Already with the first release of data by OGLE on 12-Aug, the event OGLE 2006-BLG-450 was considered to be due to a binary lens, where a caustic entry had already taken place, but was not covered densely enough in order to clearly reveal the nature of the event. Since extended caustics, which are characteristic for lens binaries or multiples (including stars orbiting planets), form closed curves, caustic passages occur in pairs of entries and exits, provided the size of the caustics exceeds the angular size of the observed source star. Therefore, a further observed rise should indicate a caustic exit to occur. But OGLE 2006-BLG-450 reached a maximum that did not correspond to caustic exit near HJD=2453962.7 (15-Aug, 5:00 UT) and was considered falling before a strong rise was first reported by OGLE, whereas PLANET Anomaly Alert 2006 #14 (issued 17-Aug, 7:00 UT) stated that earlier PLANET data collected with the SAAO 1.0m (Sutherland, South Africa) after HJD=2453964.337 (16-Aug, 20:05 UT) showed a brightening by 0.1 mag, indicating a smooth rise after HJD=2453963.7 (16-Aug, 5:00 UT). Further data collected with the Danish 1.54m at ESO LaSilla (Chile) revealed a continuation of this rise with increasing slope, which tended to become constant in the most recent data until HJD=2453964.724 (17-Aug, 5:22 UT). While this could have been the anticipated caustic exit, models without caustic passages looked possible.
Further PLANET data, now collected with the Canopus 1.0m (near Hobart, Tasmania) showed OGLE 2006-BLG-450 falling between HJD=2453964.853 (17-Aug, 8:28 UT) and HJD=2453964.987 (17-Aug, 11:41 UT), as reported in PLANET Anomaly Update 2006 #14-1 (issued 17-Aug, 14:05 UT), but the characteristic slope discontinuity marking the end of a caustic exit had not been observed (yet?).
However, as announced in PLANET Anomaly Update 2006 #14-2 (issued 18-Aug, 6:15 UT), PLANET data collected with the SAAO 1.0m (Sutherland, South Africa) and the Danish 1.54m at ESO LaSilla (Chile) show a flat and later slowly falling light curve for OGLE 2006-BLG-450 after HJD=2453965.231 (17-Aug, 17:33 UT), favouring a previous caustic exit to have occured.
Our List of monitored events provides current data as well as an updated light curve.
After OGLE had pointed to a rise by 0.35 mag during one night that was not matched by an ordinary light curve, PLANET collected data with the Canopus 1.0m (near Hobart, Tasmania) after HJD=2453934.913 (18-Jul, 9:55 UT). In PLANET Anomaly Alert 2006 #10 (issued 18-Jul, 13:45 UT), we reported that the the first three points show a rise by 0.35 mag during 1.6 hrs at increasing slope, while the fourth point is about 0.9 mag below the preceding one.
Together with the previous OGLE data, the observations appear to follow the characteristic rise to a caustic exit for a small, strongly blended source. Since for faint stars, microlensing essentially works as a caustic-crossing detector, such light curves are not unusual.
A light curve is available and data can be accessed through our List of monitored events.
As reported in PLANET Anomaly Alert 2006 #9 (issued 16-Jul, 15:15 UT), a sequence of five data points collected by PLANET with the Canopus 1.0m (near Hobart, Tasmania) after HJD=2453932.889 (16-Jul, 9:20 UT) show a rise by ~ 0.6 mag that follows the characteristic shape of a fold-caustic entry. The presence of extended caustics is a unique signature of companions to the lens star.
In PLANET Anomaly Update 2006 #9-1 (issued 16-Jul, 19:25 UT), we reported that further PLANET data collected with the Canopus 1.0m (near Hobart, Tasmania) and the Perth 0.6m (Bickley, Western Australia) reveal a caustic peak at HJD=2453933.17 +/- 0.01 (16-Jul, 16:05 +/ 15 min). Since no change of the sign of curvature indicating an approach to a plateau region between fold-caustic passages had been observed by then, it was considered more likely that the source has touched a cusp resulting in a single caustic peak. However, as reported in PLANET Anomaly Update 2006 #9-2 (issued 17-Jul, 5:35 UT), further data collected with the Perth 0.6m (Bickley, Western Australia) and the Danish 1.54m at ESO LaSilla (Chile) reveal flattening towards a plateau region and show that the source entered the caustic already at HJD=2453932.78 +/- 0.01 (16-Jul, 6:45 UT +/- 15 min), after which a brightening by about 1.7 mag until the reported caustic peak was reached. The strong brightening strongly disfavours a planetary origin.
