INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 45, May 2020

Gravitational Microlensing

by Exoplanets and Exomoons

200216_Spatium_45_2020_(001_016).indd 1 16.05.20 11:11 Editorial

Since Copernicus (1473–1543) was world view abruptly. They seemed not able to measure brightness not to be inhabited as previously Impressum ­variations of the stars due to Earth’s assumed, and life appeared not to revolutionary motion around the be a matter of course. Astrophys- Sun, he inferred that stars must be ics and interdisciplinary science ISSN 2297–5888 (Print) very far away from Earth. During recognised the very special condi- ISSN 2297–590X (Online) the 16th and 17th Centuries schol- tions required to create life. More- ars began gradually to realise that over, these conditions need to be Spatium the Universe, i.e., the sphere of very selective and restricting, de- Published by the fixed stars enclosing our Solar pending not only on physical and Association Pro ISSI ­System, might not be finite but chemical parameters but particu- ­infinite. Philosophical arguments larly on stable and stabilising hab- by Thomas Digges (1546–1595) or itable environments in space and Giordano Bruno (1548–1600) sup- on ground. ported this view. Galileo (1564– Today, the existence of extra-ter- Association Pro ISSI 1642) provided evidence from ob- restrial life seems to be quite likely Hallerstrasse 6, CH-3012 Bern servations of the Milky Way using for statistical reasons. However, it Phone +41 (0)31 631 48 96 his telescope. is very hard to find evidence from see In his Cosmotheoros posthumously bio-markers identified in exoplan- www.issibern.ch/pro-issi.html published in 1698, Christiaan Huy- etary spectra as long as it is not clear for the whole Spatium series gens (1629–1695) estimated the dis- what life actually is. Furthermore, tances to the fixed stars by compar- our Solar System and particularly President ing the apparent magnitudes of the our Earth-Moon System have a Prof. Dr. Adrian Jäggi, Sun and Sirius. He found Sirius to special history resulting from a University of Bern be 27 664 times farther away from ­series of very unlikely events that Earth than the Sun. Already in 1690, happened successively in an assor- Editor he speculated­ about the existence of tative and even choreographic way. PD Dr. Andreas Verdun ­planets orbiting other stars than the Thereby it is supposed that the CH-3086 Zimmerwald Sun carrying extra-terrestrial life. Moon is ascribed a critical role for He thus may be considered the the origin and evolution of life on Printing “­inventor” of extrasolar planets. Earth. Stämpf li AG During the 17th and 18th centu- The topic of this issue is based on CH-3001 Bern ries, the value of the solar parallax the talk “Exoplanets: In Search of was measured using Mars opposi- Terra-2” by Prof. Dr. Joachim tions and transits of Venus with Wambsganss held on October 31, ­increasing accuracy. At least since 2018, in the Pro ISSI seminar se- that time it was realised that our ries. He pointed out that gravita- solar system and the cosmos must tional microlensing takes the po- be huge. As a reaction to this “hor- tential not only to detect Earth-like ror vacui” every celestial body was exoplanets, but even Moon-like thought to be inhabited, even the exomoons. He referred to results Title Caption Sun and the stars. The “plurality published by Liebig and Wambs- Simulation of the caustic structure of the worlds” became an estab- ganss (2010), from where material (magnification pattern) resulting from lished world picture prevailing un- was used for this issue. the configuration consisting of a host til the 20th century. star (left), exoplanet (right, larger structure), and exomoon, (right, Space probes sent out to almost all Andreas Verdun smaller structure within the exo­ solar system bodies demolished this Zimmerwald, December 2019 planet’s caustic structure).

