The MONS main camera and the two star trackers will obtain millimagnitude-level photometry for about 6000 stars. Given the recent success of ground-observations of the planet-transiting star HD209458, the monitoring of stars known to have planets (from radial velocity studies) becomes a very interesting scientific objective for MONS. But even more interesting is the possibility of discovering planetary transits in other stars.

We have used the preliminary photometric accuracy estimates to evaluate the capabilities of MONS for detecting planetary transits across the stars being monitored. The estimated performance of the Faint Star Tracker (FST) has been assumed for our tests. This is the detector aboard MONS that can reach fainter magnitudes and, thus, more stars can be observed simultaneously.

Here are real ground-based observations of the transit of HD209458. These were obtained by Edward Guinan with a 0.8-m Automatic Photometric Telescope located in Arizona (USA). The accuracy of a single measurement is of the order of 4 millimagnitudes and a synthetic model of the transit event has been fit to the data.

Would MONS have been able to detect such event? The short answer is YES, but some considerations need to be made.

Some numerical estimates for MONS

Actually, the photometric accuracy of MONS is not very high at the magnitude of HD209458 (V~8), BUT, the orbital period of the transiting planet is so short that MONS would have been able to observe almost ten transits during one month. So, what are the real chances? Well, they depend on how many times MONS can observe the target because the noise of the folded transit keeps decreasing with each subsequent observation. Here is a figure that illustrates this:

  These are the thresholds for detection (at the 3σ level) of transiting planets by the FST of MONS. The visual magnitude of the target star is represented in the abscissa axis, and the ratio of the radius of the planet (in Jupiter radii) to the radius of the star (in solar radii) is shown in the ordinate axis. The solid lines depict the detection limit and the green-shaded regions cover the domain of positive detections. Thresholds for 1, 2, 3, 4, and 5 observations of the same transit are shown. Note how the size of the smallest detectable planet decreases when the number of observed transits increases. According to this plot, for the size of the planet and the visual magnitude of HD209458 (represented here as an orange diamond), a 3s positive detection would have been made after five observed transits.

Let's go one step further and assume that MONS stays on the same field for 60 days (which is about the upper limit of what is currently being considered). Then, the number of times that a transit can be observed depends obviously on the orbital period of the planet. The previous plot can be modified to look like this:

  Here, the abscissa axis is now the orbital period of the planet and the ordinate axis, as well as the color coding remains the same used in the previous plot. The threshold lines represent detection limits for stars of 3rd, 6th, and 8th magnitude. Note how the chances of detecting a transit decrease with increasing orbital period because of the smaller number of transits that can potentially be observed. Note also that a transiting planet such as that in HD209458 is easily detectable even in stars fainter than 8th magnitude due to the short orbital period. Finally, this plot remarkably shows that MONS is theoretically able to detect even Saturn-sized planets (R~0.84 RJ) in short orbital periods around bright stars (brighter than 6th magnitude).

This last plot is based exactly on the same assumption as the previous one, but it has been presented in a way that allows us to make some numerical estimates with the currently-known planet-hosting stars:

  This diagram plots the threshold detection limits as a function of the orbital period of the planet and the magnitude of the host star. The lines trace the limits for different values of the ratio of the planet radius (in Jupiter radii) to the radius of the star (in solar radii). Color coding is the same as in the previous two plots. Orange diamonds locate all the stars that are known to have planets from radial velocity measurements (66 as of April 2001) within the plot ranges. Note that MONS would be able to detect transits in about 1/3 of the currently known planet-bearing stars, should the planets have a similar ratio of radii to that of HD209458 (Rp/RS~1.2). Finally, note also that MONS stands real chances of detecting planets as small as Saturn (Rp/RS~0.8) in 6 of the stars.


These calculations implicitly assume that all planets orbiting stars may transit across the stellar disk. This is obviously not true and only orbital inclinations within about 10 degrees of the line of sight can produce transits. A rough statistical estimate indicates the chances of this occurrence are about 1 out of 10. More precise information on this is given in a PASP article, that describes the Vulcan Photometer, and a paper by Tim Brown presented at the Third MONS Workshop. Based to these estimates, MONS could expect to detect as many as 2 or 3 new planets during its mission!!

But the analysis will not be straightforward. First, star spots and other sources of stellar variability will make the combination of transit observations harder and less accurate. Secondly, once a transiting object is identified and its cyclic pattern verified, more observations are needed to certify its nature as an extrasolar planet. A lack of a secondary eclipse (which would suggest that the eclipsing object is indeed a star in a grazing orbit) must be checked. Even if the candidate passes this test, the eclipse events could be due to two identical stars that experience grazing eclipses every half an orbital period. In such case, only spectroscopic observations can unveil the true nature of the system. Results from other experiments (Vulcan, HST observations of 47 Tuc) indicate that most of the candidates will end up being classified as eclipsing binaries with stellar components.

The path to the discovery of an extrasolar planet is not a smooth one, but the rewards are well worth the effort. The long-term observing sequences of MONS combined with its photometric accuracy will certainly serve to monitor known transiting planets, but also have the potential to provide us with a few new promising candidates.

Ignasi Ribas (