Using data taken by NASA's Spitzer Space Telescope, astronomers at the University of Arizona have spotted an eruption of dust around a young star, possibly the result of a smashup between large asteroids. This type of collision can eventually lead to the formation of terrestrial planets.
A few months after scientists began tracking the star, called NGC 2547-ID8, it surged with a huge amount of fresh dust between August 2012 and January 2013.
"We think two big asteroids crashed into each other, 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," said Huan Meng, the study's lead author and a graduate student in the UA's Department of Planetary Sciences.
While Spitzer has observed dusty aftermaths of suspected asteroid collisions before, this is the first time scientists have collected data before and after a planetary system smashup. The viewing offers a glimpse into the violent process of making rocky planets like Earth.
Rocky planets begin life as dusty material circling around young stars. The material clumps together to form asteroids that occasionally run into each other. Although the asteroids often are destroyed, some grow over time and transform into proto-planets. After about 100 million years, the objects mature into full-grown, terrestrial planets. Our moon is thought to have formed from such a giant impact between proto-Earth and a Mars-size object when the sun was somewhere between 20 and 100 million years old.
In the new study, Spitzer – which includes technology developed at the UA – set its heat-seeking infrared eyes on the dusty star NGC 2547-ID8, which is a solar-type star that is about 35 million years old and lies 1,200 light-years away in the Vela constellation. Beginning in May 2012, the telescope began watching the star, sometimes daily.
A dramatic change in the star came during a time when Spitzer had to point away from NGC 2547-ID8 because the sun was in the way. When Spitzer started observing the star again five months later, team members were shocked by the data they received.
"We not only witnessed what appears to be the wreckage of a huge smashup, but have been able to track how it is changing – the signal is fading as the cloud destroys itself by grinding its grains down so they escape from the star," said Kate Su, an associate astronomer at the UA Department of Astronomy and Steward Observatory and co-author on the study.
"Imagine two asteroids, each 100 miles across, coming at each other at 40,000 miles per hour," said George Rieke, a UA Regents' Professor of Astronomy who led one of the instrument-developing teams on the Spitzer telescope project and a co-author on the study. "When they collide, much of the rock vaporizes and creates a cloud of gaseous minerals, which then condense into new, sand-like particles. Those particles smash into each other at incredible speeds, grinding each other down in the process."
A very thick cloud of dusty debris now orbits the star in the zone where rocky planets form. As the scientists observe the star system, the infrared signal from this cloud varies in proportion to what is visible from Earth. For example, when the elongated cloud is facing us, more of its surface area is exposed and the signal is greater. When the head or the tail of the cloud is in view, less infrared light is observed. By studying the infrared oscillations, the team is gathering first-of-its-kind data on the detailed process and outcome of collisions that create rocky planets like Earth.
"We are watching rocky planet formation happen right in front of us," Rieke said. "This is a unique chance to study this process in near real time."
Since terrestrial planet formation is a messy process that takes more than tens of millions of years, scientists rely on computer simulations to understand the process. The observations reported here open an avenue to compare on those simulations with how it happens in the real world, Rieke said.
The team is continuing to keep an eye on the star with Spitzer. They will see how long the elevated dust levels persist, which will help them calculate how often such events happen around this and other stars, and they might see another smashup while Spitzer looks on.
After Spitzer's expected end of operations later this decade, astronomers will catch a glimpse of the dust around these stars with the James Webb Space Telescope, or JWST, currently under construction and planned for launch in late 2018. JWST, too, will use technology developed at the UA to observe the most distant objects in the universe: a mid-infrared-wavelength camera developed by Rieke and a near-infrared-wavelength camera developed by Regents' Professor of Astronomy, Marcia Rieke, his wife.
"JWST will let us see if the dust clouds have dissipated and also let us probe the composition of the dust and gas in these systems much more powerfully than was possible with Spitzer," Su said. "Combining work with both telescopes over 20 to 25 years will provide us with a detailed look at how planets like Earth are assembled."
The results of this study are posted online on the website of the journal Science.