The Universe Has More Earth-Like Planets Than We Thought

A new study by UCLA scientists suggests that Earth-like planets are not uncommon in the universe. The study, published in the October 18 issue of Science, is the first to look in detail at the chemistry of rocky exoplanets to determine whether it is similar to or different from that of Earth

‘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,’ says study co-author Edward Young of UCLA.


A New Method for Geochemistry Analysis


In the new study, the researchers worked out a new method of analyzing the geochemistry of planets outside of our solar system. Led by UCLA graduate student Alexandra Doyle, they looked at the elements found in rocks that were asteroids and rocky planet fragments that orbited six white dwarfs.


In particular, they focused on six most common rock elements: iron, oxygen, magnesium, aluminum, silicon and calcium. They then applied mathematical calculations and formulas to determine the geochemistry of the white dwarf rocks mathematically and compare them to calculations for rocks from Earth and Mars.


‘Understanding the rocks is crucial because they reveal the geochemistry and geophysics of the planet,’ says Doyle. Young commented that what the team is doing – studying geochemistry in rocks from other stars – is almost unheard of.


‘Learning the composition of planets outside our solar system is very difficult,’ explains co-author Hilke Schlichting, also of UCLA. ‘We used the only method possible—a method we pioneered—to determine the geochemistry of rocks outside of the solar system.’


Telescope Data Gets a New Look


The researchers focused on white dwarfs, incredibly dense stellar core remnants, with masses comparable to that of the sun, and volumes comparable to that of Earth. Doyle and her team focused on the elements present in the atmosphere of white dwarfs, because those same elements were in the bodies that orbited the white dwarf – asteroids or planet fragments. The closest white dwarf observed is about 200 light-years away from Earth.


‘Observing a white dwarf is like doing an autopsy on the contents of what it has gobbled in its solar system,’ Doyle says.


The team used data collected by telescopes from the W.M. Keck Observatory in Hawaii. ‘[I]n these data, I also see other materials, such as silicon, magnesium, carbon and oxygen—material that accreted onto the white dwarfs from bodies that were orbiting them,’ Doyle said.


What Sharing Electrons Means

For example, when iron is exposed to oxygen, a chemical bond is formed and electrons are shared. The process is called oxidation, evident when metal turns to rust. ‘Oxygen steals electrons from iron, producing iron oxide rather than iron metal,’ Young explains. ‘We measured the amount of iron that got oxidized in these rocks that hit the white dwarf. We studied how much the metal rusts.’


Rocks from Earth, Mars and other celestial bodies in our solar system contain a high level of oxidized iron and are similar in their composition. ‘We measured the amount of iron that got oxidized in these rocks that hit the white dwarf,’ Young says.


A planet’s oxidation affects its atmosphere, the composition of its core and the rocks it has on the surface. ‘All the chemistry that happens on the surface of the Earth can ultimately be traced back to the oxidation state of the planet,’ Young explains. ‘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.’


Are Exoplanets Habitable?


The scientists found that the rocks they analyzed are very similar to rocks from Earth and Mars. ‘They are Earth-like and Mars-like in terms of their oxidized iron. We're finding that rocks are rocks everywhere, with very similar geophysics and geochemistry,’ Doyle says.


‘It’s always been a mystery why the rocks in our solar system are so oxidized. It's not what you expect. A question was whether this would also be true around other stars. Our study says “yes”. That bodes really well for looking for Earth-like planets in the universe,’ Young adds.


If white dwarf rocks have a similar amount of oxidation as Earth, Schlichting said, then you can extrapolate that the planet has similar magnetic fields and similar plate tectonics. In short, it has the same key ingredients for habitability. ‘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,’ she says.


The success of this study resulted from astrophysicists and geochemists working together, Young says. ‘The result 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.’

Kat Jones