Using NASA's infrared Spitzer Space Telescope, Massachusetts Institute of Technology (MIT) skygazers detected a telltale disk of what could be early planetary debris encircling a pulsar, as the remnants of a supernova are called.

The cloud of gas and particles, which orbits the pulsar at 1.6 million kilometers (one million miles), may eventually stick together by gravitational force to form a planet that could be the size of the Earth, the astronomers suggested.

This process of planet-building by agglomeration is a well-studied phenomenon -- but until now it has only been seen with the birth of stars, not their death.

"Pulsars emit a tremendous amount of high-energy radiation, yet within this harsh environment we have a disk that looks a lot like those around young stars where planets are formed," Deepto Chakrabarty, MIT's principal investigator in the research, said on Wednesday.

"We're amazed that the planet-formation process seems to be so universal," he added.

The discovery, which is published on Thursday in the weekly British journal Nature magazine, fills in the missing piece of a puzzle that is more than 13 years old.

In 1992, Pennsylvania University astronomer Aleksander Wolszczan discovered three planets, two of them Earth-sized, orbiting a pulsar named PSR B1257+12.

They were the first planets detected outside our Solar System, but the nagging question was why they were there and had not been obliterated by the star's explosion.

The pulsar spotted by the Spitzer telescope, named 4U 0142+61, is 13,000 light years distant from Earth in the constellation of Cassiopea, near the celestial north pole, opposite the Big Dipper (also called the Plough, or Ursa Major).

One light year is the distance a beam of light travels over a year at a rate of 300,000 kilometers (186,000 miles) per second.

Pulsars are extremely compact, dense and fast spinning remnants of a supernova after it explodes.

They are also called neutron stars and have a mass 1.4 times that of the Sun but concentrated in a sphere a mere 15 kilometers (9.5 miles) in diameter. A spoonful of material from a pulsar would weigh around two billion tonnes.

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