SPATIUM 45 2

200216_Spatium_45_2020_(001_016).indd 2 16.05.20 11:11 Gravitational Microlensing by ­Extrasolar Planets and Moons

By Prof. Dr. Joachim Wambsganss, Astronomisches Rechen-Institut (ARI), Zentrum für Astronomie der Universität Heidelberg (ZAH) and International Space Science Institute (ISSI), Bern

Light Curve bly enhance the probability for the Figure 1 presents the observational discovery of extra-terrestrial life. data of the microlensing event Anomalies The role and importance of our OGLE-2005-BLG-390 – the dis- Moon for the origin of life on covery of a cool 5.5 Earth mass Earth was analysed by Benn (2001). exo­planet (Beaulieu et al. 2006). He points out five possible criteria The signal of the exoplanet is The first time an extrasolar planet that may trigger and influence the super­imposed on the light curve (or exoplanet) was discovered with evolution of life: (1) stabilisation of produced by the host star, a small- the method of gravitational micro- the planet’s rotational axis, (2) amplitude blip at day 10; it is lensing was in 2004. In 2006, a elimination of primordial atmo­ zoomed out in the extra diagram cool planet of 5.5 Earth masses sphere, (3) generation of the plan- on the top right side of the dia- was discovered with this method et’s magnetic field, (4) generation gram. This light curve illustrates (Figure 1, Beaulieu et al. 2006). of large tides, and (5) generation of the difficulty of the detection of Micro­lensing is a very sensitive longer-period tides. These aspects exomoons. In order to be able to technique for detecting exoplanets certainly support the conditions of detect the much less significant with (projected) distances between life, so exomoons may play an es- ­microlensing effects caused by an roughly 0.5 and 10 AU from their sential role for the origin and evo- exomoon, tiny deviations in the host stars and with masses between lution of life on their host exo­ structure of the light curve need roughly Earth mass and 10 Jupiter planets. This is why searching for to be identified, which is only pos- masses. Very frequent photometric microlensing effects through exo- sible if the resolution in time and measurements with observation moons might be an important is- photometric magnitude is much rates of about one frame per sue for the quest of extra-terrestrial higher than in this example (which 15 minutes and visual photometric life. was state of the art at the time). precision of about 20 mmag (milli- magnitudes)1 may in the future even allow to detect microlensing Figure 1: The light curve of the OGLE-2005-BLG-390 microlensing event events caused by extrasolar moons ­(differently coloured data points from different observatories) and best-fit model (or exomoons). This means that not plotted as a function of time. (Credit: Beaulieu et al. 2006) only the light curve of a micro­ lensing host star and the deviation of this light curve caused by an exo­planet have to be considered, but also anomalies superimposed on the exoplanet’s signal caused by its moon (or moons) have to be an- alysed. Simulations of convolved caustics and magnification patterns resulting from microlensing effects by exoplanets and exomoons sug- gest that massive extrasolar moons can in principle be detected. The detection of exomoons orbiting exoplanets of Earth-like masses in habitable zones would considera-

1 The (visual) brightness of a star is measured in “magnitudes”. By definition, 0 mag is the brightness of the star Vega (a Lyrae).

