Planets Around White Dwarfs

Stars of mass lower than about eight solar masses do not become neutron stars. Rather the fusion in their cores fizzles out before the chain of fusion reactions terminates with formation of an iron care, and the outer envelope of the bloated red giant star is shed, the core contracts and cools passively, forming a so-called white dwarf. White dwarfs do not generate energy internally and are purely pressure supported. Consequently they have high densities, though not as high as neutron stars, and radii comparable with that of the Earth. White dwarfs emerge very hot, and luminous, but cool rapidly and after a hundred million years or so, depending on their exact mass and composition, the luminosity of the white dwarf is much less than its progenitor star.

In the process of shedding its envelope, any inner planets orbiting the red giant star are swallowed up and destroyed. Calculations show that when the Sun ends its life, Mercury and Venus will certainly be destroyed, Mars will probably survive, and the prospects for the survival of the Earth are so marginal that the outcome is dominated by the uncertainties in the modeling. The outer giant planets all survive.

The giant planets are moderately warm, and radiate in the infrared from their intrinsic thermal emission rather than by reflecting the light of their parent star. Since the white dwarf is much fainter than the original star, the contrast between

Fig. 8.3. Hubble Space Telescope Near Infrared Camera image of the immediate surroundings of a nearby DAZ white dwarf. The irregular spread out structure in the upper left is the residual light from the white dwarf after it was masked out by a coronograph, and the image self-subtracted. Two images were taken at different camera roll angles, and then digitally rotated into alignment and subtracted from each other. The light from the white dwarf is suppressed by a factor of several hundred, permitting high contrast searches for close low luminosity objects. The small dot to the left is about ten thousand times fainter than the star and about one arcsecond away, and was consistent in luminousity and colour with a few jupiter mass companion, but subsequent imaging revealed it to be a background object (Debes, private communication).

Fig. 8.3. Hubble Space Telescope Near Infrared Camera image of the immediate surroundings of a nearby DAZ white dwarf. The irregular spread out structure in the upper left is the residual light from the white dwarf after it was masked out by a coronograph, and the image self-subtracted. Two images were taken at different camera roll angles, and then digitally rotated into alignment and subtracted from each other. The light from the white dwarf is suppressed by a factor of several hundred, permitting high contrast searches for close low luminosity objects. The small dot to the left is about ten thousand times fainter than the star and about one arcsecond away, and was consistent in luminousity and colour with a few jupiter mass companion, but subsequent imaging revealed it to be a background object (Debes, private communication).

any outer giant planets and the white dwarf is much lower, typically by a factor of 1,000-10,000, than the contrast between the progenitor star and any planets. Consequently, giant planets that survive the death of their parent star and are still orbiting the remnant white dwarf are comparatively easy targets for direct imaging by high resolution infrared telescopes. A number of projects are currently underway to attempt such imaging of nearby white dwarfs, (e.g. Burleigh et al., 2002; Debes et al., 2005). See Hansen (2004) section 8.5 for a review.

One particularly intriguing aspect of white dwarf astronomy is so-called "DAZ" white dwarfs. These are moderately cool white dwarfs with hydrogen atmospheres, which show evidence for metal absorption lines, In a few instances, there is evidence for warm dust disks around the white dwarfs, containing the equivalent of a few cubic kilometers of metals. Theoretical models suggest the metal absorption lines, and the associated debris disks, came from tidal disruption of a planetesimal, an asteroidal or cometary body (Debes & Sigurdsson, 2002; Jura, 2003). Any surviving planetesimal must come from the outer system, and to get it into a radial orbit which will lead to tidal disruption of the object and a warm disk or dust contamination of the white dwarf atmosphere, requires a planet to perturb the orbit of the planetesimal. It is therefore conjectured that the DAZ white dwarfs are particularly promising targets for imaging any planets which might be in orbit around them. Current telescopes can marginally detect several Jupiter mass planets, if any such are present, and searches to date have not found any. The next generation of telescopes, in particular, the James Webb Space Telescope, will be able to easily image any giant planet around nearby white dwarfs, and in fact map out the orbit of the planet directly from its motion relative to the star over several years. With some additional effort, spectroscopy of giant planet atmospheres would also be possible, testing theories of giant planet atmospheres and carrying out pathfinding science for observation of planets around main sequence stars.

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Fig. 8.4. Conceptual image showing the dynamical evolution of some outer planetary systems after the end of the stellar main sequence, as the star becomes a white dwarf. The outer planets spiral outwards in their orbits as the star loses mass during the giant phase. For some orbital configurations, secular evolution drives the orbits of the planets to become chaotic, on time scales of hundred million years or more. The planets' orbits cross and there is a strong dynamical rearrangement, including possible collisions and ejections. The planets settle down into a new orbital configuration, typically this will include an outer planet on a wide eccentric orbit, penetrating the region where Kuiper belt-like objects might be founds, while any surviving inner planets might be scattered into closer orbits, repopulating the orbital region where planets were swallowed during the giant phase (Debes & Sigurdsson, 2002).

Fig. 8.4. Conceptual image showing the dynamical evolution of some outer planetary systems after the end of the stellar main sequence, as the star becomes a white dwarf. The outer planets spiral outwards in their orbits as the star loses mass during the giant phase. For some orbital configurations, secular evolution drives the orbits of the planets to become chaotic, on time scales of hundred million years or more. The planets' orbits cross and there is a strong dynamical rearrangement, including possible collisions and ejections. The planets settle down into a new orbital configuration, typically this will include an outer planet on a wide eccentric orbit, penetrating the region where Kuiper belt-like objects might be founds, while any surviving inner planets might be scattered into closer orbits, repopulating the orbital region where planets were swallowed during the giant phase (Debes & Sigurdsson, 2002).

Fig. 8.5. Hubble Space Telescope Near Infrared Camera image of the immediate surroundings of a nearby DAZ white dwarf. This white dwarf is close to the galactic plane where there are a lot of infrared sources, six sources were found which were close in angular separation and had colours and magnitudes consistent with giant planets, none have been confirmed to date as physically associated companions (Debes, private communication).

Fig. 8.5. Hubble Space Telescope Near Infrared Camera image of the immediate surroundings of a nearby DAZ white dwarf. This white dwarf is close to the galactic plane where there are a lot of infrared sources, six sources were found which were close in angular separation and had colours and magnitudes consistent with giant planets, none have been confirmed to date as physically associated companions (Debes, private communication).

An additional channel for detecting planets around white dwarfs comes from the possibility of planet formation in excretion disks formed by merging white dwarfs (Livio et al., 1992). Some binary stars evolve to form binary white dwarfs in very tight orbits, which may eventually merge. The process would lead to a very massive white dwarf, or possibly a pulsar, if the combined mass of the white dwarfs is high enough and they can merge without exploding, then in the process an "excretion disk" may form, analogous to the pulsar case. As in the pulsar case, planets may conceivably form in close orbits about the new massive white dwarf, where they may be detected, in an orbital region that would otherwise be scoured clean of planets during the formation of the white dwarfs themselves.

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