How polluted white dwarfs expanded what exoplanets are possible

Generally, geologists like Keith Putirka spend their careers peering down at rocks and pondering the rules of planetary evolution, not looking up at the stars.

But when Putirka, a professor of volcanology at California State University, Fresno, learned of a certain kind of star known as a “polluted white dwarf,” he realized there was a way to do both.

Polluted white dwarfs are dense, compact stars that have recently digested one of the rocky planets that formerly orbited them, and the mineral composition of those planets can be seen, for a time, in the spectral information from the star’s light. By examining polluted white dwarfs in our Solar System’s neighborhood, Putirka could learn what those exoplanets had been made of, and how similar they were to Earth or other rocky planets around our Sun.

The answer, as Putrika and his colleague Astronomer Siyi Xu detail in a paper published Tuesday in Nature, is not very similar at all.

“They're not Mars-like, they're not like the Moon, they're not like Mercury,” Putirka tells Inverse. “They look like stuff that is not anything like what is in our inner Solar System.”

It’s a finding that could change how geologists and astronomers understand planet formation and evolution and send scientists back to their labs to envision what a much wider, potentially weirder menagerie of exoplanets might look like.

What’s new — White dwarfs are small, dense zombie stars around the size of Earth, but were once stars not unlike our Sun. When they age, they swell into a red giant, incinerating closely orbiting planets and disrupting the orbits of the survivors.

When a red giant sheds its outer layers and leaves behind a white dwarf, the disrupted planets can swing too close to the dense star’s gravity and get pulled to pieces, raining rocky material onto the star. Putirka and Xu were able to break down their spectra to determine their composition.

Putirka and Xu examined 23 polluted white dwarfs within 650 light years of our Sun and measured the composition of the stars and the material falling into them. Then they compared those compositions to Earth and the other rocky planets of our Solar System, as well as local asteroids and meteorites.

The values of magnesium, calcium, silicon, and iron in those systems differed widely from each other, and from the rocky planets of our Solar System.

“So our inner Solar System doesn't give us the complete range of the kinds of rocks and minerals that we might expect exoplanets to be made out of,” Putirka says. “We have to think a little more broadly about the kinds of compositions that might evolve around other stars.”

But the study also upends some assumptions about how planets form and what information can be gleaned by studying polluted white dwarfs.

Astronomers have studied polluted white dwarfs for around 100 years, according to Putirka, and noted some such stars showed signs of lots of calcium in their spectra. “They don't necessarily think in terms of mineral and rock types, because it's just not what their training is,” he says.

But astronomers knew enough geology to know continental crust, like Earth’s, is high in calcium and theorized that, perhaps, the calcium was coming from the crusts of planets swallowed by the white dwarfs.

Putirka and Xu showed that this is not the case.

“Silica is a much better identifier of continental crust than calcium,” Putirka says, and while the stars they studied had high calcium levels, they had low levels of silicon, and high levels of magnesium, which is more typical of the Earth’s mantle. “If you take a planet and throw it into a star and the whole thing dissolves, it’s kind of what you would expect to see,” he says. “The crust is just a tiny fraction, about half a percent by weight of the material.”

The other assumption the paper upends is that planets will tend to reflect their stars. The Earth, our Solar System’s other rocky planets, and chondritic asteroids all show mineral profiles similar to heavier elements in the Sun’s spectra. The idea was that as planets and their stars form out of the same solar nebula, they would share similar mineral compositions.

The polluted white dwarfs and their planets do not.

“We thought they did. That was a working assumption,” Putirka says. “It seems to work pretty well in our Solar System. But it might not work so well in other solar systems.”

Why it matters — The findings are important both for understanding exoplanets and Earth’s place in the larger scheme of planetary bodies in the universe.

A wider range of mineral compositions on exoplanets could mean a wider range of behavior, different evolutionary paths to the development of planetary crusts, oceans, and atmospheres, all of which resulted — on Earth — from the partial melting of the mantle early in our planet’s history.

Life as we know it on Earth, for instance, depends a great deal on continental crust and plate tectonics, Putirka says. Plate tectonics is part of our planet’s water cycle, affects the development of the oceans, and, of course, gave life someplace to live other than the oceans.

“You and I don’t exist without continental crust,” Putirka says. “Biological evolution would have taken some other path if there was no dry land.”

What’s next?— Geologists and planetary science have done a whole lot of experiments with rocks and heat and pressure chambers to try and understand how planets evolve geologically. But all of those experiments used mineral compositions based on Earth and our Solar System’s planets.

“I think one of the things is that we need to do just a whole new set of experiments looking at the behavior of materials that are more extreme than what we've looked at so far for modeling planetary behavior,” Putirka says.

Researchers can synthesize rocks based on the compositional values he and Xu detected in these distant exoplanets and heat, stress, compress and otherwise test them to get a sense of how crusts and oceans, and atmosphere might develop on these strange new worlds.

“When we do those experiments there might be some surprises,” Putirka adds. “We won't know until we do them.”

ABSTRACT: Prior studies have hypothesized that some polluted white dwarfs record continent-like granitic crust—which is abundant on Earth and perhaps uniquely indicative of plate tectonics. But these inferences derive from only a few elements, none of which define rock type. We thus present the first estimates of rock types on exoplanets that once orbited polluted white dwarfs—stars whose atmospheric compositions record the infall of formerly orbiting plane- tary objects—examining cases where Mg, Si, Ca and Fe are measured with precision. We find no evidence for continental crust, or other crust types, even after correcting for core formation. However, the silicate mantles of such exoplanets are discernable: one case is Earth like, but most are exotic in composition and mineralogy. Because these exoplanets exceed the compositional spread of >4,000 nearby main sequence stars, their unique silicate compositions are unlikely to reflect variations in parent star compositions. Instead, polluted white dwarfs reveal greater planetary variety in our solar neighborhood than currently appreciated, with consequently unique planetary accretion and differentiation paths that have no direct counterparts in our Solar System. These require new rock classification schemes, for quartz + orthopyroxene and periclase + olivine assemblages, which are proposed here.