Miércoles 5 de Julio de 2006, Ip nº 160

Star's second dusty disc hints at giant planet
Por David Shiga

A nearby star, Beta Pictoris, is surrounded by two dusty discs – not one - a newly released image from the Hubble Space Telescope reveals. The tilt of the second disc with respect to the first suggests it was formed by a giant planet.

In 1984, Beta Pictoris became the first star known to be orbited by a dusty disc of debris, called a circumstellar disc. Discs are now known or suspected around more than 100 stars. Stars quickly clear dust away from their vicinity by the force of their radiation, so the presence of a dusty disc suggests collisions between asteroids or comets are replenishing it.

The newly released image was made by combining dozens of images taken with the Hubble Space Telescope's Advanced Camera for Surveys (ACS), which has recently broken down (see Hubble's key camera stops working). The raw images were shot by the ACS in October 2003, and astronomers have been processing them to bring out faint details.

Astronomers had previously seen what appeared to be a warp in the disc, and a study in 2000 suggested the warp could actually be a second disc that was tilted with respect to the first.

"With the extra imaging processing, we were able to definitively show that this warp is a visual blend of two separate discs," says David Golimowski of Johns Hopkins University in Baltimore, Maryland, US, who led the new study.

Tilted orbits
The newly imaged disc is smaller than the main disc, suggesting it lies closer to the star, and it is tilted by 4° with respect to the main disc. Astronomers believe it formed after a giant planet between 1 and 20 times the mass of Jupiter was scattered out of the main disc by gravitational interactions with other bodies there.

The researchers believe the planet's gravity then pulled small rocky or icy bodies out of the main disc and into the plane of its orbit. There, they collided to generate the dust in the second disc.

"The finding suggests that planetary systems could be forming in two different planes," says Golimowski. If that is the case, then our own solar system is not so odd, with each planet's orbit tilted slightly with respect to the others. Mercury's orbit is tilted by 7° compared to that of the Earth, for example.

And there may be even more discs around Beta Pictoris. Infrared images taken from the Keck II Observatory in 2002 suggest there is a third disc that is tilted by 14° with respect to the main disc – but in the opposite direction to the second disc. This one is too close to the star to be detected by Hubble, but it is also thought to be caused by an unseen planet.

Kamikaze comets
The Hubble images also show the second disc is relatively red, suggesting the dust particles are even smaller than suspected. Finer dust is more quickly blown away from the star, so its abundance in the star's vicinity means that "the rate of its production must be very, very rapid," Golimowski told New Scientist.

This fine dust could come from the frequent collisions between planetary building blocks. But it could also come from comets being thrown into Beta Pictoris itself, he says. The comets would evaporate, leaving only gas and dust behind.

Evidence for this process around Beta Pictoris dates back to the late 1980s. That was when astronomers first saw signs of evaporated metals in the star's spectrum, which were attributed to comets vaporising near the star.

Journal reference: Astronomical Journal (vol 131, p 3109)

  28/06/2006. New Scientist Magazine.