New method used to study planets’ geochemistry implies that Earth is not unique
Earth-like planets may be common in the universe, a new UCLA study implies. The team of astrophysicists and geochemists presents new evidence that the Earth is not unique. The study was published in the journal Science on Oct. 18.
“We have just raised the probability that many rocky planets are like the Earth, and there’s a very large number of rocky planets in the universe,” said co-author Edward Young, UCLA professor of geochemistry and cosmochemistry. “We’re studying geochemistry in rocks from other stars, which is almost unheard of,” Young said.
The scientists, led by Alexandra Doyle, a UCLA graduate student of geochemistry and astrochemistry, developed a new method to analyze in detail the geochemistry of planets outside of our solar system. Doyle did so by analyzing the elements in rocks from asteroids or rocky planet fragments that orbited six white dwarf stars.
“Learning the composition of planets outside our solar system is very difficult,” said co-author Hilke Schlichting, UCLA associate professor of astrophysics and planetary science. “We used the only method possible — a method we pioneered — to determine the geochemistry of rocks outside of the Milky Way.”
White dwarf stars are dense, burned-out remnants of normal stars. Their strong gravitational pull causes heavy elements like carbon, oxygen and nitrogen to sink rapidly into their interiors, where the heavy elements cannot be detected by telescopes. The closest white dwarf star Doyle studied is about 200 light-years from Earth and the farthest is 665 light-years away. “By observing these white dwarfs and the elements present in their atmosphere, we are observing the elements that are in the body that orbited the white dwarf,” Doyle said. The white dwarf’s large gravitational pull shreds the asteroid or planet fragment that is orbiting it, and the material falls onto the white dwarf, she said. “Observing a white dwarf is like doing an autopsy on the contents of what it has gobbled in its solar system. All the chemistry that happens on the surface of the Earth can ultimately be traced back to the oxidation state of the planet,” Young said. “The fact that we have oceans and all the ingredients necessary for life can be traced back to the planet being oxidized as it is. The rocks control the chemistry.”
“If extraterrestrial rocks have a similar quantity of oxidation as the Earth has, then you can conclude the planet has similar plate tectonics and similar potential for magnetic fields as the Earth, which are widely believed to be key ingredients for life,” Schlichting said. “This study is a leap forward in being able to make these inferences for bodies outside our own solar system and indicates it’s very likely there are truly Earth analogs.”
Young said his department has both astrophysicists and geochemists working together. “The result,” he said, “is we are doing real geochemistry on rocks from outside our solar system. Most astrophysicists wouldn’t think to do this, and most geochemists wouldn’t think to ever apply this to a white dwarf.”
Co-authors are Benjamin Zuckerman, a UCLA professor of physics and astronomy, and Beth Klein, a UCLA astronomy researcher.
The research was funded by NASA.