Other Worlds

Until recently, astrophysicists' ideas for how planets formed were designed to explain the one set of planets that they knew of --- the 8 around our Sun (at the time, it was 9, because weren't sure that Pluto was part of the Kuiper Belt). There were to key facts that this model explained. First, all of the planets orbit in the same plane around the sun, and in the same direction. To explain this, astrophysicists assumed that the planets formed out of a disk of material that was left over after the Sun formed from an interstellar cloud. Second, the planets appear well organized: the rocky planets, Mercury, Venus, Earth, and Mars, lay close to the Sun; the gas giants, Jupiter, Saturn, Uranus, and Neptune, lay farther away; and at the greatest distances there were small icy bodies that formed the Kuiper Belt (Pluto's home) and the Oort Cloud (the source of comets). This was explained by considering the temperature of the proto-planetary disk, which decreased as one considered locations farther from the Sun. At the largest distances, the disk had little material, so only small, icy bodies could coalesce. Closer in, the disk was thicker, and gas giants could form around large, icy cores. Still closer, the disk became hot enough that the ice disappeared (sublimated), so the planets that formed were small and rocky.

For centuries, the Solar System was the only set of planets that we knew about, and the only set that we could build models to explain. Then, twenty years ago (in 1989) the first planet around another star was discovered. Today (as of May 2009), 347 planets are known around other stars (http://exoplanet.eu/catalog.php).

It took so long to discover planets around other stars, because planets are incredibly faint compared their stars --- the Earth shines with roughly one ten-billionth of the intensity of the Sun. A variety of techniques have been used to identify planets:

(1) Most of the planets (321) have been found by measuring their gravitational pull on their host stars. As the planets orbit around their stars, the star moves a similar, much smaller orbit around the center of mass of the stellar system. We can measure the movement of the star at Earth by measuring the Doppler shift in its spectrum. The measurements need to be extremely precise (velocities of about 10 meters per second, or about 20 miles per hour), and it is only recently that the technology was developed to make this possible.

(2) Another 59 planets have been discovered when they pass across the face of the star (referred to as a transit). We can only see planets pass across the face of the star for a very small fraction of all planetary systems, because viewing it requires that the Earth happen to lie in the same plane as the orbits of the extra-solar planets. The transits occur regularly, once every time time planet orbits the stars. During the transit, the planet blocks a tiny fraction (less than about 1%) of the light from the star. It has only recently been possible to identify these transits, thanks to the wide availability of small, computerized telescopes and cameras.

(3) The picture above shows three of the 11 planets that have been identified in images. Finding planets this way particularly difficult, because the star is so much brighter than the planets. To make the image above, the light from the star has been blocked out by placing an opaque piece of material, called a coronagraph, in the path that the starlight travels through the telescope. The other speckles of color are the left-over starlight, which isn't completely blocked by the coronagraph. Each or the red, blue, and green speckles illustrate a different wavelength of light. The pattern of left-over light is different for each wavelength, while the planets can be identified as speckles of light that lay at the same place at each wavelength.

(4) Another 15 planets have been found by other means. Seven have been found around stars that pulse, allowing astronomers to look for the reflex motion of the star by timing the rate of the pulsations. Eight have been found when the planet and parent stars "gravitationally lens" background stars. These rare events occur when the nearby star passes in front of the more distant one, and the gravity of the star and planets focuses the light from the more distant star like a lens.

Astronomers consider finding planets important because it helps them understand how our own world came into being. The planets that we have found so far tend to be very large, and very close to their host stars. This is not what we expected based on models for our own Solar System, in which the largest planets form fairly far away. Astronomers have not agreed on how these large, close extra-solar planets formed. This is in part because most of the techniques we use to find planets are only capable of identifying planets that are large and close to their host stars, because these are the ones that cause their host stars to move the most. Nonetheless, based on the evidence that we have, some astronomers suspect that planets tend to be dragged in toward the star as the form in the proto-planetary disk. The problem with this idea is that it doesn't explain why the large planets in our Solar System did not migrate inward.

In the future, astronomers hope to identify stars with smaller planets, particularly ones like Earth that lie where liquid water can exist. At the moment, we don't know whether Earth is unusual for lying at just the right distance from the Sun, or whether the manner in which planets for inevitably leaves planets where liquid water can form. For many astronomers, the ultimate goal of studying extra-solar planets is to learn how many places there are in our where life can form.

For astronomy, the 20th century centered on the study of cosmology --- the origins and structure of the Universe. I expect the study of extra-solar planets to be the central theme of astronomy in the 21st century. It may culminate in