by Laurance Doyle - Astronomer
We have discussed to date seven methods for detecting extrasolar planets in this series. During that time another two dozen extrasolar giant planets have been discovered, and the Kepler Mission, which will detect Earth-like planets around Sun-like stars, has been accepted by NASA as a Discovery Program. Within the next decade, therefore, we should have an idea if other "Earths" exist. Could there be a more exciting time than the beginning of such a Renaissance in our perspective of our place in the universe?
This is the final article in the Detecting Other Worlds series. Today we will discuss the detection of extrasolar planets using radio telescopes. Jupiter, for example, puts out radio signals due to its huge magnetic field. An extremely simplified model of magnetic fields requires two components: a metallic core and movement. Jupiters hydrogen core is metal-like and the planet itself rotates about twice as fast as the Earth, giving it a magnetic field that can, for example, deliver 5 million amps of electric current to its nearest large moon, Io.
It is interesting that, at a radio frequency of about 10 Megahertz, energetic particles in Jupiters magnetosphere can outshine the quiet (i.e. starspot inactive) Sun! Thus the possibilities of imaging a star with a Jupiter-like planet might not be that difficult even though their brightness ratios are significantly different. (See the Rayleigh criterion discussion in my essay on Direct Imaging.) In the infrared, Jupiter is, of course, a billion times fainter than the Sun.
So how might one go about detecting "Jupiters" around other stars by their radio emissions? Several authors have suggested that an array of about 100 eight-meter antennas - in particular, millimeter-wave telescopes - located in the very dry east Antarctic Plateau could detect "Jupiters" at a wavelength of about 1 millimeter (300 Gigahertz frequency), and at a distance of about 4 parsecs (about 13 light years) in a matter of days. Unlike the radio velocity method, the farther from their parent star the "Jupiters" are, the more easily detected (resolved) they would be.
The detection of such "Jupiters" would be interesting since such a large mass in another solar system may be needed to clear it of comets (see my essay on Circumstellar Habitable Zones). For example, if Jupiter were not in our Solar System, we would have a large cometary impact on Earth every few ten-thousand years, as opposed to every tens-of-millions of years.
As far as detecting evidence of exobiology, radio is a very good candidate. Of the various methods (weve talked about ozone detection, albedo changes due to forests, and so on), the most unequivocal for the remote detection of exobiology remains the SETI (search for extraterrestrial intelligence) method. The main endeavor, of course, is to detect narrow-band radio signals from another civilization.