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Other Worlds

by Louise Good

artist's conception of an exoplanet

Artist's conception of an exoplanet. Art by Karen Teramura.

How do you find a planet around a distant star? Are other solar systems like ours? Are any of these planets habitable? These were some of the questions addressed by planet hunter John A. Johnson at the June 30 Frontiers of Astronomy Community Lecture entitled "Other Worlds" at IfA Manoa.

The discovery of the first planets outside our solar system (exoplanets, also called extrasolar planets) took place about 15 years ago. An article in the March 2000 issue of the magazine Astronomy, whose cover proclaimed "Extrasolar Planets: 33 and Counting," piqued Johnson's interest, and he decided to go to graduate school at the University of California, Berkeley, where one of the authors of the article, Geoff Marcy, taught. Marcy, one of the world's premier planet hunters, became Johnson's thesis advisor.

John A. Johnson
John Johnson

Johnson, a National Science Foundation astronomy and astrophysics postdoctoral fellow working at IfA, explained two methods of finding exoplanets. The first looks for a Doppler shift (lengthening or shortening of the wavelength of the star's light) that occurs because the planet's gravity causes its star to move in a very small orbit. The second is the transit method, in which the planet travels across the face of its star, causing a small dip in the amount of light from the star. The Doppler method works best when one of the largest telescopes in the world, such as the Keck telescopes on Mauna Kea, is used because the large telescopes enable scientists to detect smaller planets that are farther from their stars. The transit method uses much smaller telescopes to screen many stars for possible planet transits, and then such discoveries must be confirmed using the Doppler method. The advantage of the transit method is that in addition to finding out the planet's orbital period and a lower limit to its mass, it enables scientists to calculate the precise mass of the planet and its radius, and therefore, its density. This is important because it allows scientists to test theories about the interior structure of planets.

A large fraction of the 303 planets discovered so far are "hot Jupiters" or "hot Neptunes," large gaseous planets that orbit very close to their stars. These are relatively easy to detect because they have the largest Doppler shifts. The first exoplanet discovered orbits its star, 51 Pegasi, in about 4 days, has a mass about half of Jupiter's, and is 1,800 degrees C (about 3,300 degrees F). None of the planets discovered so far is as small as Earth, and although some are in the "habitable zone," the distance from a star where liquid water can exist, all of these are gas giants. Although 25 solar systems--stars with more than one planet--have been discovered, very few of them resemble the architecture of our solar system, in which the planets are in well-spaced, nearly circular orbits in the same plane.

Johnson explained that the occurrence of planets around stars is intimately related to the properties of the stars, which provides important clues to how planets form. This is because the stars we see today serve as a "fossil record" of the primordial disks from which planets form. Stars with a higher metal content (with a higher percentage of elements heavier than hydrogen and helium) are more likely to be orbited by planets. Similarly, very hot, massive stars are more likely to have planets, but it is very difficult to detect planets around these stars. However, as these stars age, they become cooler and rotate more slowly, making the detection of planets easier.

So far, no one has ever seen or taken a picture of an exoplanet, but this may become possible in less than ten years, according to Johnson. New technology will also help with the discovery of Earth-size planets.

Johnson is using a new CCD camera built by IfA astronomer John Tonry and mounted on the UH 2.2- meter telescope to observe a secondary occultation (planet going behind its star) of the planet around WASP-3. If this experiment succeeds, he will be able to estimate the brightness and temperature of this planet.

More information about Johnson's research.