HD 172555 is a white-hot A7V star located relatively close by, 95 light years from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

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 had been at least the size of Earth's moon, and severely damaged the larger one, which was at least the size of Mercury. Evidence of the collision was detected by NASA's Spitzer Space Telescope.

Giant hypervelocity impact debris

HD172555 was first recognized in the 1980s as being unusually bright in the mid-infrared by the

IRAS The Infrared Astronomical Satellite (Dutch: ''Infrarood Astronomische Satelliet'') (IRAS) was the first space telescope to perform a survey of the entire night sky at infrared wavelengths. Launched on 25 January 1983, its mission lasted ten mo ...

sky survey. Follow-up, ground-based observations by Schütz et al. and the Spitzer Space Telescope, also in 2004, 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 The Beta Pictoris Moving Group is a young moving group of stars located relatively near Earth. A moving group, in astronomy, is a group of stars that share a common motion through space as well as a common origin. This moving group is named for B ...

, HD172555 is coeval with that more famous system, approximately 20 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 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 (Johns Hopkins, Hopkins, or JHU) is a private research university in Baltimore, Maryland. Founded in 1876, Johns Hopkins is the oldest research university in the United States and in the western hemisphere. It consi ...

Applied Physics Laboratory in Laurel, MD using the

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...

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 Tektites (from grc, τηκτός , meaning 'molten') are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz ...

) as well as copious amounts of vaporized rock (

silicon monoxide Silicon monoxide is the chemical compound with the formula SiO where silicon is present in the oxidation state +2. In the vapour phase, it is a diatomic molecule. It has been detected in stellar objects and has been described as the most common o ...

or SiO gas) and rubble (large dark pieces of dust) in a region at 5.8+/-0.6 AU from the HD172555 (inside the

frost line The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the s ...

of that system). The material had to have been created in a hypervelocity impact between two large bodies; relative velocities at impacts less than 10 km/s 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: * Massive hypervelocity impacts happen in young planetary systems. There are a number of examples of such impacts in the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

(Hartmann & Vail 1986): Mercury's high density;

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

' retrograde rotation; Earth's

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

; Mars' North/South hemispherical cratering anisotropy; Vesta's igneous origin (Drake 2001);

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...

' rotation 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 lunar soils (Warren 2008). * Rocky protoplanets, and possibly planets, exist in the HD172555 system, at about 20 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. Follow-up VISNIR observations of the system published in 2020 have shown that the majority of observed fine dust is composed of very fine grains 1-4 micrometers diameter. as expected from a recent hypervelocity impact. In 2021, a

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

ring at ~6 AU separation from the star was also found in the system by ALMA, further reinforcing a giant impact scenario for explaining the system's structure. The large amount of CO gas detected would likely have been sourced from the colliding planets' atmospheres.

References

External links

When worlds collide
Phil Plait,

Discover Discover may refer to: Art, entertainment, and media * ''Discover'' (album), a Cactus Jack album * ''Discover'' (magazine), an American science magazine Businesses and brands * DISCover, the ''Digital Interactive Systems Corporation'' * D ...

website, Blogs / Bad Astronomy. August 10, 2009.

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

animation of what the collision may have looked like. Retrieved 2009-08-11

Giant hypervelocity impact debris

See also

References

External links

Further reading