HD 172555
Encyclopedia
HD 172555 is a white-hot A5V star
located relatively close by, 95 light years from Earth
in the direction of the constellation
Pavo
. 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
.
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. 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.
The material in the disk was analyzed in 2009 by Carey Lisse, 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:
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
located relatively close by, 95 light years from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
in the direction of the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Pavo
Pavo (constellation)
Pavo is a constellation in the southern sky. Its name is Latin for peacock. It is one of twelve constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman and it first appeared on a 35-cm diameter celestial globe published in 1597 in...
. 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
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
.
Giant Hypervelocity Impact Debris
HD172555 was first recognized in the 1980s as being unusually bright in the mid-infrared by the IRASIRAS
The Infrared Astronomical Satellite was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths....
sky survey. Follow-up ground based observations by Schütz et al. (2004) and the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
, 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
Beta Pictoris
Beta Pictoris is the second brightest star in the constellation Pictor. It is located 63.4 light years from our solar system, and is 1.75 times as massive and 8.7 times as luminous as the Sun. The Beta Pictoris system is very young, only 8–20 million years old, although it is already in the main...
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. 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.
The material in the disk was analyzed in 2009 by Carey Lisse, of the Johns Hopkins University
Johns Hopkins University
The Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university based in Baltimore, Maryland, United States...
Applied Physics Laboratory in Laurel, MD using the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
on board the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
, 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
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
) and flash-frozen magma (tektite
Tektite
Tektites are natural glass rocks up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular.Tektites are among the "driest" rocks, with...
) as well as copious amounts of vaporized rock (silicon monoxide
Silicon monoxide
Silicon monoxide is the chemical compound with the formula SiO. In the vapour phase it is a diatomic molecule. It has been detected in stellar objects and it has been described as the most common oxide of silicon in the universe....
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:
- Mass hypervelocity impacts happen in young solar systems. There are a number of examples of such impacts in our own solar system (Hartmann & Vail 1986): Mercury’s high density; Venus’ retrograde spin; Earth’s Moon; Mars’ North/South hemispherical cratering anisotropy; Vesta’s igneous origin (Drake 2001); Uranus’ spin axis located near the plane of the ecliptic. Local geological evidence for widespread impact melting includes tektites found on Earth and glass beads found in lunarsoils (Warren 2008).
- Rocky protoplanets, and possibly planets, exist in the HD172555 system, at about 12 Myr after its formation.
- If the collision happened within the last few thousand years, there is likely a protoplanet in the HD172555 system with a liquid magma surface. This is not unexpected; a simple calculation of the gravitational binding energy of the Earth, shows that the energy released in assembling the Earth is about 10x the amount needed to melt it.
External links
- When worlds collide. Phil Plait, DiscoverDiscover (magazine)Discover is an American science magazine that publishes articles about science for a general audience. The monthly magazine was launched in October 1980 by Time Inc. It was sold to Family Media, the owners of Health, in 1987. Walt Disney Company bought the magazine when Family Media went out of...
website, Blogs / Bad Astronomy. August 10, 2009. NASANASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
animation of what the collision may have looked like. Retrieved 2009-08-11