Rocky planets outside our solar system, known as exoplanets, are composed of ‘exotic’ rock types that don’t even exist in our planetary system, a study shows.
Researchers have used telescope data to analyse white dwarfs – former stars that were once gave life just like our Sun – in an attempt to discover secrets of their former surrounding planets.
The experts found that some exoplanets have rock types that don’t exist, or just can’t be found, on planets in our solar system.
These rock types are so ‘strange’ that the authors have had to create new names for them – including ‘quartz pyroxenites’ and ‘periclase dunites’.
Some of these rocks would dissolve more water than rocks on Earth, which might have impacted how oceans formed on these planets prior to their demise.
In this illustration, rocky debris, or the pieces of a former, rocky planet, spiral inward towards a White dwarf. An NSF geologist and an astronomer studied the atmospheres white dwarfs that have been ‘polluted” by such debris. They discovered exotic rock types that don’t exist in our solar systems.
The NSF’s NOIRLab, an Arizona-based astronomical research center, led the new study.
Siyi Xu, an astronomer at NOIRLab, said that while some exoplanets once orbited polluted dwarfs look similar to Earth, many have rock types that are unusual to our solar system.
“They have no direct counterparts within the solar system.”
Astronomers have found thousands of exoplanets that orbit other stars in our galaxy so far.
Since the first exoplanet discovery in the early 1990s, approximately 4,374 have been confirmed in 32334 systems.
According to NASA’s online databases, most of these exoplanets appear to be gaseous (like Jupiter or Neptune) rather than terrestrial.
Proxima Centauri, located around 4.2 lightyears from the Sun, is the nearest exoplanet.
It’s not possible to know the exact composition of exoplanets or if they resemble Earth.
To find out more, Xu teamed up with Keith Putirka, a geologist at California State University, Fresno to study the atmospheres and properties of what are called polluted white dwarfs.
White dwarfs are the dense stellar remains of dead stars that have been reduced to Earth’s size by exhausting their nuclear fuel.
They contain foreign material, such as asteroids and planets, that once orbited the star, but fell into the white dwarf’s atmosphere and became ‘contaminated’ or “polluted”.
Nearly 98 percent of the stars in our universe will eventually end up becoming white dwarfs. This includes our Sun.
Artist rendering of an exoplanet, with its star in a background. Astronomers have so far discovered thousands of exoplanets that orbit other stars in the Milky Way galaxy.
Scientists can identify the elements that would not naturally exist in white dwarf’s atmospheric (other than hydrogen or helium) and determine what the rocky objects of the star were made from.
The team looked at 23 polluted white dwarfs, all within about 650 light-years of the Sun, where calcium, silicon, magnesium and iron had been detected, using the W. M. Keck Observatory in Hawai’i, the Hubble Space Telescope and other observatories.
The list of polluted white dwarfs studied included WD 1145+017, approximately 570 light-years from Earth in the constellation of Virgo.
The scientists used the measured abundances to reconstruct the minerals or rocks that would be formed from them.
They discovered that white dwarfs had a wider range of compositions than any other inner planets in the solar system. This suggests that their planets may have a wider range of rock types.
‘Some rock types might melt at much lower temperatures and produce thicker crust than Earth rocks, and some rock types might be weaker, which might facilitate the development of plate tectonics,’ said Putirka.
Studies of polluted white dwarfs in the past had revealed elements from rocky bodies such as calcium, aluminum, and lithium.
These are’minor’ elements, which typically make up a small portion of an Earthrock. Therefore, measurements of’major’ elements (which make up a large percentage of an Earthrock), particularly silicon, are required to determine what type of rock types would have been found on those planets.
The team examined polluted dwarfs using the W. M. Keck Observatory, Hawai’i (pictured), and other observatories
The white dwarfs’ atmospheres showed high levels of magnesium, and low levels in silicon. This suggests that the rocky material detected was likely to have come from the interiors of the moons, not their crust.
Previous studies of polluted dwarfs have shown that there was evidence of continental crust on the rocky planets once orbiting these stars.
Interestingly, Putirka and Xu found no evidence of crustal rocks, although this does not completely rule out that the planets had continental crust or other crust types.
Putirka stated, “We believe that crustal rocks exist, but we are unable see them, probably because they occur in too small a fraction of other planetary components like the core or mantle to be measured.”
The study has been published in the journal Nature Communications.