* Astronomy

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White Dwarf Planetary Systems

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White dwarf Debris Disks

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RE: White dwarf planets

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Title: Transit surveys for Earths in the habitable zones of white dwarfs
Authors: Eric Agol (University of Washington)

To date the search for habitable Earth-like planets has primarily focused on nuclear burning stars. I propose that this search should be expanded to cool white dwarf stars that have expended their nuclear fuel. I define the continuously habitable zone of white dwarfs, and show that it extends from ~0.005 to 0.02 AU for white dwarfs with masses from 0.4-0.9 solar masses, temperatures less than 10,000 K, and habitable durations of at least 3 Gyr. As they are similar in size to Earth, white dwarfs may be completely eclipsed by terrestrial planets that orbit edge-on, which can easily be detected with ground-based telescopes. If planets can migrate inward or reform near white dwarfs, I show that a global robotic telescope network could carry out a transit survey of nearby white dwarfs placing interesting constraints on the presence of habitable Earths. If planets were detected, I show that the survey would favour detection of planets similar to Earth: similar size, temperature, rotation period, and host star temperatures similar to the Sun. The Large Synoptic Survey Telescope (LSST) could place even tighter constraints on the frequency of habitable Earths around white dwarfs. The confirmation and characterization of these planets might be carried out with large ground and space telescopes.

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Title: Ancient planetary systems are orbiting a large fraction of white dwarf stars
Authors: B. Zuckerman, C. Melis, B. Klein, D. Koester, M. Jura

Infrared studies have revealed debris likely related to planet formation in orbit around ~30% of youthful, intermediate mass, main sequence stars. We present evidence, based on atmospheric pollution by various elements heavier than helium, that a comparable fraction of the white dwarf descendants of such main sequence stars are orbited by planetary systems. These systems have survived, at least in part, through all stages of stellar evolution that precede the white dwarf. During the time interval (~200 million years) that a typical polluted white dwarf in our sample has been cooling it has accreted from its planetary system the mass of one of the largest asteroids in our solar system (e.g., Vesta or Ceres). Usually, this accreted mass will be only a fraction of the total mass of rocky material that orbits these white dwarfs; for plausible planetary system configurations we estimate that this total mass is likely to be at least equal to that of the Sun's asteroid belt, and perhaps much larger. We report abundances of a suite of 8 elements detected in the little studied star G241-6 that we find to be among the most heavily polluted of all moderately bright white dwarfs.

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Astronomers keep turning up new exoplanets, and as the count rises, they keep edging closer to finding worlds like our own pale blue dot. Astronomer Jay Farihi thinks Earth-like worlds might be even more common in the universe than previously expected, based on evidence from rocky planets few astronomers are studying: The ones that don't exist anymore.
Farihi research subjects are white dwarfs. In our galaxy, about 90 percent of stars will end their lives in this incredibly dense state once the star sheds its outer material and only the core remains. This is the fate of our sun. White dwarfs usually have atmospheres composed of the light elements helium and hydrogen, as the heavy elements have settled to the core. But about 20 percent of white dwarfs are different, showing heavy elements-what astronomers call "metals"-in their atmospheres.
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Looking for Planets Around White Dwarfs
Our Solar System is such a nice, ordered place. Eight planets have been happily circling our sun for 4.6 billion years, and the whole system is stable enough to life to have formed on at least one planet. And the solar system looks like it will continue to be a nice, stable place to live, at least for another six billion years. At that time, the sun will run out of hydrogen fuel, swell up into a red giant star (swallowing Mercury, Venus, and maybe Earth in the process), expel about half of its mass as a planetary nebula, and then shrink into a white dwarf. When the sun loses mass, its gravitational pull will weaken a little. This will cause the remaining planets to move outwards in their orbits a little bit, but they should remain bound to the white dwarf sun, forever circling the ashes of our star.

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B1257+12

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RE: White dwarf planets

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Astronomers have turned to an unexpected place to study the evolution of planets - dead stars.
Observations made with NASAs 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.

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