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An artist’s conception of a body about the size of our Moon slamming into a body the size of Mercury. As the bodies hit each other at speeds exceeding 10 km per sec (about 22,400 mph), a huge flash of light is emitted, and their rocky surfaces are vaporized and melted, spraying hot matter everywhere. |
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| Observation data Epoch J2000.0 Equinox J2000.0 |
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|---|---|
| Constellation | Pavo |
| Right ascension | 18h 45m 26.9011s |
| Declination | -64° 52′ 16.533″ |
| Apparent magnitude (V) | 4.8 |
| Astrometry | |
| Distance | 95.34±1.86 ly (29.23±0.57 pc) |
| Details | |
| Mass | 2.0[1] M☉ |
| Luminosity | 9.5[1] L☉ |
| Age | ~12×106[1] years |
| Database references | |
| SIMBAD | data |
HD 172555 is a white-hot A5V star located relatively close by, 95 light years from Earth in the direction of the constellation Pavo.[2] Spectrographic evidence indicates a relatively recent collision between two planet-sized bodies that destroyed the smaller of the two, which was about the size of Earth's moon, and severely damaged the larger one, which is about the size of Mercury. Evidence of the collision was detected by NASA's Spitzer Space Telescope.[3]
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HD172555 was first recognized in the 1980s as being unusually bright in the mid-infrared by the IRAS sky survey. Follow-up ground based observations by Schütz et al. (2004) and the Spitzer Space Telescope, also in 2004 (Chen et al. 2006), confirmed the unusually strong nature of the infrared spectral emission from this system, much brighter than what would be emitted normally from the star's surface. As part of the Beta Pictoris moving group, HD172555 is coeval with that more famous system, approximately 12 million years old, and is the same kind of white-hot star as Beta Pic, about twice as massive as our Sun and about 9.5 times as luminous.[1] Comparison with current planetary formation theories, and with the very similar Beta Pic system, suggests that the HD172555 is in the early stages of terrestrial (rocky) planet formation. But what makes HD 172555 special is the presence of a large amount of unusual silicaceous material – amorphous silica and SiO gas – not the usual rocky materials, silicates like olivine and pyroxene, which make up much of the Earth as well.[4]
The material in the disk was analyzed in 2009 by Carey Lisse,[5] of the Johns Hopkins University Applied Physics Laboratory in Laurel, MD using the infrared spectrometer on board the Spitzer Space Telescope, and the results of the Deep Impact and STARDUST comet missions. Analysis of the atomic and mineral composition, dust temperature, and dust mass show a massive (about a Moon’s mass worth) amount of warm (about 340K) material similar to re-frozen lava (obsidian) and flash-frozen magma (tektite) as well as copious amounts of vaporized rock (silicon monoxide or SiO gas) and rubble (large dark pieces of dust) in a region at 5.8+/-0.6 AU from the HD172555 (at the edge of any terrestrial habitability zone, equivalent to a location in the inner part of the asteroid belt in our solar system). The material had to have been created in a hypervelocity impact between two large bodies; relative velocities at impacts less than 10 km/sec would not transform the ubiquitous olivine and pyroxene into silica and SiO gas. Giant impacts at this speed typically destroy the incident body, and melt the entire surface of the impactee.
The implications for the detection of abundant amorphous silica and SiO gas are the following: