The PLANET collaboration (Probing Lensing Anomalies NETwork) is a worldwide group of astronomers with the primary goal of detecting and characterizing microlensing anomalies, especially those that could be due to planets orbiting Galactic lenses. In order to study enough microlensing events frequently enough to detect short-lived planetary anomalies that could occur at any time, PLANET must have round-the-clock access to a network of 1m-class telescopes at different longitudes (Fig. 11).
| Fig. 11 --- The observatories that currently take part in PLANET monitoring are located in Chile (European Southern Observatory), South Africa (South African Astronomical Observatory), Western Australia (Perth Observatory) and Tasmania (Canopus Observatory). This allows nearly 24-hour coverage of microlensing events toward the inner regions of the Milky Way. Click on figure for a zoom. |
The telescopes must be located in the southern hemisphere to allow good visibility to the Galactic center, where the high density of lenses and sources produces the most microlensing events. The group has been in existence since 1995 (Fig. 12). Currently, four telescopes in the southern hemisphere dedicate large amounts of observing time to the project, and scientists from The Netherlands, South Africa, The United States, Australia and New Zealand are part of the team. While the microlensing survey teams are looking for events, the four PLANET telescopes can be continually monitoring the known events precisely and frequently --- searching for anomalies.
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| Fig. 12 --- The telescopes observing in 1996 and 1997 for the PLANET collaboration. Clockwise beginning at the top: Perth 0.6m in Western Australia, Canopus 1m in Tasmania, Dutch 0.9m at La Silla, Chile, and SAAO 1m in South Africa. Click on figures for a zoom. |
What happens during a PLANET observing season? During May through September when the center of our Galaxy is visible, PLANET begins with the current electronic alerts of on-going microlensing events announced by the microlensing survey teams. At a virtual PLANET "homebase" located at different PLANET stations during different times of the season, decisions are made about which events to monitor. Data are reduced as soon as they are taken at the various observatories and then sent to homebase for daily review. In this way, tens of events are monitored every 1-2 hours during the observing season. If an anomaly is detected, PLANET issues an "anomaly alert" to the general community through email and using its Internet website. In the 1997 season two such anomaly alerts were made by PLANET. The center of our Galaxy is so full of stars that the light from one can be confused with the light from another, making precise observations challenging. Precise measurements are nevertheless necessary since most of the anomalies will be due to the gentle distortions just outside the caustic regions rather than due to caustic crossing themselves. In addition, observations must continue in all sorts of weather conditions, including light cloud, turbulent atmosphere, and bright full moon (Fig. 13).
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| Fig. 13 --- The field (piece of sky that can be imaged) of the Dutch 0.9m telescope at ESO in Chile is 4 arcminutes on a side. Here, the same field in the direction of the center of the Milky Way is shown as it appeared on three different nights. Left: In good conditions between 5000 and 10000 stars can be measured in such a field. Middle: In turbulent conditions, light from neighboring stars can be confused, causing blurring and making precise measurements difficult. Right: Since the light curves must be monitored continually, observations must proceed even when the moon is risen and near the field with the microlensing event. Click on figures for a zoom. |
During the three years since its inception, PLANET has monitored several tens of microlensing light curves with a precision of 1 to 5% every 1 to 2 hours. Since every field contains between 5000 and 10000 other stars, information on the variability (or lack thereof) of hundreds of thousands of other stars has been collected at the same time. Indeed, hundreds of new variable stars have been found by PLANET, including many that are quite faint, vary rapidly, or have small changes in their brightness (Fig. 14).
| Fig. 14 --- A few of the hundreds of variables discovered by the PLANET team. Unlike microlensing events, normal variable stars are usually vary periodically and often experience color (temperature) variations as well. The stars shown here continually repeat these patterns with a period that can be read from the horizontal axis (in days) of each light curve. Note that some are quite faint, some vary rapidly (few hours), and others vary by small amounts (few hundredths of a magnitude). Click on figure for a zoom. |
Most of the microlensing curves appear normal, and follow the expectations of the simple lens, simple source model quite well (Fig. 15).
| Fig. 15 --- This normal microlensing light curve monitored by the PLANET collaboration in 1995 demonstrates how precisely a simple microlens follows theoretical predications, both in very red (I-band, top panel) and considerably bluer (V-band, bottom panel) filters. The arrows mark measurements that were taken up to a year after the event was over. Click on figure for a zoom. |
Three events, however, show clear signs of anomalies indicative of binary lenses (Fig. 16). Two of these binary pairs are likely to have stellar masses; the ratio of their individual masses is not extreme enough to indicate a planetary system. The third system binary lens system monitored by the PLANET collaboration is still under study.
| Fig. 16 --- This light curve is likely to be caused by a two stars in a close binary orbit that are microlensing a distant background star. The bottom three panels show observations in red and blue filters by the GMAN and MACHO collaborations. The top panel shows the first month of PLANET observations in 1995 for this event in a very red filter; black points are from the Dutch 0.9m telescope at ESO in Chile and red points are from the SAAO 1m in South Africa. The event has not been observed to vary again since the occurrence of this binary anomaly. Click on figure for a zoom. |
For the first time, another of these binary lenses enabled astronomers to use a microlens both as a powerful telescope (to increase the sensitivity to the distant source star) and as astronomical microscope (to resolve small details on the surface of the source). As the caustic structure of this binary lens passed directly in front the source star some 8 kiloparsec distant, the amount of light originating from different parts in the atmosphere of the background giant star could be studied by PLANET. This technique is likely to become an important way to study the atmospheres of different kinds of stars; most of the observational data on stellar atmospheres comes from the one star that is nearest to us: our Sun.