A regularly updated light curve is displayed on our List of monitored events, where the corresponding data can be found, too.
As reported in PLANET Anomaly Alert 2006 #6 (issued 29-Jun, 12:45 UT), PLANET data collected with the Canopus 1.0m (near Hobart, Tasmania) and the Danish 1.54m at ESO LaSilla (Chile) after HJD=2453914.98 (28-Jun, 11:30 UT) seem to deviate from a model light curve through the previous points assuming a single lens and a point-like source star. Similar behaviour in the OGLE data had previously been reported by the OGLE team. Trying to revise the model to account for all data left us with a small deviation for earlier epochs as well as unreasonable blend ratios.
While a 10-15% deviation lasting about 1.5 days, compatible with the presence of a planet, was initially considered to be a viable explanation, further data showed that the suspected deviation is rather a peak asymmetry, most likely being due to the lens being a stellar binary.
In PLANET Anomaly Update 2006 #6-1 (issued 1-Jul, 23:20 UT) and PLANET Anomaly Update 2006 #6-2 (issued 2-Jul, 2:25 UT), we reported rapid brightening, mentioned earlier by OGLE. PLANET data collected with the Canopus 1.0m (near Hobart, Tasmania) allows to determine the beginning of the caustic entry to have occured at HJD=2453918.22 +/- 0.02 (1-Jul, 17:20 UT +/- 50 min).
After an excellent coverage of the caustic entry after its peak and the plateau region with data from the Danish 1.54m at ESO LaSilla (Chile), the Canopus 1.0m (near Hobart, Tasmania), and the Boyden 1.5m (Bloemfontein, South Africa), we observed a rise, reported in PLANET Anomaly Update 2006 #6-3 (issued 5-Jul, 7:50 UT), the expected fold-caustic exit peak, reported in PLANET Anomaly Update 2006 #6-4 (issued 5-Jul, 14:45 UT), and finally the end of the caustic exit, where the trailing limb of the caustic has been found to have exited at HJD=2453922.159 +/- 0.001 (5-Jul, 15:49 UT +/- 1 min), observed with the Canopus 1.0m (near Hobart, Tasmania), as reported in PLANET Anomaly Update 2006 #6-5 (issued 5-Jul, 17:10 UT).
The large magnification in the plateau region also favours a stellar binary lens over a planetary explanation. However, with a coverage of two caustic passages, the nature of the event should be well-constrained and a reliable measurement of the stellar brightness profile is expected.
A light curve is displayed on our List of monitored events.
After the OGLE team pointed to ongoing microlensing of a source that shows periodic variability in OGLE 2006-BLG-357, PLANET/RoboNet added this event to its monitoring programme. A model by Lukasz Wyrzykowski (on behalf of the OGLE team) based on a Fourier analysis of the variable baseline yields a period of about 0.75 days, so that rapid variations of the observed magnitude can be expected while the source undergoes microlensing. According to that model, the expected peak magnification is A0 ~ 1.8, reached at HJD = 2453913.489 (26-Jun, 23:45 UT).
A light curve is displayed on our List of monitored events, and more information is available from Lukasz' Variable Events Warning system (VEWS) pages.
In PLANET Anomaly Alert 2006 #5 (issued 17-Jun, 8:35 UT), we reported that PLANET data collected on OGLE 2006-BLG-304 (identical to OGLE 2006-BLG-307) with the SAAO 1.0m (Sutherland, South Africa) during the recent night showed a rise consistent with the approach to a fold-caustic, while subsequent data collected at the Danish 1.54m at ESO LaSilla (Chile) confirmed this by initially showing an even steeper rise. This rise, up to the caustic exit peak, has also been reported by the OGLE team. Further Danish 1.54m data cover the caustic exit peak and revealed the event falling fast with a caustic exit to be expected very soon.
