Methods other than microlensing are being used to search for planets outside our own Solar System. Some of these have been successful, and a handful of candidate planets have been identified. How successful will microlensing be and what can it add to the knowledge that has already been gained?
A planet orbiting a distant star can make its presence known by altering the speed (radial velocity technique) or position (astrometry) of its parent star, by dimming the light from the parent star if it happens cause an eclipse (transit method), through its own luminous radiation (direct detection), or --- if the parent star is a pulsar --- by altering the period of the cyclic pulses of the parent pulsar (pulsar timing method). Most of the extrasolar planets detected to date have been found through the radial velocity technique. Over ten planetary candidates have been discovered by this method, the first by the Swiss astronomers Major and Queloz around a nearby star called 51 Peg (Fig. 17). Although the discovery was scrutinized carefully by many astronomers who initially expressed doubts, most researchers now feel that the 51 Peg system is a large planet orbiting quite close to its mother Sun-like star. Since then, American scientists have detected several other planets or brown dwarfs orbiting nearby stars. After decades of searching for planets around other stars, these very recent discoveries reopen old astronomical questions of how planets form, how often, and under what circumstances.
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Fig. 17 --- The gravitational force field of a massive planet causes its
parent star to wobble slightly. This wobble is detectable as a periodic
change in the velocity with which the parent star moves toward and
then away from the observer. This periodic velocity signal is shown
here for the first planet detected via this method around the star
known as 51 Peg. (Courtesy: http://www.physics.sfsu.edu/~gmarcy/planetsearch/planetsearch.html) Click on figure for a zoom. |
It is also reasonable to ask whether our own Solar System is truly typical. Our own Jupiter, for example, orbits the Sun at a distance of 5 astronomical units (5 AU), that is, five times the Earth-Sun distance. The extrasolar planets discovered by the radial velocity method are all near Jupiter's mass or heavier, but they orbit their parent stars at considerably smaller distances (Fig. 18). At the moment, it is not clear whether this means that Jupiter-mass planets in small orbits are more typical than Jovian planets at larger radii. This is because the radial velocity technique is not yet sensitive enough to detect planets at large radii unless they are considerably heavier than Jupiter.
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Fig. 18 --- Most of the planets found so far by the radial velocity
technique and fairly massive and close to their parent star, despite
have Jupiter-like masses, most lie at orbital distances comparable
to that of the Earth from the Sun (1 AU). (Courtesy: http://www.physics.sfsu.edu/~gmarcy/planetsearch/planetsearch.html) Click on figure for a zoom. |
Microlensing offers a way to provide a partial answer to the question ``Is our Solar System unique?'' because it is sensitive to planets as small as Neptune's mass at orbital radii of a few AU. In the future, the technique may also be capable of detecting Earth-mass planets, but that will probably require special-purpose telescopes and computers dedicated to the project. In the meantime, it is possible to collect the microlensing data needed to search for Jupiter-mass planets orbiting at Jupiter-like distances from their parents stars. This is precisely what has occupied the PLANET collaboration for the last three years.
Microlensing has a few disadvantages as a planet detection method:
On the other hand, microlensing provides a good complement to other techniques, precisely because of its many strong advantages:
Taken together, these advantages and disadvantages make it clear that although microlensing is not appropriate for detailed studies of individual nearby planetary systems, it is ideally suited to statistical studies designed to discover how many planets exist around typical stars in the disk of the Milky Way, including planets that are similar to those in our own Solar System. This, together with the planetary mass and orbital range over which it can detect planets makes complementary to other planet search techniques (Fig. 19).
| Fig. 19 --- Different methods for detecting extrasolar planets are sensitive to planets of different masses and orbital separations. Gravitational microlensing complements other techniques nicely. With current technology, gravitational lensing is sensitive to a wider range of planetary masses and fills part of the gap in planetary mass and orbital radius from more traditional methods. Microlensing is clearly sensitive to planets like our own Jupiter (J), and nearly sensitive to planets with orbital radii as large as that of our own Saturn (S). Click on figure for a zoom. |
After the analysis of its first three years of data, the PLANET collaboration expects to be able to detect or place limits on the numbers of normal lenses (stars) in the Galaxy that have large planets, with masses comparable or larger than that of Jupiter, in their lensing zones. Beginning in 1998, the PLANET team will have increased sensitivity to the detection of planets, both through the commissioning of specially designed cameras in Chile and South Africa, and due increases in the amount of telescope time that is available to the group. In the 1998 and 1999, this worldwide network of astronomers expects to be observing microlensing events continuously for about 5 months out of the year, weather permitting. If planets exist in the lensing zones of most stars in the Galaxy, PLANET expects to be able to detect a handful of planets every year. Astronomers around the world are now beginning to talk about the possibility that advances accompanying the first years of the new millennium may see the first microlensing discoveries of Earth-mass planets orbiting distant Suns.