Astronomers have discovered that rocky exoplanets might be even weirder than previously thought. Observations of the remains of rocky exoplanets around white dwarfs have revealed unusual rock types that are not found anywhere in the solar system.
The number of confirmed exoplanets in our galaxy is in the thousands and there are even more candidate ones (including one in the famous Whirlpool galaxy). But studying rocky planets when they are whole is currently very difficult. A team of astronomers had the clever idea to look at planetary remains after they were destroyed by their stars.
As reported in Nature Communications, the team looked at polluted white dwarfs. These are stellar remnants, the exposed degenerate core of a star. The atmospheres of these objects are predominantly made of hydrogen and helium but can become "polluted" when materials from former planets, asteroids, and moons fall into them.
Astronomer Siyi Xu and geologist Keith Putirka looked at 23 polluted white dwarfs within 650 light-years. There are precise estimates of how much magnesium, silicon, calcium, and iron are present in these objects which allowed the team to reconstruct what the rocks that they originated from were like.
The team found no evidence of continental crust (the outer part of the planets) in the polluted material, probably due to its relatively small proportion among the debris. Deeper layers of rock are more abundant and not what they expected. One case had a mantle that is Earth-like but most are very exotic in composition and mineralogy.
"While some exoplanets that once orbited polluted white dwarfs appear similar to Earth, most have rock types that are exotic to our Solar System," Xu from NSF's NOIRLab, said in a statement. "They have no direct counterparts in the Solar System."
The duo had to come up with some new names for some of the rock types they expect to have made these ancient and now-obliterated planets. Among these new names are "quartz pyroxenites" and "periclase dunites" which are some really excellent sci-fi sounding names. But the importance of these new types of rock are their properties. These worlds will be very different from our own.
"Some of the rock types that we see from the white dwarf data would dissolve more water than rocks on Earth and might impact how oceans are developed," Putirka, from California State University, added. "Some rock types might melt at much lower temperatures and produce thicker crust than Earth rocks, and some rock types might be weaker, which might facilitate the development of plate tectonics."
The rocky worlds of the Milky Way might be a lot more exotic than our previous understanding might have suggested.
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