In fact, as reported in PLANET Anomaly Update 2006 #5-1 (issued 17-Jun, 9:15 UT), data from the Danish 1.54m at ESO LaSilla (Chile) revealed that the source star completed the caustic exit at JD=2453903.8629 +/- 0.003 (17-Jun, 8:42:35 UT +/- 25 sec).
For further details, please refer to our List of monitored events providing links to data and an updated light curve.
In PLANET Anomaly Alert 2006 #4 (issued 7-Jun, 22:00 UT) and PLANET Anomaly Update 2006 #4-1 , we reported that PLANET data collected with the SAAO 1.0m at Sutherland (South Africa), the Canopus 1 m telescope at Hobart, Tasmania (Australia) and the Danish 1.54m at ESO LaSilla (Chile) reveal that the source star in event MOA 2006-BLG-003 underwent a caustic crossing. The apparent caustic entry started at HJD=2453861.85, whereafter the source became 1.5 mag brighter over the next approx. 12 hours. In PLANET Anomaly Update 2006 #4-3 , we reported that the caustic exit took 0.435 days with the trailing limb exiting the caustic at JD=2453900.87.
For further details, please refer to our List of monitored events providing links to data and an updated light curve.
In order to determine the nature of the event, further dense monitoring is required and encouraged. The source star might have hit a caustic created by a stellar lens binary, but this kind of deviation also does not seem unlikely to have been caused by a planet orbiting the (single) lens star.
For further details, please refer to our List of monitored events providing links to data and an updated light curve.
In PLANET Anomaly Update 2006 #2-1 (issued 7-May, 6:55 UT), we reported that PLANET data collected with the Danish 1.54m telescope at ESO LaSilla (Chile) and the Perth 0.6m telescope (Western Australia) reveal that the source star in event OGLE 2006-BLG-207 transited a caustic structure within just 1 to 1.5 days, where two peaks separated by a local minimum at around 2.45 mag above baseline occured.
While further observations are required in order to fully determine the nature of this event, there are some indications pointing to the presence of a giant planet around the lens star.
Our light curve plot as well as data can be found on the List of monitored events.
As reported in the 26-Jan-2006 issue of Nature, the PLANET/RoboNet campaign, together with OGLE and MOA, found evidence for a cool rocky/icy planet with only 5 Earth masses (uncertain within a factor of two) around a red dwarf.
The event OGLE 2005-BLG-390 was reported by the OGLE Early-Warning System (EWS) on 11-Jul 2005, and was then monitored with the telescopes constituting the PLANET/RoboNet network. The light curve initially followed the characteristic brightening expected for a single isolated lens star and a point-like source star, which in this case is a G2-4 giant, and reached a peak magnification of about 3 on 31-Jul. However, on 10-Aug, an anomalous rise by 0.15 mag was observed by PLANET/RoboNet with the Danish 1.54m at ESO LaSilla (Chile), while an OGLE point from this night showed the same trend. By succeeding in monitoring the second half of this anomaly, lasting about half a day, with the Perth 0.6m (West Australia), and previously obtaining a dense coverage of the peak region of the event, we were able to conclude that the lens star is orbited by a low-mass planet, which is designated OGLE 2005-BLG-390Lb. The MOA collaboration was able to identify the source star on its frames and confirmed the observed deviation.
From the light curve, we infer an event time-scale tE = 11.0 d, in which the source star moves by one angular Einstein radius θE relative to the lens star on the sky, making OGLE 2005-BLG-390 one of the shorter events. Moreover, we find t∗ = 0.28 d, in which the source moves by its own angular radius, which we estimate from the observed luminosity and colour to be θ∗ = (5.25 ± 0.73) μas. This yields a proper motion &mu = 210 μas d-1 = 7.6 mas yr-1 and the angular Einstein radius as θE ~ 210 μas. The planet affects the light curve only by means of two dimensionless parameters, which are the mass ratio q = 7.6 × 10-5 and d = 1.610, where d θE is ithe current angular separation from its parent star (the orbital period exceeds by far the duration of the observed anomaly).
