Planetary debris found in white dwarf atmospheres

In summary: I think the definition is terrible. Exoplanets are technically not planets. An object at a particular place and time is a planet but the same object at another time or place is not. I understand why they did it (although I would argue that the problem is not, as Wkkipedia claims, with Eris but rather with Sedna) but I think the solution is no better.
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I read this interesting article today, the original paper here. Abstract:
White dwarfs that accrete the debris of tidally disrupted asteroids provide the opportunity to measure the bulk composition of the building blocks, or fragments, of exoplanets. This technique has established a diversity in compositions comparable to what is observed in the solar system, suggesting that the formation of rocky planets is a generic process. Whereas the relative abundances of lithophile and siderophile elements within the planetary debris can be used to investigate whether exoplanets undergo differentiation, the composition studies carried out so far lack unambiguous tracers of planetary crusts. Here we report the detection of lithium in the atmospheres of four cool (<5,000 K) and old (cooling ages 5-10 Gyr) metal-polluted white dwarfs, where one also displays photospheric potassium. The relative abundances of these two elements with respect to sodium and calcium strongly suggest that all four white dwarfs have accreted fragments of planetary crusts. We detect an infrared excess in one of the systems, indicating that accretion from a circumstellar debris disk is on-going. The main-sequence progenitor mass of this star was 4.8±0.2M⊙, demonstrating that rocky, differentiated planets may form around short-lived B-type stars.
 
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  • #2
As far as I know, most exoplanets we discover today are around "small" hosting stars, similar to our Sun. Categories F, G and K are most common:
https://en.wikipedia.org/wiki/Planet-hosting_stars#Type_of_star,_spectral_classification

On the other hand, planetary formation around O-type stars is not possible:
Observations using the Spitzer Space Telescope indicate that extremely massive stars of spectral category O, which are much hotter than the Sun, produce a photo-evaporation effect that inhibits planetary formation.

So I find this paper as a fascinating evidence that formation of planetary systems should have been possible around B stars.
 
  • #3
I would not expect planets around O's. They don't live long, and in that time you need to form exoplanets, and (since the Pluto Purge) give them time to clear the neighborhood.
 
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  • #4
Good point. I wonder whether the objects orbiting a B star would have enough time to clear neighborhoods, so IAU could call them (exo)planets. For a star with 5 solar mass, the lifetime is only about 170 million yrs. Could be that enough time?
 
  • #5
lomidrevo said:
Good point. I wonder whether the objects orbiting a B star would have enough time to clear neighborhoods, so IAU could call them (exo)planets. For a star with 5 solar mass, the lifetime is only about 170 million yrs. Could be that enough time?
Vanadium 50 said:
I would not expect planets around O's. They don't live long, and in that time you need to form exoplanets, and (since the Pluto Purge) give them time to clear the neighborhood.
Shoemaker Levy-9 hit Jupiter. There are millions of Jupiter Trojans. 99942 Apophis is still flying by Earth again. Either there are no planets or the definition needs to mean the object has the potential to clear its orbit.

The radiation pressure will be 1000 times higher if luminosity is 1000 times higher. So larger dust particles would blow out on hyperbolic orbits. The time scale for Poynting-Robertson effect would drop too. That should have some effect on planet formation. The neighborhood would clear out fairly quickly.
 
  • #6
stefan r said:
Either there are no planets or the definition needs to mean the object has the potential to clear its orbit.

I think the definition is terrible. Exoplanets are technically not planets. An object at a particular place and time is a planet but the same object at another time or place is not. I understand why they did it (although I would argue that the problem is not, as Wkkipedia claims, with Eris but rather with Sedna) but I think the solution is no better.

There are better ways to clarify this than adding "potentially" in front of "clearing the neighborhood", such as comparing the mass of the planet to the sum of all other bodies in its "neighborhood". By that standard, Earth (the most planetty planet) is 1.700,000 and Ceres is 0.33. There are no objects between Mars at 5000 and Pluto at 1/3.
 

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