Astronomers Find Possible Young Planetary Systems
(From the July/August 1998 issue of StarDate magazine)
Rocky planets like Earth may orbit two fairly young, nearby stars, while very unEarth-like planets may orbit two others, according to a series of discoveries reported this spring. The findings support the current theory of planet formation, but suggest that subtle differences between systems can produce diverse results.
The discoveries show that planetary systems form from disks of gas and dust surrounding newborn stars. Dust grains coalesce into ever-larger chunks, which sometimes grow large enough to sweep up leftover gas.
The newly discovered systems are no more than a few hundred million years old, compared with about 4.5 billion years for our own solar system. Observations of the other systems are like frames from a movie showing several early steps in the planetary-formation cycle. It's like looking back in time to the formation of Earth and the other planets in our solar system. A frame-by-frame look at the discoveries:
• HR 4796A, at an age of just 10 million years, is the youngest of the bunch. One-half of a binary star system, it is encircled by a doughnut-shaped ring of gas and dust several times wider than our own solar system. The "hole" in the doughnut contains very little dust -- a sign that it may have been cleared out by newly formed planets. The planets closest to HR 4796A are likely to be small, dense, and rocky, like Earth and the other inner planets in our own solar system.
• Scientists first discovered a disk around Beta Pictoris, a 30-million-year-old star, in 1983. Images from Hubble Space Telescope showed a lump in the disk, as though a giant planet were hidden inside. Recent observations by a ground-based radio telescope confirmed the lump, which is about 10 times as far from Beta Pictoris as Pluto is from the Sun. The possible planet is larger and farther from its parent star than most astronomers would have expected, suggesting that there are few limits on planetary size and location.
• Fomalhaut, one of the brightest stars of the autumn sky, also appears to have a wide gap in its inner disk. The gap may have been cleared of dust by rocky planets like Earth. Fomalhaut is about 200 million years old, and may provide a close approximation of our own solar system at the same age.
• Vega is the oldest of the bunch, at an age of up to 350 million years. Like Beta Pictoris, it shows a large lump of dust far from the star itself -- in this case, about twice the distance from the Sun to Pluto. The lump may be a jumbo planet surrounded by dust, or something else entirely. Vega is the brightest star in the summer sky.
Just to mix things up a bit, a team of European astronomers says it has discovered a dust ring around a dying star in the constellation Monoceros. The star is a red giant -- a several-billion-year-old star that has consumed most of its hydrogen fuel -- in a binary system known as the Red Rectangle. Red giants eject large quantities of gas and dust into space. In most cases, this material drifts away from the star. But in the Red Rectangle, the gravity of the companion star may cause the expelled matter to remain near the red giant, where it is forming a disk that could someday give birth to planets. -- — Damond Benningfield
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