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A Decade of Discovery
All About Hubble
THE SEARCH FOR PLANETARY SYSTEMS

Our Earth and solar system were born 4.5 billion years ago. Clues to our genesis are fragmentary, so astronomers must look at the birth of stars in neighboring stellar "maternity wards" to see a replay of the events that created our Sun and planets.

By studying debris disks, composed of dust and gas, whirling around developing stars, the NASA Hubble Space Telescope's high resolution has opened up a new level of detail that reinforces earlier theories and offers some new surprises about planet birth. The ultimate goal, which only may be realized by a next-generation space telescope, will be to inventory planets and solar systems around other stars and ultimately identify Earth-like planets.

In 1994 Hubble discovered dozens of dust disks around young stars in the great Orion Nebula. Dubbed "proplyds" these disks are widely considered to be precursors to full-fledged planetary systems. Such disks were proposed in the eighteenth century by Immanuel Kant to explain the simple fact that all the planets of the solar system lie in nearly the same plane, and so were born from a primordial disk that provided the raw material for planet growth.

The Hubble images may long be remembered as the first tantalizing visual evidence that planets might be ubiquitous in the galaxy. Dozens of stars in the Orion region — which itself is just one of thousands of star birth regions in the Milky Way Galaxy — have these disks. Though it is not clear if they will go on to condense into planetary systems, their abundance alone is firm visual evidence that the first baby steps toward planet formation are very common.

The Hubble telescope has also spied these disks around isolated young stars ranging in age from 1 million to 10 million years. Astronomers have used the Hubble's arsenal of cameras to capture these disks in ultraviolet to near-infrared light. By chronicling these disks at different stages of a star's early life, astronomers are adding information to the planet-making recipe.

The telescope's sharp vision analyzed disks around eight young stars in the nearby constellation Taurus. Images taken in near-infrared and visible light show that material falling onto the disks are driving outflowing jets of gas from the developing stars. Many of these disks are 8 to 16 times the diameter of Neptune's orbit.

Hubble's Wide Field and Planetary Camera 2 spotted the first example of an edge-on disk in a young double-star system. These images offer further evidence that planet formation should be possible in binary star systems. Theory holds that gravitational forces between the two stars tend to tear apart fragile planet-forming disks. Astronomers also found evidence of dust in the disks clumping together and beginning to make larger bodies.

Looking at a disk around a slightly older star, astronomers using Hubble's Space Telescope Imaging Spectrograph found evidence of more clumping of material. The 2- to 4-million-year-old star, called AB Aurigae, is in constellation Auriga. These clumps are much farther away from the star than Pluto, our outermost solar system planet, is from our Sun.

Turning its gaze to a fully developed star about 10 million years old, Hubble's Near Infrared Camera and Multi-Object Spectrometer captured images of a 6.5-billion-mile-wide dust ring. This ring around the star, called HR4796A, resembles Saturn's rings, but on a larger scale. The gaps between the rings could be the result of unseen bodies sweeping out lanes. The star is 220 light-years from in the constellation Centaurus.


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