This area will cover relevant news of the threat to the planet from Near Earth Objects (NEOs) including concepts and designs for mitigation. All opinions are those of the author.

10 August 2009

Evidence of Planetary Collision in Another Solar System

Image credit: NASA/JPL-Caltech

From the NASA/JPL-Caltech News Release...

This artist's concept shows a celestial body about the size of our moon slamming at great speed into a body the size of Mercury. NASA's Spitzer Space Telescope found evidence that a high-speed collision of this sort occurred a few thousand years ago around a young star, called HD 172555, still in the early stages of planet formation. The star is about 100 light-years from Earth.

Spitzer detected the signatures of vaporized and melted rock, in addition to rubble, all flung out from the giant impact. Further evidence from the infrared telescope shows that these two bodies must have been traveling at a velocity relative to each other of at least 10 kilometers per second (about 22,400 miles per hour).

As the bodies slammed into each other, a huge flash of light would have been emitted. Rocky surfaces were vaporized and melted, and hot matter was sprayed everywhere. Spitzer detected the vaporized rock in the form of silicon monoxide gas, and the melted rock as a glassy substance called obsidian. On Earth, obsidian can be found around volcanoes, and in black rocks called tektites often found around meteor craters.

Shock waves from the collision would have traveled through the planet, throwing rocky rubble into space. Spitzer also detected the signatures of this rubble.

In the end, the larger planet is left skinned, stripped of its outer layers. The core of the smaller body and most of its surface were absorbed by the larger one. This merging of rocky bodies is how planets like Earth are thought to form.

Astronomers say a similar type of event stripped Mercury of its crust early on in the formation of our solar system, flinging the removed material away from Mercury, out into space and into the sun. Our moon was also formed by this type of high-speed impact: a body the size of Mars is thought to have slammed into a young Earth about 30 to 100 million years after the sun formed. The sun is now 4.5 billion years old. According to this theory, the resulting molten rock, vapor and shattered debris mixed with debris from Earth to form a ring around our planet. Over time, this debris coalesced to make the moon.

Link: JPL News Release

Link: NASA Multimedia (Planetary Smash Up)

Link: NASA Animation (25 MB .mov file)

Link: CalTech Press Release



C. M. Lisse et al 2009 ApJ 701 2019-2032

C. M. Lisse1,8, C. H. Chen2, M. C. Wyatt3, A. Morlok4,9, I. Song5, G. Bryden6 and P. Sheehan7
1 JHU-APL, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
2 STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA
3 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
4 The Open University, Milton Keynes, MK7 6AA, UK
5 Department of Physics and Astronomy, The University of Georgia, Athens, GA 30602, USA
6 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
7 Department of Physics and Astronomy, University of Rochester, Rochester, NY, USA
8 Address for correspondence: Planetary Exploration Group, Space Department, Johns Hopkins University, Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA.
9 Current address: CRPG-CNRS, UPR2300, 15 rue Notre Dame des Pauvres, BP20, 54501 Vandoeuvre les Nancy, France.
E-mail:,,,,, and

ABSTRACT. The fine dust detected by infrared (IR) emission around the nearby β Pic analog star HD172555 is very peculiar. The dust mineralogy is composed primarily of highly refractory, nonequilibrium materials, with approximately three quarters of the Si atoms in silica (SiO2) species. Tektite and obsidian lab thermal emission spectra (nonequilibrium glassy silicas found in impact and magmatic systems) are required to fit the data. The best-fit model size distribution for the observed fine dust is dn/da = a –3.95±0.10. While IR photometry of the system has stayed stable since the 1983 IRAS mission, this steep a size distribution, with abundant micron-sized particles, argues for a fresh source of material within the last 0.1 Myr. The location of the dust with respect to the star is at 5.8 ± 0.6 AU (equivalent to 1.9 ± 0.2 AU from the Sun), within the terrestrial planet formation region but at the outer edge of any possible terrestrial habitability zone. The mass of fine dust is 4 × 1019-2 × 1020 kg, equivalent to a 150-200 km radius asteroid. Significant emission features centered at 4 and 8 μm due to fluorescing SiO gas are also found. Roughly 1022 kg of SiO gas, formed by vaporizing silicate rock, is also present in the system, and a separate population of very large, cool grains, massing 1021-1022 kg and equivalent to the largest sized asteroid currently found in the solar system's main asteroid belt, dominates the solid circumstellar material by mass. The makeup of the observed dust and gas, and the noted lack of a dense circumstellar gas disk, strong stellar X-ray activity, and an extended disk of β meteoroids argues that the source of the observed circumstellar materials is a giant hypervelocity (>10 km s–1) impact between large rocky planetesimals, similar to the ones which formed the Moon and which stripped the surface crustal material off of Mercury's surface.

Print publication: Issue 2 (2009 August 20)
Received 2008 November 24, accepted for publication 2009 June 16
Published 2009 August 7

Link: Abstract August 20 issue of the Astrophysical Journal

Link: Discover Magazine Blog

Link: article

Link: YouTube Video (Planetary Smash-Up)

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