Saturday, February 14, 2015

Why We Orbit the Sun

The formation of the universe and relevant theories such as the Big Bang are commonly discussed topics among astronomers, physicists, and even ordinary people. However, the formation of our immediate solar system is often not given much attention. The most widely accepted theory about how our solar system (the Sun and the eight orbiting planets) came to be is called the “solar nebula theory.” This theory quickly gained popularity among astronomers as well as the public. It is important to note that this theory is applicable to the rest of the universe, not just our solar system.

We begin with a massive, relatively cold cloud of gas and dust called a nebula. Gravitational force causes the dust to collapse inward. It is believed that the nebula is initially spinning slowly. As the nebula collapses, the system spins faster by conservation of angular momentum. Clumps of matter run into each other to form larger clumps called planetesimals and eventually become full-size planets. Over time, the random velocities and directions of the objects average out to form one rotating disk, with a protosun in the center. As the nebula continues to condense, the energy it contains from the gas pressure is released as heat, which explains the intense temperature of our sun. The objects in the disk continue to run into each other, eventually forming our “clean” solar system that we know today.

There is good evidence to believe that the solar nebula theory is correct. First of all, this theory accounts for the fact that all of our planets lie on one plane, and they all spin and revolve in the same direction. It also successfully explains why there is a sun in the center and how it became a shining star. At the center of the nebula, the temperature and density was hot enough to begin nuclear fusion.

Unfortunately, the solar nebula theory fails to explain certain observations. If, indeed, the sun formed by the collapse of clouds, we would expect the sun to spin faster and the planets to spin slower than they do. We would expect this because the sun contains approximately 99.9% of the solar system’s mass, but only accounts for 1% of its angular momentum. One proposed explanation is that solar wind leaving the Sun carried much of the initial angular momentum away with it. Even today, we observe the Sun’s rotational speed slowing down. Nevertheless, the nebular hypothesis continues to be the dominant theory today. We have recently taken a great number of photos of faraway nebulae where new stars are constantly being formed, providing visual support of the nebular hypothesis.
- Sarah Shy