Since we grew up on a planet and feel it is essential to our existence, we are particularly interested in how planets fit into story of star formation. However, planets outside the solar system are extremely difficult to spot. Remember that we only see planets in our own system because they reflect sunlight and are close; when we look at the other stars We find that the amount of light a planet reflects is a depressingly small fraction of the light emitted by its star. Also, from afar, the planets lose themselves in the light of their much brighter parent stars.
Disks around Protostars: Planetary Systems in Formation
It is much easier to discover the scattered raw material that the planets could be composed of than to discover planets after they are fully formed. From our study of the solar system, we know that planets are formed by the accumulation of gas and dust particles in orbit around a newly created star. Each dust particle is heated by the young protostar & radiates in the infrared region of the spectrum. Before a planet is formed, we can detect such radiation from all scattered individual dust particles that are destined to become parts of the planets. We can also see the silhouette of the disk when it blocks bright light coming from a source behind.
Once the dust particles come together and form some planets (and maybe some moons), most of the dust hides in the planets where we cannot see it. All we can discover now is radiation from the outer surfaces, covering an area that is drastically smaller than the huge, dusty disk from which they were formed. Therefore, the amount of infrared radiation is greatest before the dust particles combine to form planets. For this reason, our search for planets begins with finding infrared radiation from material needed to make it.
A disk of gas and dust appears to be an integral part of star formation. Observations show that almost all very young protostars have disks and that the disks are between 10 and 1000 AU in size (for comparison: the average diameter of Pluto’s orbit, which can be considered roughly the size of our own planetary system, is 80 AU, while the outer diameter of the Kuiper belt of smaller ice bodies is about 100 AU.) The mass contained in these disks is typically 1 to 10% of the mass of our own sun, which is more than the mass of all planets combined in our solar system. Such observations already show that most of the stars are beginning their lives.with enough material in place to form a planetary system.
The Timing of Planet Formation and Growth
By observing how disks change over time, we can estimate how long it will take for planets to form. As we have seen, when we measure the temperature and luminosity of a protostar we can put it on an HR diagram. By comparing the real star with our models of how protostars should evolve over time, we can estimate their ages. Then we can see how the disks we are observing change with the age of the surrounding stars.
What such observations show is that when a protostar is less than 1 to 3 million years old, its disk extends from very close to the star’s surface to tens or hundreds of AU away. In older stars we find disks with outer parts that still contain a lot of dust, but the inner regions have lost most of their dust. In these objects the disk looks like a donut with the protostar centered in its hole. The inner, dense parts of most disks have disappeared by the time stars are 10 million years old.
Calculations show that the formation of one or more planets could create a donut-like dust distribution. Suppose a planet forms some AU from the protostar, presumably due to the accumulation of matter from the disk. As the planet grows in mass, the process clears a dust-free region in its immediate vicinity. The calculations also show that any small dust and gas particles that were originally in the region between the protostar and the planet are not carried by the planet. , then it will fall on the star very quickly in about 50,000 years.
Matter outside the orbit of the planet, on the other hand, is not moved towards the hole by the gravitational forces exerted by the planet. If the formation of a planet actually creates and maintains the holes in the disks that surround very young stars, then the planets must form. in 3 to 30 million years. This is a short period of time compared to the lifespan of most stars and shows that planet formation can be a rapid by-product of star formation.
Calculations show that accretion can determine the rapid growth of planets: small, dusty particles orbiting the disk collide and stick together, and larger clusters grow faster as they attract and trap the smallest. As soon as these lumps reach a size of about 10 centimeters, they enter a dangerous stage in their development. At this size, unless they can reach a diameter of more than 100 meters, they are subject to drag forces created by friction with the gas inside. The disk and its orbits can quickly disintegrate and plunge them into the host star. Therefore, these bodies must quickly reach a diameter of nearly 1 kilometer in order to avoid a fire target. At this stage they are considered planetesimals. Once that size is reached, the largest survivors will continue to grow by enlarging smaller planetesimals; Ultimately, this process leads to a few large planets.
When the growing planets reach a mass more than 10 times the mass of the earth, their gravity is strong enough to trap and hold the hydrogen gas remaining in the disk. At that point they will rapidly increase in mass and radius and reach a huge planet dimensions. However, this assumes that the rapidly developing central star with its ever-increasing vigorous-wind has not yet driven away the gas in the disk. From observations we see that the disk can fly in 10 million years, so the growth of a giant planet must also be astronomically very fast.
Debris Disks and Shepherd Planets
Dust around newly formed stars is gradually built into the growing planets in the newly formed planetary system or thrown into space through gravitational interactions with the planets. The dust will disappear after about 30 million years, unless the disc receives new material all the time. Local comets & Asteroids are the most likely sources of new dust. As planet-sized bodies grow, they stir up the orbits of smaller objects in the area. These tiny bodies collide at high speed, break apart, and produce tiny particles of silica dust and ice that can hold the disc. supplied with the debris from these collisions.
Over several hundred million years, the number of comets and asteroids will gradually decrease, the frequency of collisions will decrease and the supply of fresh dust will decrease. Keep in mind that the heavy bombardment in the early solar system ended when the sun was only about 500 million years old. Observations show that the dusty “debris disks” around stars also become largely undetectable when stars reach an age of 400 to 500 million years. However, a small amount of cometary material is likely to remain in orbit, as will our Kuiper Belt, a flattened cometary disk outside of Neptune’s orbit.
In a young planetary system, even if we cannot see the planets directly, the planets can concentrate the dust particles in clumps and arcs that are much larger than the planets themselves and easier to visualize. This is similar to how Saturn’s tiny moons shepherd the particles. in the rings and create large arches and structures in the rings of Saturn
Currently, debris disks, many with such clumps & arcs, have been found around many stars, such as HLTau, located about 450 light-years from Earth in the constellation Taurus. For some stars, the brightness of the rings varies with position; Around other stars there are bright arcs and holes in the rings. The brightness indicates the relative dust concentration because what we see is infrared (thermal radiation) from the dust particles in the rings. More dust means more radiation.