By looking at rocky debris near small, cooled corpses of stars called white dwarfs, researchers have concluded that planets like ours may be common around other stars. The Cosmic Companion talks to lead researcher Alexandra Doyle.
Earth-like planets may be common around alien stars, a new study from UCLA reports. This new finding answers one of the greatest questions in exoplanet research, and could help us better understand the future of our own solar system.
Alexandra Doyle, a graduate student of geochemistry and astrochemistry at UCLA, led a team which developed a new method to study the chemical makeup of planetary systems orbiting alien stars.
By examining fragments of rocky planets and asteroids orbiting white dwarf stars, her team was able to identify the composition of planets which were found (or still exist) within those systems. They found rocks and asteroids impacting these stellar corpses were affected by oxygen in a similar way to the manner many bodies in our own inner solar system react.
“These exoplanetary rocks that orbited these white dwarfs we studied most resemble the oxidation states of Earth, Mars, asteroids and chondrites [stony meteorites] in our Solar System. That is to say that they do not resemble Mercury, which is less oxidized,” Doyle tells The Cosmic Companion.
The objects typically resembled bodies within our own solar system in another way, as well — their masses.
“[T]hese rocks that previously orbited the white dwarfs are about ‘asteroidal’ in size, or perhaps just fragments of planets themselves, Doyle continues.
Dwarf stars are stars too!
White dwarfs are corpses of stars like our sun, which have depleted their supply of usable hydrogen and helium, and have collapsed to a body roughly the size of the Earth.
White dwarf stars started life much like our own Sun, with similar masses, temperatures, and surface colors. As they run out of hydrogen to fuse in their cores, energy production ceases, and the stars begin to shrink. This provides the conditions necessary for helium to begin fusing into carbon and oxygen, releasing energy once again. The star then swells up into a red giant.
When this happens to the Sun in roughly five billion years, the dying, reddened Sun will swallow the planets Mercury and Venus, and it may swallow the lifeless remains of the Earth. Once the supply of fusible helium runs out, the star (like our Sun) will collapse to a body roughly the size of Earth.
As the stellar corpse shrinks, its gravitational pull draws carbon, oxygen, and nitrogen found near the outer layers of a star into its interior. Once there, these heavy elements are invisible to detectors here on Earth.
White dwarfs alone in space should show just hydrogen and helium, with minimal quantities of heavier elements. Anything other than those two elements would come from bodies orbiting these dead stars.
“Learning the composition of planets outside our solar system is very difficult. We used the only method possible — a method we pioneered — to determine the geochemistry of rocks outside of the solar system,” said co-author Hilke Schlichting, UCLA associate professor of astrophysics and planetary science, and co-investigator on the study.
Studying the chemical composition of white dwarfs and their atmospheres allowed Doyle and her team to determine the makeup of planets and asteroids in those stellar systems.
“Observing a white dwarf is like doing an autopsy on the contents of what it has gobbled in its solar system,” Doyle explained.
It’s Like Reverse-Engineering a Car Crash from a Video
Data collected in previous astronomical searches, including studies conducted at the Keck Observatory in Hawaii, was scoured for clues about exoplanets. From this raw information, researchers were able to calculate the composition of debris near these deceased stars.
Doyle found more than she bargained for in the study, finding silicon, magnesium, carbon and oxygen left over from rocky bodies. Moreover, oxidized iron (also known as rust) was seen in rocks striking the surface of the white dwarfs.
During oxidation, metals share their electrons with atoms of oxygen, forming a chemical bond. Rocks from the Earth and Mars all show a similar chemical composition and contain significant amounts of oxidized iron. The one exception to this rule of thumb is the planet Mercury, where the surface is not as oxidized as the Earth or Mars, due to its proximity to the Sun. Little is known about the surface of Venus, where hellish conditions and a dense atmosphere have hindered most observations.
Until now, astronomers were unsure how similar the chemical composition of exoplanets are to worlds in our own Solar System. The answer now appears to be that rocks here are very similar to stars in other systems. This suggests that planets with geology like that found close to home could be common around other stars.
“The biggest takeaway here is that these rocks look very similar to the rocky bodies in our Solar System,” Doyle tells The Cosmic Companion.
Thinking Outside the Rocks
The laws of geology and the effects of oxygen played significant roles in the development of our own world, as well as other bodies in our 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. 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,” Edward Young, UCLA professor of geochemistry and cosmochemistry at UCLA, states.
The presence of rocky material in alien solar systems that looks like similar bodies here makes finding rocky planets around other suns more likely.
“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,” Young explains.
Long after the Sun has died, and the inner solar system decimated, the remains of our dead planetary system could resemble those examined in this study. Doyle imagines a fascinating death photo of our solar system for our readers:
“When our Sun evolves into a white dwarf it might change the orbits of smaller bodies in our Solar System (e.g. asteroids). And those small bodies could be sent on trajectories towards the Sun (now a white dwarf), where the white dwarf Sun will rip them to shreds. This will be relatively close to the white dwarf Sun. This debris disk around is analogous to Saturn’s rings, if Saturn were smaller,” Doyle speculates.
Detailed analysis of this study was published in Science.
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