Artist impression of a white dwarf star in orbit with pulsar PSR J2222-0137. It may be the coolest and dimmest white dwarf ever identified. [B. Saxton (NRAO/AUI/NSF)]
Artist impression of a white dwarf star in orbit with pulsar PSR J2222-0137. It may be the coolest and dimmest white dwarf ever identified. [B. Saxton (NRAO/AUI/NSF)]B. Saxton (NRAO/AUI/NSF)

A team of researchers has detected the faintest and the coldest white dwarf star close to Earth. Over the ages, the carbon content of the ancient lunar remnants has crystallized thus forming an Earth-size diamond in the solar system.

"It's a really remarkable object. These things should be out there, but because they are so dim they are very hard to find, " said David Kaplan, a professor at the University of Wisconsin-Milwaukee in a statement.

The stellar gem was found by Kaplan and his colleagues using National Radio Astronomy Observatory's (NRAO) Green Bank Telescope (GBT) and Very Long Baseline Array (VLBA) and other observatories.

White dwarfs are super-dense stars composed mostly of carbon and oxygen and over billions of years these stars gradually cool and fade away. The object detected is around 11 billion years old. The Milky Way is about the same age as that of the object.

Pulsars are fast spinning neutron stars, the superdense remnants of huge stars that have blasted as supernovas. As the neutron stars spins, beams of radio waves pass by  space. When one of these beams passes by  Earth, the radio waves and pulses can be detected by radio telescopes.

The pulsar, PSR J2222-0137 was the first object to be detected and was found using GBT by Jason Boyles, at West Virginia University in Morgantown. Further observations revealed that the pulsar was spinning over 30 times per second close to a unusually cool white dwarf companion star, which was earlier identified as either another neutron star .

Astronomers, knowing how bright a white dwarf star should emerge at such a  distance with such precision, believed that these stars should be observed in optical and infrared light. But, neither the Southern Astrophysical Research (SOAR) telescope in Chile nor Keck telescope, the 10-meter telescope in Hawaii were able to detect it.

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"Our final image should show us a companion 100 times fainter than any other white dwarf orbiting a neutron star and about 10 times fainter than any known white dwarf, but we don't see a thing. If there's a white dwarf there, and there almost certainly is, it must be extremely cold,"said Bart Dunlap, a graduate student at the University of North Carolina at Chapel Hill and one of the team members.

The astronomers calculated that the white dwarf would be as cooler as 3,000 degrees Kelvin (2,700 degrees Celsius).

Astronomers considered that such a  star would be a result of extremely crystallized carbon and not actually a diamond. Other stars have also been identified, which suggest that Earth-like objects are not rare, but can be extremely difficult to detect with a low intrinsic brightness.