—caused by the dust formed immediately after the collision—and its rapid decline) the dust cloud will be gone entirely in just a year or two.
Illustration by Ron Miller