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Planets Form Even Around Dead Stars
This artists concept depicts a type of dead star called a pulsar and the surrounding disk of rubble discovered by NASAs Spitzer Space Telescope.Can planet formation really be this easy? Scientists have found new planets forming in the most unlikely of places: around the burnt-out shell of a type of dead star called a pulsar.

Image right: This artist's concept depicts a type of dead star called a pulsar and the surrounding disk of rubble discovered by NASA's Spitzer Space Telescope. (+ High Resolution) Credit: NASA/JPL-Caltech/R. Hurt (SSC)

The budding solar system was discovered with the Spitzer Space Telescope's infrared array camera, which was built by NASA Goddard Space Flight Center in Greenbelt, Md.

If planets can form in this hostile environment, they can form just about anywhere. The discovery bodes well for finding warm, wet, rocky planets like Earth that can sustain life.

Planets seem to be forming, quite literally, from the ashes of the exploded star. The scientists, led by Dr. Deepto Chakrabarty of the Massachusetts Institute of Technology in Cambridge, Mass., identified dusty rubble orbiting the pulsar. The dust lanes are similar to those in our solar system 4.5 billion years ago when our sun was born.

This is the first time scientists have detected planet-building materials around a star that died in a fiery blast. Without a hot star to warm the emerging planets, life as we know it would be impossible. The finding highlights the fact, however, that dust plus gravity can equal rocky planet formation in a seemingly endless cycle of stellar birth and death.

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

The result appears in the April 6 issue of Nature. Chakrabarty's team, which includes Zhongxiang Wang and David Kaplan of MIT. The dust is too cold to radiate visible light, like our sun, but it is a strong source of lower-energy infrared light.

The pulsar, named 4U 0142+61, is 13,000 light-years away in the Cassiopeia constellation. This was once a large, bright star about 10 to 20 times more massive than our sun. The star probably survived for about 10 million years, until it collapsed under its own weight about 100,000 years ago and blasted apart in a supernova explosion.

Some of the debris, or fallback, from that explosion eventually settled into a disk orbiting the shrunken remains of the star---what is now pulsar. The disk orbits at a distance of about one million miles from the pulsar and probably contains about 10 earth-masses of material. The Earth, in comparison, is about 90 million miles from the sun.

A pulsar is incredibly dense, packing the mass of our sun into a sphere only about 12 miles wide. Pulsar 4U 0142+61 is an X-ray pulsar, which means it pulsates with X-ray light as it spins. Deadly X-ray light further reduces the chance for life in this emerging solar system. But the X-rays could help the dust stick together into clumps and ultimately rocky planets by altering their chemical composition.

Any planet around a star that gave rise to a pulsar would have been incinerated when the star blew up.

The pulsar disk discovered by Spitzer might represent the first step in the formation of a new, more exotic type of planetary system, similar to the one found by Dr. Aleksander Wolszczan of Penn State in 1992. Wolszczan found three planets circling a pulsar called PSR B1257+12. Those pulsar planets, two the size of Earth, were the first planets of any type ever discovered outside our solar system. Astronomers had indirect evidence the pulsar planets were born out of a dusty debris disk, but nobody had directly detected this kind of disk until now.

"I find it very exciting to see direct evidence that the debris around a pulsar is capable of forming itself into a disk. This might be the beginning of a second generation of planets," said Wolszczan.

Life, it seems, goes on.

Christopher Wanjek
NASA Goddard Space Flight Center