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. 

Rocky debris, the pieces of a former rocky planet that has broken up, spiral inward toward a white dwarf in this illustration. Studying the atmospheres of white dwarfs that have been 'polluted' by such debris, an NSF astronomer and a geologist have identified exotic rock types that do not exist in our solar system

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.


An exoplanet can be any planet that is not in our solar system. Most orbit stars, but rogue planets orbit the galactic centre and are untethered from any star. 

A white dwarf is a remnant of a smaller star that has run low on nuclear fuel.

While large stars – those exceeding ten times the mass of our Sun – suffer a spectacularly violent climax as a supernova explosion at the end of their lives, smaller stars are spared such dramatic fates.

Stars such as the Sun reach the end stages of their lives and exhaust their fuel. They then expand into red giants and eventually expel their outer layers into the universe.

Only the hot and dense core of the former Star – a white dwarf, is all that remains.

White dwarfs are approximately equal in mass to the Sun, but roughly the same radius as Earth. They are therefore extremely dense.

The gravity of a white dwarf’s surface is 350,000 times greater than that of Earth’s.  

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.   

Artistic rendering of what an exoplanet might look like, with its star in the background. So far, astronomers have discovered thousands of exoplanets orbiting other stars in our galaxy, the Milky Way

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.


Astronomers have discovered a new type of ‘waterworlds exoplanets. These planets are hot, ocean-covered, hydrogen-rich and could support life.

They were dubbed “Hycean” worlds by the University of Cambridge team, who claim they “greatly increase our chances of finding alien life”. 

Hycean worlds can be habitable and are more numerous than Earth-like ones. They are hot, ocean-covered, hydrogen-rich worlds that are easier to spot with current telescopes.

Scientists believe the discovery of biosignatures indicating life outside of our Solar System could be possible in the next two or three year.

Continue reading: Scientists discover a new class of exoplanets

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 studied polluted white dwarfs using the W. M. Keck Observatory in Hawai'i (pictured) and other observatories

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. 


Through its approximately 10-billion-year lifespan, the Sun has only lived for 4.6 billion years.

When hydrogen fuel at the centre of a star is exhausted, nuclear reactions will start move outwards into its atmosphere and burn the hydrogen that’s in a shell surrounding the core. 

The star’s outer surface begins to cool and expand, becoming much redder. 

The star will become a red giant over time and reach a size of more than 400 times its initial size.

As they expand, the red giants engulf their orbiting planets. This could also mean the fiery end for all the inner planets in our Solar System, including the Earth.

Don’t worry though, this won’t happen for another 5,000,000,000 Years.

Once swelled into a red giant, engulfing the inner planets and searing the Earth’s surface, it will then throw off its outer layers, and the exposed core of the Sun will be left as a slowly cooling white dwarf. 

This stellar ember is extremely dense and will pack a large portion of the Sun’s mass into an approximately Earth-sized sphere. 

Source: ESA/National School’s Observatory