Editing White dwarf (section)

== Habitability ==
A search has been proposed for [[transit (astronomy)|transits]] of hypothetical Earth-like planets around white dwarfs with surface temperatures of less than {{val|10000|u=K}}. Such stars that could harbor a [[habitable zone]] at a distance of {{circa}}&nbsp;0.005 to 0.02&nbsp;[[Astronomical unit|AU]] that would last upwards of 3&nbsp;billion years.<!-- see page 2 of the paper--> This is so close that any habitable planets would be [[tidally locked]]. As a white dwarf has a size similar to that of a planet, these kinds of transits would produce strong [[eclipse]]s.<ref>
{{cite journal
 |bibcode=2011ApJ...731L..31A
 |arxiv= 1103.2791
 |doi= 10.1088/2041-8205/731/2/L31
 |title=Transit Surveys for Earths in the Habitable Zones of White Dwarfs
 |date=2011
 |last1=Agol |first1=Eric
 |journal=The Astrophysical Journal Letters
 |volume=635 |issue=2 |pages=L31
 |s2cid= 118739494
}}</ref>

Newer research casts some doubts on this idea, given that the close orbits of those hypothetical planets around their parent stars would subject them to strong [[tidal force]]s that could render them uninhabitable by triggering a [[greenhouse effect]].<ref>
{{cite journal
 |bibcode=2013AsBio..13..279B
 |arxiv= 1211.6467
 |doi= 10.1089/ast.2012.0867
 |title=Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary
 |date=2011
 |last1=Barnes |first1=Rory |last2=Heller |first2=René
 |journal=Astrobiology
 |volume=13 |issue=3 |pages=279–291
 |pmid=23537137 |pmc=3612282
}}</ref> Another suggested constraint to this idea is the origin of those planets. Leaving aside formation from the [[accretion disk]] surrounding the white dwarf, there are two ways a planet could end in a close orbit around stars of this kind: by surviving being engulfed by the star during its red giant phase, and then spiralling inward, or inward migration after the white dwarf has formed. The former case is implausible for low-mass bodies, as they are unlikely to survive being absorbed by their stars. In the latter case, the planets would have to expel so much orbital energy as heat, through tidal interactions with the white dwarf, that they would likely end as uninhabitable embers.<ref>
{{cite journal
 |bibcode=2013MNRAS.432..500N
 |arxiv=1211.1013
 |doi=10.1093/mnras/stt569
 |title=On the orbits of low-mass companions to white dwarfs and the fates of the known exoplanets
 |date=2013
 |last1=Nordhaus |first1=J. |last2=Spiegel |first2=D.S.
 |journal=Monthly Notices of the Royal Astronomical Society
 |volume=432 |issue=1 |pages=500–505
 |doi-access=free
 |s2cid=119227364
}}</ref>