Scientists have directly observed the rocky guts of exoplanets, which are worlds from different star systems, by watching the fallout of these objects crashing into the corpses of dead stars.
This mind-boggling technique has revealed that exoplanets are similar in composition to planets in our own solar system, implying that worlds like Earth may be plentiful in our galaxy, according to a study published on Thursday in Science.
“It’s pretty cool because this is really the only way to measure the geochemistry of exoplanetary bodies directly,” said lead author Alexandra Doyle, a graduate student of geochemistry and astrochemistry at UCLA, in a phone call.
Co-author Edward Young, a professor of geochemistry and cosmochemistry at UCLA, added that the study represents “the first time such an advanced way of looking at the geochemistry of these bodies has been used,” in the same call.
We are living through a golden age of exoplanet discoveries. Thousands of exoplanets have been detected, including an Earth-sized world orbiting the closest star to the Sun. But it is still extremely difficult to capture details about the interior composition and dynamics of these worlds. Unlike other planetary properties such as mass or atmospheric composition, a planet’s geochemistry cannot be deduced just by looking at an object passing in front of its host star.
White dwarfs, as it turns out, can help plug this information gap. These objects are the remains of stars that have blown up and collapsed into tiny, dense spheres about the size of Earth (our own Sun will embark on this transition in about five billion years).
The pyrotechnic deaths of these stars scramble the orbits of many objects in their stellar systems, such as asteroids and planets. Some of these worlds may end up hurtling toward the star’s posthumous white dwarf, which tears them apart over the course of about 100,000 to one million years.
“Whenever a body comes really close to it, a white dwarf will completely shred it and that dust and debris will fall and accrete onto the star,” Doyle explained. “White dwarfs are really the only way that you can measure the rocks directly because we know that when we’re observing them, the elements that we’re seeing—iron, silicon, magnesium—are directly from the rock itself.”
Doyle’s team observed six of these “polluted” white dwarfs located between 200 and 665 light years from Earth. The researchers were particularly interested in quantifying the iron content of rocks smashed up by the white dwarfs. Iron is a key indicator of “oxygen fugacity,” which is a measurement of the past oxidation levels of a planetary body.
“The more oxygen you have around when you make the rocks, the more iron ends up in the rocks, as opposed to in metal,” said Young. “That’s what we’re measuring in the white dwarfs—the amount of iron that was in the rock when it hit the white dwarf.”
It is still not known why planets in our solar system seem to have been enriched with so much oxygen, which has made it difficult to make assumptions about the oxygen content of alien systems. But it is clear that oxidation deeply influences whether a world will develop a magnetic field, plate tectonics, and other processes that are crucial to life here on Earth.
“Oxygen fugacity is as important as pressure and temperature constraints for determining which minerals will be dominant in the interior of a planet,” said Doyle. ”So, this can have huge implications for parameters that are important for habitability.”
The researchers discovered that the same chemical compounds found in Earth, Mars, and asteroids in our solar system—including iron—are also abundant in the objects that crashed into these six white dwarf systems. “Rocks are rocks everywhere, we think,” Young said.
In other words, bodies that are similar to Earth and Mars appear to be common throughout the Milky Way, which boosts the odds that extraterrestrial life may have evolved elsewhere in the galaxy.
Doyle and her colleagues plan to keep observing polluted white dwarfs to build an even more robust model of the contents of exoplanets.
“The prospect of doing the kind of chemistry on planets that we do inside this solar system outside the solar system, I wouldn’t have thought would be possible,” said Young.
“But it is possible,” he continued. “We’re actually doing true geochemistry on rocks from several planetary systems. To me, that’s really exciting because now everything we’ve learned about the solar system has relevance beyond the solar system.”