SPATIUM 45 3

200216_Spatium_45_2020_(001_016).indd 3 16.05.20 11:11 Classical trast in brightness or magnitude planet applies to the spectroscopic between exoplanet and its host star. method. Here (projected) radial Methods In addition, such telescopes are not velocities are extracted from intended to be scheduled for sur- ­Doppler shifted spectral lines of to Detect vey or monitoring observation the order of a metre per second (or campaigns. smaller). To be successful, the host Extrasolar star must be bright enough in or- Planets der to produce a high enough flux Astrometry for an extremely high resolution spectrum containing a sufficient The gravitational pulls of exoplan- number of spectral lines, so that the Let us briefly summarise the ad- ets acting upon their host star force Doppler shifts can be analysed vantages and limits of the five clas- the star to move on a Keplerian or- properly. Although this method sical methods to detect exoplanets, bit around the barycentre of the was and still is very successfully in particular with respect to the exo­system. If only one exoplanet used for the detection of exoplan- possibility of the detection of is present, the star’s motion is quite ets, it seems unimaginable that it exomoons. regular and can be measured (at will be able to detect exomoons: least with the Gaia satellite) if the The effect of the barycentre mo- exoplanet’s mass is of the order of tion of exoplanet plus exomoon Direct Imaging the mass of Jupiter. However, if cannot be disentangled into those more than two exoplanets are or- two components. Radial velocity By about 2025 (hopefully) three biting their host star, this motion measurements are most sensitive new ground-based optical tele- can become quite complicated. for exoplanets with very small scopes will become operational: Since the gravitational effect of an semi-major axes to their host stars The Giant Magellan Telescope exomoon upon the host star is van- because then their gravitational (GMT), the Thirty Meter Tele- ishingly small, there is no chance pull is strongest on the stars’ scope (TMT), and the European for this astrometric method to de- velocity. Extremely Large Telescope (ELT). tect exomoons at all. The ratio be- Equipped with newest generation tween the masses of host star and of advanced and dedicated techno­ exomoon is so large that the angu- Transit logy in active and adaptive optics, lar resolution needed to detect a it is expected that these telescopes variation in the host star’s position The transit method uses the sim- will provide images in the optical would not be sufficient in any case. ple geometric effect that – if we see and near infrared wavelength re- The strongest astrometric effects of the planetary system edge-on – the gion with an angular resolution of exoplanets are expected for large planet passes periodically in front more than ten times the resolution distances from their host stars of the host star and reduces its of the Hubble Space Telescope (which – on the other hand – means brightness slightly. That means (HST). Although these instru- very long orbital periods). transit observations are restricted ments are designed to provide al- to the rare cases where the line-of- most diffraction-limited images sight coincides with the orbital which allow for direct imaging of Spectroscopy plane of the exoplanet system, i.e., (some) exoplanets, their angular if the system is inclined by 90 ° resolution will not be sufficient to The same argument concerning w.r.t. the tangential plane to the separate exoplanet from exomoon the effects of the gravitational force celestial sphere. This means a seri- images due to the close distance of of an exomoon on the motion of a ous limitation of the transit method such moons from their host plan- host star due the exomoon’s mo- in the sense that only about 1% or ets and due to the immense con- tion around a (detectable) exo- less of all planetary systems are de-

SPATIUM 45 4

200216_Spatium_45_2020_(001_016).indd 4 16.05.20 11:11 tectable with this method. How- exoplanet-plus-exomoon system the very rare occasion that a fore- ever, this can be corrected statisti- would pass regularly in front of the ground star passes directly in front cally. In addition to very frequent star, so a massive exomoon would of a background star and ­focusses observations of the same field in affect the exact periodicity of the the light of the background star such the sky, highly accurate photomet- exoplanet transits and cause so- that for a short time the background ric measurements are required in called transit timing variations star brightens in a very characteris- order to detect small planets: the (TTV). However, it will be cer- tic way. The chance for this spatial dip in the light curve is propor- tainly very challenging to detect coincidence to happen is very tional to the square of the ratio of exomoons with the transit method, roughly one in a million. That planet radius to stellar radius. The even with satellites like CHEOPS, means that a million stars have to transit method, however, can because many other phenomena be monitored regularly in order to ­potentially detect exomoons in could cause similar effects in the detect one (stellar) microlensing two ways: the straightforward light curve, e.g., anomalies from event. And even if every star had a way is that occasionally a moon other small exoplanets. planet, this planet would be de- would be leading or tracing a tected in only one out of one hun- planet transit and cause its one dred stellar microlensing events. (very tiny) transit before or after Gravitational (Micro-) Exoplanets can be detected with the planet transit. Another way for Lensing microlensing when they are in a (massive) exomoons to show up in certain (projected) distance range the transit light curves is by mod- Gravitational microlensing is a to their host stars: If they are too ulating the exact periodicity of the method which also employs photo­ close-in, the star-plus-planet sys- transit: only the barycentre of the metric measurements. It searches for tem acts as a single-mass object with the combined mass. If the planet is very far away from its host Figure 2: Sensitivity of astrometry, spectroscopy, and microlensing for exo­planets star, the lensing system acts as two (mass and separation). single lenses. Only if the planet is in a (projected) distance range ­between about 0.5 to 10 AU (cf. ­Figure 2), the star-plus-planet sys- tem acts as a binary lens system and the exoplanet might be detected. Interestingly, this distance range overlaps largely with the so-called

Jupiter habitable zone.

