Birth of the Planets

Let's put the nebular theory and the condensation theory together, as most current astronomers do.

Here is a possible portrait of the formation of our solar system: A cloud of interstellar dust, measuring about a light-year across, begins to contract, rotating more rapidly the more it contracts. With the accelerating rotation comes a flattening of the cloud into a pancake-like disk, perhaps 100 A.U. across—100 times the current distance between the earth and the sun.

The original gases and dust grains that had formed the nebular cloud have contracted into condensation nuclei, which begin to attract additional matter, forming clumps that rotate within the disk.

The clumps encounter other clumps and more matter, growing larger by accretion. Accretion is the gradual accumulation of mass, and usually refers to the building up of larger masses from smaller ones through the mutual gravitational attraction of matter.

Close Encounter

Startling new infrared and radio telescope evidence supporting the condensation theory was reported in April 1998 by astronomers working at the Keck Observatory, Mauna Kea, Hawaii, and at Cerro Tololo Inter-American Observatory in Chile. Studying a star known as HR4796A, 220 light-years from the earth, the astronomers discovered a vast dust disk forming around it. A doughnut-like hole, slightly larger than the distance between the sun and Pluto, surrounds the star, and the disk itself extends more than twice the distance of the doughnut hole.

While astronomers did not detect any planets in this very distant object, they believe the hole in the disk may be caused by the gravitational force of one or more inner planets. In effect, then, astronomers believe they are seeing a distant planetary system in the making.

In 1984, a disk was observed around another star, Beta Pictoris, but astronomers are even more excited about the 1998 discovery, because they are more certain of the young age of HR4796A than they are of the age of Beta Pictoris. At a mere 10 million years old, HR4796A is believed to be the right age for planetary formation.

Further evidence for proto-planetary disks around young, low-mass stars was provided by the Hubble Space Telescope images of objects called proplyds, short for proto-planetary disks. This image shows a number of proplyds in the Orion nebula.

(Image from CSC/STScI and NASA)

Star Words

Planetesimals are embryonic planets in an early formative stage. Planetesimals, which are probably the size of small moons, develop into protoplanets, immature but full-scale planets. It is the protoplanets that go on to develop into mature planets.

Star Words

Planetesimals are embryonic planets in an early formative stage. Planetesimals, which are probably the size of small moons, develop into protoplanets, immature but full-scale planets. It is the protoplanets that go on to develop into mature planets.

Astro Byte

Astro Byte

Astronomers estimate that the evolution from a collection of planetesimals to 9 protoplanets, many protomoons, and a protoso-lar mass at the center of it all consumed about 100 million years. After an additional billion years, it is believed that the leftover materials had assumed their present orbits in the asteroid belt, the Kuiper Belt, and the Oort Cloud. The high temperatures close to the protosun drove most of the icy material into the outer solar system where (with the exception of periodic cometary shows) it remains to this day.

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Telescopes Mastery

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