by Laurance Doyle - Astronomer
Its fortunate for exobiologists and those who study circumstellar habitable zones that most stars do not much vary in their brightness. One particular brightness variationa sort of "wink" of the starprovides the only present means for detecting and studying Earth-sized extrasolar planets. This is known as the photometric transit method, and it relies upon a planet orbiting across the disc of its parent star in our line of sight.
When big things move across little things, we call it an "occultation." When little things move in front of big things we call it a "transit." And when two things about the same size move in front of each other, we call it an "eclipse." The transit method records a drop in brightness when a planet moves across the disc of its star. Since stars are far away, one cannot directly see a planetary transit. Rather, astronomers measure the decrease and increase in the stars brightness with respect to time. The result is called the star's "light curve."
Since our Sun is much closer, we can see a planet transit across it. Mercury recently transited the Sun. A rare eventobservable from Europe on June 8, 2004will be the first transit of Venus across the disc of the Sun since 1882. Such transits (as the Venus transit that Captain Cook was sent to observe from the South Pacific in 1769) allowed for a better determination of the scale of our Solar System and the distance of the Earth from the Sun.
The main limitation of the photometric transit method is the small likelihood that a planet's orbit will be correctly aligned to pass between us and its star. The probability that this occurs is less than 1%. To overcome this, one can either observe stars that are known to be "edge-on," or observe several stars at once. The first approach is being done by a group called the TEP (Transits of Extrasolar Planets) Network, and the second is slated for future spacecraft missions, such as NASAs Kepler Mission and the ESAs Eddington Mission.
To see how small a planet one could detect, the TEP Network used 1-meter telescopes in California, Russia, Greece, France, the Canary Islands, New York, and New Mexico to observe a small double star system, called CM Draconis, for over 1000 hours. (This took about 6 years due to weather conditions, observing time, and culling data that wasnt good enough for their purposes).
The special characteristic of CM Draconis is that its two stars orbit each other, with a period of about 1 1/4 days, across our line of sight. That is, they eclipse each other in what is known as an "eclipsing binary" system (Ill discuss these systems in my next planet detection essay.)
What is also special about CM Dra is that together the two stellar discs are about 12% the size of the Sun's single disc area. This is because the two stars that comprise CM Dra are tiny, red stars. This system has the observational advantage of being edge-on to Earth, so planets can be expected to transit both stars. Also, since the system is small, a transiting planet will yield a bigger drop in relative brightness.
After observing CM Dra for many years, several candidate planets were discovered. The candidates were selected by generating all possible planetary transits that could occur and then matching the possible light curves with the observed light curve, which was obtained by measuring CM Dra images and comparing its brightness to other, more constant "standard" stars in the same images. Of the candidates, only one persisted until recently when its apparent transits did not show up at the predicted time. One explanation is that we may have detected a planet that is not quite along our line of sight and so might only precess across the stars' discs.
What we were able to prove is that this method can be used to detect very small planets. The candidate had a predicted size of 2.3 Earth radii, which is smaller than 1% the size of Jupiter! It has a period of about 21-days, which puts it in the circumstellar habitable zone of the CM Dra system (see previous articles on circumstellar habitable zones). We had at least accomplished the first search for potentially habitable planets.
As the first effort was nearing its completion, two groups (one led by Tim Brown of the High Altitude Observatory in Colorado, and the other by Greg Henry of the University of Tennessee) detected the transit of a giant planet with a period of about 3 days across the disc of a solar type star named HD209458. From the amount of light blocked during the transit (about 1.8%!), the planet's size was determined. This was the first time that the size of an extrasolar planet was measured, and it was a whopperabout 2.7 times the size of Jupiter! It was all puffed up from being so close to its star. The planet was predicted to be there by Geoff Marcy and his radial velocity group (see the "wobble" method) but this planet actually crossed the star.
Near future missions that will use the photometric transit method are expected to look at 100,000 stars continuously. Hundreds of Earth-size planets could be found soon after these missions are launched around 2006. The photometric transit method will allow us to find other habitable places in the Galaxy before the decade us up! What could be more exciting?