To first order, the lensing effect is independent of the mass of the exo ­planet (cf. Figure 2), and it is also Mass in M independent of the type of host star. In principle microlensing could also detect exoplanets around white dwarfs, neutron stars or black holes. And in particular for statistical analyses microlensing is AU well suited: Cassan et al. (2012) showed that on average every star Separation has at least one planet.

SPATIUM 45 5

200216_Spatium_45_2020_(001_016).indd 5 16.05.20 11:11 Principles of object, the separation between the (cf. Figure 4), it depends only on the two images S1 and S2 is too small mass of the lens, the distances ob- Gravitational to be resolved with optical tele- server-lens DL, observer-source DS scopes; however, the combined and lens-source DLS and the impact (Micro-)Lensing brightness of S1 and S2 is higher parameter . The lens equation is than the brightness of S because of thus a mapping from the lens plane the focussing effect. (This can be to the source plane, it maps the According to Einstein’s Theory expressed differently: gravitational ­images of a source star to its actual of General Relativity, the mass lensing changes the apparent solid position in the source plane. Here ­distribution in the Universe tells angle of a source, it does not affect we focus on the triple lens case space-time how to curve, and the surface brightness; the magni- with host star, exoplanet and exo- space-time tells the masses how fication is the ratio of the total solid moon. The triple-lens equation is to move. Curved space-time angle of the images and the appar- a tenth-order polynomial equation means, that light rays do not pro- ent solid angle of the unlensed in . ceed along straight lines but follow source.) Since the three involved so-called null-geodesics. Conse- objects – observer O, lens L and One approach to solve the lens quently, light rays (Figure 3) emit- source S – move relative to each equation numerically, in particu- ted by a source S and detected by other, this magnification is varia- lar if the number of lenses is higher an observer O are bent around a ble with time in a perfectly pre-­ than two, is to generate a magni- massive object L, which acts as a determined way (only depending fication map in the source plane. lens. The closer the light ray passes on the impact parameter) so that This is a two-dimensional map the lens, the more it is bent. With microlensing events can be de- of magnification as function of the observer situated in O, the tected photometrically. The geo- ­position. A one-dimensional cut source S is not visible, but rather metric relation between the posi- through such a magnification pat-

two images, S1 and S2, at shifted tions of the lens, the source and tern is a light curve of the source positions. If the lens is a stellar mass the images is called “lens equation” due to its motion relative to the lens. The method to produce a magnification pattern is known as inverse or backwards ray shooting: Figure 3: The bending of light rays Figure 4: Parameters defining the lens one “shoots” light rays from the emitted by a source S passing through equation in the case of gravitational observer to the lens plane and cal- curved space-time produced by lens L. (­micro-)lensing. (Credit: Wambsganss (Credit: Wambsganss 1998) 1998) culates the angle of deflection due to the individual objects acting as microlenses. Then these deflected light rays are followed until they hit the source plane. The rays are “collected” in the source plane in small squares. These “pixels” con- tain various numbers of rays (or photons); the number of rays per pixel is directly proportional to the magnification of a source with the size and shape of such a square. This two-dimensional density dis- tribution of light rays can be easily visualised and is called “magnifi- cation pattern”.

SPATIUM 45 6

200216_Spatium_45_2020_(001_016).indd 6 16.05.20 11:11 lens equation is more complex, but it remains true that there are solu- tions to this equation for which the magnification is infinite. In this case, the caustic is a closed curve, or set of closed curves, and at those curves the magnification diverges formally to infinity. An example magnification map and the corre- sponding caustic in this case are shown in Figure­ 5. Figure 5: (Left) The magnification pattern of a two-body lens calculated by ­inverse ray shooting: The brighter the green scale, the higher the magnification. There are some general rules about (Right) The corresponding caustic consisting of cusps and folds defining the caustics: (1) They are closed curves ­positions where the lens equation becomes singular resulting in formally infinite with an inside and an outside. (2) magnification. (Credit: https://microlensing-source.org/) They have zero width. (3) The magnification diverges to infinity at the caustic. (4) The magnifica- An important feature of gravita- where the lens equation becomes tion inside of the caustic is always tional lensing is the occurrence of singular. For a point lens, the caus- greater than the magnification a “caustic”. Caustics are well tic is degenerated to a single point: outside of the caustic. (5) The caus- known in optics. In microlensing, perfect alignment between source tic lines are called folds, the loca- the caustic corresponds to the and observer results in an annulus tions where curved folds connect ­positions of the source for which shaped image, the so-called Ein- are called cusps. The magnifica- the solution of the lens equation re- stein ring. For a two-body lens tion pattern also depends on the sults in infinite magnification, i.e., (such as a star plus exoplanet), the mass ratio of the bodies defining

