Planets Around Other Stars
The abundance and characteristics of planets around other stars has long been a topic of great interest in astronomy. It has direct implications on our understanding of our own solar system and planet. It is also directly linked with the broader philosophical question “are we alone in the universe?”. If other stars commonly have planets, this would greatly increase the likelihood of us discovering life elsewhere in the universe.
Solar System Formation
The figure above summarizes our current theory of solar system formation. The sun forms from a gravitationally collapsing ball of gas and dust that flattens to form a spinning disk. The disk reaches an equilibrium size while the protosun continues to collapse in the center. Note that there is a temperature gradient in the disk – it is hotter in regions near the protosun and coolest in the outer regions.
Remember that there are no liquids in space – just solids and gases. Thus, the dust grains found in the inner solar system where it is hot are materials with high melting points (which have high densities). The region where Venus formed had a temperature of about 900 K, thus Venus formed out of materials that are solid at 900 K. In the outer solar system it is sufficiently cool for many substances to be solid. Dust grains collect into clumps and the clumps collect into planetesimals. Some planetesimals in the outer solar system become large enough to gravitationally hold onto gas which allows them to grow rapidly. About this time nuclear reactions begin in the sun and the radiation pressure and solar wind push the remaining gases out of the solar system leaving the large planetesimals. Over time the planetesimals either merge or are ejected from the solar system, leaving the few planets in nearly circular orbits.
The solar nebula theory of planet formation does a good job of explaining the origin of what we see in our solar system today. We get a disk-shaped solar system with planets in nearly circular orbits revolving in a common direction around the sun. The condensation sequence explains why there are two groups of planets in the solar system: large, low-density Jovian planets in the outer parts and small, high-density Terrestrial planets in the inner parts. At the core of this theory is the idea that planets form as a natural byproduct of star formation. Thus, it suggests that planets should be common in our universe.
Planets around other stars proved very elusive to find. In 1995, astronomers had infrared observations of protosolar systems and had detected planets around pulsars. However, no planets around regular stars were known despite the considerable effort that had been exerted to find them. All that we knew about planets came from those in our own solar system.
All of that changed beginning in 1995 when an extrasolar planet was discovered using the radial velocity technique. As of 2009 nearly 400 extrasolar planets are known, with more being found at an increasing rate. Several large programs are underway to find extrasolar planets, including the Kepler space mission. Launched in 2009, Kepler is staring at over 100,000 stars, waiting for a planet to cross in front of one of them. After a few years Kepler should begin to provide information on the relative number of planets around other stars.