* Astronomy

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RE: White dwarf planets

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A survey of dead stars has found that many are probably surrounded by the rocky remains of asteroids and planets.
The discovery comes from a Spitzer space-telescope study of white dwarfs - the fading embers of burnt-out stars.
Astronomers showed at least one in 100 of these objects has "surface" layers that are contaminated with elements such as calcium and magnesium.

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A glimpse of the bleak fate that awaits the Earth in billions of years has emerged from research into dying stars that once blazed as brightly as the Sun.
Astronomers have discovered that at least one in 100 white dwarfs the burnt-out remnants of Sun-like stars - once had solar systems, with planets that were destroyed or deep-frozen by the death throes of their stars
The research, by a team led by Jay Farihi, of the University of Leicester, suggests millions of other solar systems have endured the destiny predicted for the Earth when the Sun dies.

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Using the infrared detectors on NASA's Spitzer Space Telescope, a team of researchers based in the U.K. and in the United States have placed new constraints on how often rocky planets like Earth may exist around stars other than our Sun.
The team of researchers, including Jay Farihi, from the University of Leicester; and Michael Jura and Ben Zuckerman, both from UCLA, looked at the environments surrounding a number of "dead" stars known as a "white dwarfs". Using Spitzer's sensitive infrared eyes, they found that at least 1-3% of all white dwarf stars of a certain age are surrounded by dust and rocky debris.

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Using NASA's Spitzer Space Telescope, an international team of astronomers have found that at least 1 in 100 white dwarf stars show evidence of orbiting asteroids and rocky planets, suggesting these objects once hosted Solar Systems similar to our own. Team member Dr Jay Farihi of the University of Leicester will present this discovery on Monday 20th April at the European Week of Astronomy and Space Science conference at the University of Hertfordshire.
White dwarf stars are the compact, hot remnants left behind when stars like our Sun reach the end of their lives. Their atmospheres should consist entirely of hydrogen and helium but are sometimes found to be contaminated with heavier elements like calcium and magnesium. The new observations suggest that these Earth-sized stars are often polluted by a gradual rain of closely orbiting dust that emits infrared radiation picked up by Spitzer.
The data suggest that at least 1% to 3% of white dwarf stars are contaminated in this way and that the dust originates from rocky bodies like asteroids (also known as minor planets). The Spitzer results imply that asteroids are found in orbit around a large number of white dwarfs, perhaps as many as 5 million in our own Milky Way Galaxy.

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Title: Infrared Signatures of Disrupted Minor Planets at White Dwarfs
Authors: J. Farihi, M. Jura, B. Zuckerman

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T < 20,000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457-086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates dM/dt > 3e8 g/s display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 1e6 g/s in the solar system zodiacal cloud and 1e10 g/s often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

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Dust made up of similar stuff as the Earth has been found in and around a handful of dead stars. The dust, which was left behind when the stars chewed up errant asteroids, suggests terrestrial planets may be common.
Six white dwarfs, the burned-out embers of Sun-like stars, showed heavy elements, or metals, in their atmospheres. That is unusual because white dwarfs contain about as much mass as the Sun squeezed into bodies the size of the Earth, giving them surface gravities 10,000 times stronger than the Sun's. That should cause heavy elements to sink towards their centres - and out of sight.
In addition, the six stars also shine more brightly than expected in infrared light. That suggests the stars are surrounded by dust, which glows at infrared wavelengths. The dusty debris is thought to be the remains of asteroids that once orbited the white dwarfs but were gravitationally torn apart when they wandered too close to the stars.

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New observations of chewed-up asteroids around old dead stars called white dwarfs bolster the idea that the Earth and other rocky planets in our solar system are far from alone in the universe.
Astronomers used NASA's Spitzer Space Telescope to peer at six such white dwarf stars and found the signature of asteroid debris circling the stars. An analysis of the light coming from the systems show the rings are made of some of the same materials as rocky bodies in our own solar system.

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Astronomers have turned to an unexpected place to study the evolution of planets -- dead stars.
Observations made with NASA's Spitzer Space Telescope reveal six dead "white dwarf" stars littered with the remains of shredded asteroids. This might sound pretty bleak, but it turns out the chewed-up asteroids are teaching astronomers about the building materials of planets around other stars.
So far, the results suggest that the same materials that make up Earth and our solar system's other rocky bodies could be common in the universe. If the materials are common, then rocky planets could be, too.
In all eight systems observed, Spitzer found that the dust contains a glassy silicate mineral similar to olivine and commonly found on Earth.
The Spitzer data also suggest there is no carbon in the rocky debris -- again like the asteroids and rocky planets in our solar system, which have relatively little carbon.
A single asteroid is thought to have broken apart within the last million years or so in each of the eight white-dwarf systems. The biggest of the bunch was once about 200 kilometres in diameter.

Source
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2MASS J09393548-2448279

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Title: Metal-rich debris discs around white dwarfs
Authors: B.T. Gaensicke, T.R. Marsh, J. Southworth, A. Rebassa-Mansergas

We have identified two moderately hot (~18000-22000K) white dwarfs, SDSSJ1228+1040 and SDSSJ1043+0855, which exhibit double-peaked emission lines in the CaII 8600A triplet. These line profiles are unambiguous signatures of gaseous discs with outer radii of ~1Rsun orbiting the two white dwarfs. Both stars accrete from the circumstellar material, resulting in large photospheric Mg abundances. The absence of hydrogen emission from the discs, and helium absorption in the white dwarf photospheres demonstrates that the circumstellar material is depleted in volatile elements, and the most likely origin of these gaseous rings are tidally disrupted rocky asteroids. The relatively high mass of SDSSJ1228+1040 implies that planetary systems can not only form around 4-5Msun stars, but may also survive their post main-sequence evolution.

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PSR B1257+12