Figure 6: (Left) Magnification pattern of a two-body lens (Right) Light curves resulting from the trajectories of the consisting of a host star (producing the central caustic) and its source for two cases depending on the impact angles repre- exoplanet (producing the planetary caustic). sented by the blue and red straight lines.

SPATIUM 45 7

200216_Spatium_45_2020_(001_016).indd 7 16.05.20 11:11 the lens. The light curve depends on the path of the source relative to the lens and particularly relative to the caustics. A real situation is shown in Figure 6.

More complicated situations are drawn in Figure 7, where 8 (A), 4 (B), and 6 (C) exoplanets are situ- ated at positions inside and outside the Einstein radius produced by the host star situated at the or­ igin. ­Figure 8 shows the caustics of a ­Saturn-mass planet (mass ratio 10-4) with a projected separation close to one Einstein radius (so-called resonant lensing): the six panels Figure 7: Geometry of three host star and exoplanet configurations A, B, and C from top to bottom show parts of with 8, 4, and 6 exoplanets, respectively. The star symbol at the origin of the co- ordinate system marks the position of the “main” lensing star. The crosses indicate the magnification patterns for the positions of the exoplanetary positions. The scale is in units of Einstein radii of ­separations of 1.105, 1.051, 1.025, the main lensing star. The dashed line shows the Einstein ring radius. 0.951 and 0.905 Einstein radii ac- cording to case C of Figure 7. ­Figure 9 displays the typical (difference) light curves obtained from the sec- ond, fourth and sixth panel; time scales is in units of the Einstein Figure 8: Resonant lensing represented Figure 9: Microlensing light curves re- time, the amplitude is given in by the caustic of a Saturn-mass exo- sulting from the three trajectories indi- planet with a projected separation close cated by the arrows of Figure 8. magnitudes. to one Einstein radius corresponding to case C of Figure 7.

SPATIUM 45 8

200216_Spatium_45_2020_(001_016).indd 8 16.05.20 11:11 Detectability of Exoplanets and Exomoons by Microlensing Figure 10: Triple lens configuration of a host star, an exoplanet and an exomoon. The mass ratios and geometric parameters are given in Table 1. (Credit: Liebig The idea to simulate the detecta- und Wambsganss 2010) bility not only of exoplanets, but mainly of exomoons, is simple: Table 1: Parameter values of a typical standard scenario. Parameters marked with One calculates different magnifi- an asterisk (*) are varied in order to evaluate their influence on the exomoon de- tection rate and to compare different triple-lens scenarios. The fixed parameters cation patterns resulting from dif- lead to values for the Einstein ring radius (1.9 AU in the lens plane). The lensed sys- ferent lens configurations (i.e., var- tem is a Saturn-mass planet at a projected separation of 2.5 AU from its 0.3 solar iations in parameter space where mass M-dwarf host star. The Earth-mass exomoon orbits the exoplanet at 0.06 AU, mass ratios, distances and angles are i.e., in 0.01 mas angular separation. changed) and generates from them theoretical light curves represent- ing different source paths and ra­ dii w.r.t. caustic or convolved situa- tions (Figures 10–12 and Table 1). Then one compares the simulated with measured light curves to identify possible exoplanets and exomoons. “Comparison” in this case means to subtract the light curve corresponding to a binary- lens magnification map and to test the significance of the deviation or residual light curve against statis- tical methods, e.g., 2-test.

