Weird Exoplanets Made of Rocks Unlike Anything Found in the Solar System

Astronomers have carried out the first estimates of rock types that exist on planets orbiting nearby stars. The team found the rock types are composed of exotic and diverse materials not found anywhere in the solar system.

While astronomers have thus far discovered over 4,000 planets outside the solar system, there has not been a good consensus on what types of material make up these worlds.

In an attempt to discover this, and find out if these worlds resemble our planet, National Science Foundation's (NSF) Noirlab astronomer Siyi Xu teamed with California State University, Fresno, geologist Keith Putirka. Their findings were published in the journalNature Communications.

"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 said in a press release from the NSF. "They have no direct counterparts in the Solar System."

The duo studied the atmospheres around white dwarfs, the stellar remnants that are left behind when stars of a similar size to the sun exhaust their nuclear fuel and undergo gravitational collapse, shedding their outer layers in supernova explosions.

These dense collapsed stellar cores are a good choice to conduct such a survey because they are composed mostly of material that was once part of the star's core, hydrogen, and helium. However, the atmospheres around white dwarfs become "polluted" when material from the rocky bodies, such as planets and asteroids, fall into them.

That means, that astronomers can discover what these rocky bodies were made of by studying these atmospheres and detecting materials that aren't supposed to be there.

Xu and Putirka selected 23 white dwarfs located within 650 light-years of the sun where elements such as calcium, silicon, magnesium and iron had previously been detected by telescopes like the Hubble Space Telescope.

They reconstructed the minerals and rocks that would have deposited the measured abundances of these elements.

The white dwarfs they studied had much more varied and exotic materials than is found in the inner solar system's rocky bodies. This suggests that the planets around the white dwarfs they studied had a wider range of rock types than is found in our solar system.

In fact, some of these rock types were so exotic, the duo had to provide new names for them, including "quartz pyroxenites" and "periclase dunites."

"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 explained. "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."

Previous research of polluted white dwarfs discovered elements including calcium, aluminum and lithium, which in small part compose rocks here on Earth. What Xu and Putirka suggest is that to know what kind of rocks made up exoplanets that once existed around these stellar remnants, measurements of major Earth-rock components like silicon need to be measured.

Because the duo detected high levels of magnesium and low levels of silicon in the atmospheres of the white dwarfs they studied, they suggest that the rocky debris that left it behind came from the interiors of exoplanets, their mantle rather than their crust.

While Putirka and Xu found no evidence of crustal rocks in their research, they can't completely rule out that the exoplanets that fell into the white dwarf had continental crust or other crust types.

Putirka added: "We believe that if crustal rock exists, we are unable to see it, probably because it occurs in too small a fraction compared to the mass of other planetary components, like the core and mantle, to be measured."

The duo's work marks the unification of astronomy and geology to learn more about the composition of planets outside our solar system and demonstrates that the Milky Way is a varied and exotic galaxy.

"I met Keith Putirka at a conference and was excited that he could help me understand the systems that I was observing," Xu concluded. "He taught me geology and I taught him astronomy, and we figured out how to make sense of these mysterious exoplanetary systems."

Rocky Exoplanets
An illustration showing rocky debris around a distant white dwarf. New research has shown that exoplanets orbiting these stellar remnants could be stranger than we knew. NSF/AURA/J. da Silva/NOIRLab