Mass and distance of the lens star are related by the measured angular Einstein radius, but neither of these quantities is obtained directly. However, we can calculate probability densities based on the likelihood for a certain value to produce the observed event with its given parameters, where mass functions for lens and source stars in the galactic bulge and disk as well as their spatial mass density and velocity distribution for a double-exponential disk and a tilted barred bulge have been assumed (Dominik, 2005, MNRAS accepted, astro-ph/0507540). Such an analysis yields the masses of lens star and planet as M ~ 0.22 Msun (2.1) and m ~ 5.5 M⊕ (2.1), respectively, and the lens distance DL = (0.85 ± 0.15) RGC, where RGC ~ 7.6 kpc denotes the Galactic centre distance. Assuming circular orbits and averaging over the unknown orbital orientation and phase, we find an orbital radius a = 2.9 AU (1.6) and a period P = 10.4 yr (2.0). Except for the lens distance, the logarithmic average is quoted and numbers in brackets indicate uncertainty factors corresponding to the standard deviation of the logarithm of the respective quantity.
OGLE 2005-BLG-390Lb appears not to be massive enough for being able to accrete a substantial amount of gas to become a gas giant planet like Jupiter or Saturn. Instead, its expected surface temperature of 50-70 K points to a icy nature, making it resemble more closely the outer icy planets Uranus or Neptune in this respect rather than the inner rocky planets Venus and Earth.
With lens and source separating at a rate &mu = 7.6 mas yr-1, cutting-edge instruments could in principle be able to resolve these and determine the lens star magnitude in the foreseeable future, which would result in a reliable measurement of its mass (and thereby that of planet OGLE 2005-BLG-390Lb) rather than being plagued with a huge uncertainty. The challenge however is with the contrast between lens and source magnitudes, which is expected to be at least 7-9 (for K-Band).
By OGLE 2005-BLG-390Lb being just the third reported discovery of a planet by microlensing, where the other two involve much easier-to-detect gas giants of a few Jupiter masses, we find a strong observational hint that cool rocky/icy planets are common. In fact, this has been predicted by simulations of planet formation and migration based on core-accretion models. With a unique sensitivity to Earth-mass planets, microlensing searches will be able to provide a census of such objects around red dwarfs, allowing to probe planet formation theory on objects that share their history with habitable planets.
More information can be found in a related ESO press release as well as in our detailed description. There are also a bunch of tailored press releases in several languages targetting different geographical regions. A full-text version of the discovery paper (2006, Nature 439, 437-440) as well as a preprint (astro-ph/0601563) can also be accessed.
After having been notified by μFUN about anomalous behaviour in the perspective high-magnification event, confirmed by OGLE, PLANET was able to activate part of its network ahead of the regular observing season, while PLANET's cooperative sites of UK-operated telescopes forming Robonet-1.0 were already in operation. Moreover, MOA also succeeding in collecting data on this event.
The light curve shows two sharp peaks, about 12 h wide, which are about 3 days apart. As first suggested by M. Jaroszynski, it can be well-explained by the distortion caused by a caustic located near the center of the lens star that results from the presence of a low-mass companion, where best-fit model parameters are a mass ratio of q ∼ 0.007 and a separation parameter of either d = 1.3 or d = 0.76, which denotes the angular instantaneous separation between planet and parent star in units of the angular Einstein radius &thetaE. The ambiguity in the separation is a general intrinsic feature. In contrast to the proposed planetary system, stellar binaries are unable to produce a light curve that is consistent with the observed data, and therefore do not provide an alternative interpretation.
With the measured event time-scale tE ∼ 70 d, the assumption that the source is at a distance DL ∼ 8.5 kpc, and models for the Galactic disk and bulge populations, we conclude that we have likely detected a planet with mass M ∼ 2.7 Mjup orbiting a lens star with mass M ∼ 0.45 Msun located at DL = (5.2 ± 1.8) kpc, where the semi-major axis of the orbit is a ∼ 2.2 AU or a ∼ 3.7 AU, corresponding to an orbital period of P ∼ 5 yr or P ∼ 11 yr.
Details are available in the Journal Article, ApJ 628, L109 (2005) . Preprint had appeared as astro-ph/0505451
A more thoroughful and detailed analysis is underway with the aim to determine the uncertainties of the estimates of mass and and orbital axis in order to confirm the planetary nature of the detected companion.
