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An unusual white dwarf discovered relatively near the solar system defies current explanation.

Called DES J2147-4035, the object is located about 90 light years from Earth. It is both extremely dim and extremely cool for an object of this kind 鈥� probably just a few thousand degrees in temperature. This is cooler than almost any other known white dwarf. Its coolness suggests it is about 10 billion years old, but because our region of the galaxy is composed of much younger stars, DES J2147-4035 has left scientists baffled.

鈥淚t鈥檚 something we鈥檝e not seen before,鈥� says Kevin Apps, an amateur astronomer in the UK who discovered the object. 鈥淭o be quite frank, we鈥檙e not really sure what it is.鈥�

White dwarfs form when stars like our sun exhaust their fuel, expelling their outer shells of material and leaving their dense cores behind. These cores then cool over time. DES J2147-4035 is estimated to be at least 70 per cent the mass of our sun, but packed into a sphere smaller than Earth. It was seen by four separate surveys of the night sky, including the European Space Agency鈥檚 Gaia space telescope, but it had gone unnoticed until now because of the sheer volume of data these surveys produce.

Its age is consistent with ancient stars that are found in an outer halo surrounding our galaxy. 鈥淏ut it鈥檚 part of the disc of the galaxy, which is not thought to contain objects of that age,鈥� says Apps. When the star that became this white dwarf first formed, it would probably have been 鈥渃lose to the age of the universe鈥�, he says 鈥� adding to the mystery.

Further analysis with telescopes could tell us more. 鈥淚t could turn out this is the nearest example of some unusual type of white dwarf that we haven鈥檛 seen before,鈥� says Apps. 鈥淚t might have not formed, evolved or even be composed of the same material as normal white dwarfs.鈥�

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