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9.18: Extrasolar Planets

he Sun is a star like other stars. So an inevitable question arises: do planets also orbit around these other suns? The search for extrasolar planets is one of the most exciting adventures in science. It addresses a central theme in the study of astronomy: our place in the universe. Life evolved on a moist, rocky planet called Earth. Perhaps it also existed in a primitive form on ancient Mars. Its presence in more extreme environments in the Solar System has not yet been ruled out. Are there other potential sites for life, far beyond our Solar System?


Astropedia Image

Astropedia Image

Artist's concept of our location in the Milky Way galaxy. Click here for original source URL

The idea that planetary material is a common byproduct of star formation gets a strong boost from observations of newly forming stars. Many of these have disk shaped systems of gas and dust, including mineral grains of silicates and ices — just like the precursor to our Solar System, the hypothetical solar nebula. But many questions are unanswered. Will these systems necessarily produce planets like Earth, or like Jupiter? What type and number of planets is typical? How does planet formation depend on the amount of rotation of the initial nebula? How varied is the chemical composition of these disks?

Several lines of evidence are converging on the idea that planets are very common around stars in our galaxy— in fact it appears that the average number of planets per star is greater than one. This means that most stars have a planet orbiting them. As we find more and more planets, the variety in their sizes and environments is posing many challenges to existing theories of planet formation. Planets have been found bigger and closer to their stars than astronomers expected, and some have been found further way than expected. It now appears that the most common planet in our galaxy is one larger than the Earth but smaller than the size of Neptune. Astronomers called planets orbiting stars beyond the Solar System extrasolar planets, or exoplanets — the terms are used interchangeably.

Astronomers use several different techniques to search for possible planetary companions of stars. In the indirect methods, astronomers look for changes in the light from the star, or a background star, due to the presence of a planet. A star will wobble due to the gravitational pull from the planet, and if the alignment with Earth is just right the star will get fainter as the planet passes between us and the star. This last effect, called a transit, is how NASA’s Kepler satellite has detected over 3500 planets and counting.

Direct detection of an extrasolar planet means measuring light from the planet itself, including taking a picture of a planet. If the light from the faint planet can be separated from the substantially brighter star light, astronomers can also take a spectrum of the planet. Images and spectra allow measurements of the temperature and atmospheric composition of planets. Although this is incredibly challenging, direct detection is how we find out what a planet is really like and how we will someday identify the signs of life on another planet beyond the Solar System.