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Friday, January 8, 1999 Published at 18:30 GMT


In the groove

HR 4796A: Offers clues into the possible presence of young planets

By BBC News Online Science Editor Dr David Whitehouse

Astronomers using the Hubble Space Telescope have observed dust disks around two stars that may indicate the presence of planets.

The immense disk of dust surrounding one of the stars has a dark band separating two lighter regions, "like a wide groove in a phonograph record."

[ image: HD 141569: First suspected of having a dust disk in 1986]
HD 141569: First suspected of having a dust disk in 1986
The scientists speculate that an unseen planet may have gravitationally carved out this 3.7 billion mile-wide gap. The star, a fairly ordinary one known as HD 141569, lies about 320 light-years away in the constellation of Libra.

Hubble shows that the disk is in two parts. A bright inner region and a fainter outer region separated by a dark band. Superficially, it resembles the largest gap in planet Saturn's rings.

"The most obvious way to form a gap in a disk is with a planet," says astronomer Alycia Weinberger. "The planet does not have to be in the gap, however. It could either be sweeping up the dust and rocks from the disk as it travels in its orbit around the star, or the gravity of the planet could knock the dust out of one part of the disk."

The vast disk is 75 billion miles across, or about 13 times the diameter of Neptune's orbit. The inner edge of the gap is 21 billion miles from the star.

Though already a fully formed star, HD 141569 is relatively young, probably only 1% through its lifetime as a stable star. It is nearly three times more massive and 22 times brighter than our Sun.

HD 141569 was first suspected of having a dust disk in 1986. Thermal radiation from it was observed earlier this year. At the distance of HD 141569, the crisp resolution of the Hubble Space Telescope shows structures as small as 1 billion miles across.

Held in place

The other star around which a dust ring has been detected is called HR 4796A. It likewise offers clues into the possible presence of young planets.

"The rings surrounding the giant planets in our own solar system are held in place by the gravitational force of moons orbiting nearby," explains Brad Smith of the University of Hawaii.

"The narrow width of the HR 4796A ring implies that it also is held in place by unseen bodies, most probably planets or protoplanets."

HR 4796A is about 70% larger than the Sun and probably less than 10 million years old. "Considering our own solar system to be middle aged, HR 4796A would be a mere infant," says Smith.

"It is especially interesting this stellar ring is so young, indicating planetary bodies have already formed in less than 10 million years." Just visible to the naked eye, HR 4796A is located 220 light-years away in the southern constellation of Centaurus.

To image the rings, Hubble scientists had to use a coronagraphic camera on Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

A coronagraph blocks out the glare of a star's light (just as the Moon blocks the Sun's light during a solar eclipse) so that much fainter surrounding material can be seen. These images add to the growing body of evidence that there may be planetary systems around many, if not most, stars similar to our Sun.

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