An examination of the chemical constituents within the atmospheres of nearby, so-called ‘polluted’ white dwarfs indicates that the majority of rocky exoplanets are notably exotic in their composition and mineralogical makeup.
The initial estimations of exoplanetary rock types, derived from the analysis of polluted white dwarfs whose atmospheric compositions serve as records of formerly orbiting celestial bodies, are presented by Putirka & Xu. Image attribution: NOIRLab / NSF / AURA / J. da Silva / M. Zamani & M. Kosari, NSF’s NOIRLab.
White dwarfs represent stellar remnants that have exhausted their nuclear fuel, having transitioned from their main sequence phase. These stars first undergo an expansion to become red giants before contracting to a dimension approximately equivalent to that of Earth.
Celestial bodies in orbit around such stars are susceptible to disintegration under the immense gravitational forces exerted by their host star. The ensuing detritus subsequently descends into the stellar atmospheres.
These peculiarly enriched white dwarfs function as sophisticated analyzers, akin to ‘cosmic mass spectrometers,’ providing remarkably direct insights into the compositional makeup of exoplanets.
The origin of this atmospheric enrichment can be attributed to either entire planetary bodies or fragmented remnants of planets, comparable to the constituents of our Solar System’s primary asteroid belt.
By identifying elements that are not naturally present in a white dwarf’s atmosphere — that is, anything beyond hydrogen and helium — astronomers can deduce the material composition of these rocky celestial objects.
In a recent investigation, Professor Keith Putirka, a geologist at California State University, collaborated with Dr. Siyi Xu from the Gemini Observatory and NSF’s NOIR Lab. Their focus was on 23 polluted white dwarfs situated within approximately 650 light-years of our Sun. These stars had their calcium, silicon, magnesium, and iron content precisely measured using advanced instrumentation, including the W. M. Keck Observatory in Hawai’i and the NASA/ESA Hubble Space Telescope, among other observing facilities.
Subsequently, the astronomers employed the quantified abundances of these elements to model the minerals and rocks that would naturally precipitate from such compositions.
Their findings revealed that these white dwarfs exhibit a considerably broader spectrum of compositions than any of the inner planets within our own Solar System, suggesting that the planets they once hosted possessed a more diverse array of rock types.
Indeed, some of these compositions were so extraordinary that the researchers were compelled to coin new nomenclature — including terms like quartz pyroxenites and periclase dunites — to categorize the novel rock formations that must have characterized those planetary bodies.
“While certain exoplanets that previously orbited polluted white dwarfs bear a resemblance to Earth, the majority are composed of rock types that are alien to our Solar System,” stated Dr. Xu.
“Some of the rock types identified through the white dwarf data exhibit properties that would result in more significant water dissolution than terrestrial rocks, potentially influencing ocean development,” Professor Putirka elaborated.
“Certain rock types may melt at considerably lower temperatures, forming thicker crusts than Earth’s rocks, and some rock formations could be less robust, which might foster the emergence of plate tectonics.”
“Prior investigations of polluted white dwarfs had identified elements originating from rocky celestial bodies, such as calcium, aluminum, and lithium.”
“However, these are considered minor constituents (typically comprising a small fraction of Earth-based rock). To accurately ascertain the rock types present on those planets, precise measurements of major elements (which constitute the bulk of Earth rocks), particularly silicon, are indispensable.”
“Furthermore, the presence of elevated magnesium levels and diminished silicon quantities detected in the white dwarfs’ atmospheres suggests that the identified rocky debris most likely originated from the planets’ internal structures — their mantles, rather than their crusts.”
Some earlier studies focusing on polluted white dwarfs had indicated the presence of continental crust on the rocky planets that formerly orbited those stars. However, this research team found no corroborating evidence for crustal rocks.
Nevertheless, the current observations do not definitively exclude the possibility that these planets possessed continental crust or other forms of crustal material.
“We posit that if crustal rock is present, it remains undetectable to us, likely because its proportion relative to the total mass of other planetary components, such as the core and mantle, is too minuscule for accurate measurement,” Professor Putirka explained.
These discoveries were disseminated this week in the esteemed journal Nature Communications.
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K.D. Putirka & S. Xu. 2021. Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhood. Nat Commun 12, 6168; doi: 10.1038/s41467-021-26403-8