Figure 11: (Left) Magnification pattern computed for the lens configuration of Table 1. The relative positions of host star, exoplanet and exomoon are sketched in the lower left. A darker shade of grey corresponds to a higher magnification. The straight line marks the trajectory of the source. (Right) Resulting light curves for two different source radii, one (thin) and three (dashed) solar radii. (Credit: Liebig und Wambsganss 2010)

SPATIUM 45 9

200216_Spatium_45_2020_(001_016).indd 9 16.05.20 11:11 To detect exomoons it is absolutely critical to have caustics by which the resulting light curves can clearly be discriminated from the exoplanet light curve. Figures 13 and 14 show convolved magnifica- tion patterns of exoplanet and exo­ moon for different angular separa- tions and different mass ratios, respectively.

In this way magnification patterns can be calculated for a whole orbit Figure 12: (Left) In the ideal case the trajectory or path of a source of reasonable size (indicated by the circles at the bottom left) will cross the magnification pattern of the exomoon around its exo- through the caustics of both the exoplanet and the exomoon (seen at the lower cusp planet. The result of such a simu- of the exoplanet caustic) as indicated by the straight line. (Right) However, the lation is shown in Figures 15 and 16. source trajectories have to be chosen independently of the (resonant) caustic fea- Actually, triple-lens magnification tures, though all light curves are required to pass through or close to the exoplanet caustic. This is realised through generating a grid of source trajectories that is only maps are generated, but only the oriented at the exoplanet caustic. (Credit: Liebig und Wambsganss 2010) caustics of the exoplanet and the exomoon are displayed for twelve orbital phases, i.e., in steps of 30 °. Light curves resulting from caus- tic and source trajectory configu- ration as displayed in Figure 16 are shown in Figure 17.

Figure 13: Exoplanet and exomoon magnification/caustic patterns for three different angular separations. (Credit: Liebig und Wambsganss 2010)

SPATIUM 45 10

200216_Spatium_45_2020_(001_016).indd 10 16.05.20 11:11 Figure 14: Same as Figure 13 for three different mass ratios. (Credit: Liebig und Wambsganss 2010)

Figure 15: Set of calculated triple-lens mag- nification patterns including caustics of an exomoon orbiting an exoplanet in steps of 30 °. (Credit: Liebig und Wambsganss 2010)

SPATIUM 45 11

200216_Spatium_45_2020_(001_016).indd 11 16.05.20 11:11 Figure 16: Triple-lens magnification maps for scenarios with mass ratios exoplanet/host star of 10-3 and exomoon/exoplanet of 10 -2 and separation angle ratios exoplanet/host star of 1.3 and exomoon/exoplanet of 1.0 in units of Einstein radii (accord- ing to the standard scenario as summarised in Table 1). Displayed are the caustic configurations for twelve different exomoon positions completing a full circular orbit in steps of 30 °. The relative positions of host star, exoplanet and exomoon are sketched in the lower left of each panel (not to scale). A darker shade of grey corresponds to a higher magnification. (Credit: Liebig und Wambsganss 2010)

SPATIUM 45 12

200216_Spatium_45_2020_(001_016).indd 12 16.05.20 11:11 Figure 17: Sample of triple-lens light curves corresponding to the source trajectories as indicated in Figure 16 by the straight lines crossing the caustics. The solid light curves were extracted with an assumed solar source size. The thin, dashed lines show the same light curves with source size of three solar radii. The magnification scale is given in negative magnitude difference, i.e., the unmagnified baseline flux of the source is 0. The time scale represents a uniform relative motion of source and lens. (Credit: Liebig und Wambsganss 2010)

SPATIUM 45 13

200216_Spatium_45_2020_(001_016).indd 13 16.05.20 11:11 Searching for strategy. The first step comprises monitors 170 million stars on a the monitoring of more than 100 regular basis using a 1.3 metre tele­ Exoplanets and million stars in the galactic bulge scope situated in Chile and can several times per week. The CCD ­detect more than 2,000 alerts per Exomoons with frames taken during this monitor- observation season (Figure 18). ing are then successively subtracted MOA (Microlensing Observations Microlensing from each other (called “difference in Astrophysics) follows a similar Surveys imaging”). In the case of a micro- strategy by using a 1.8 metre tele- lensing event, the difference im- scope in New Zealand, and is ages will contain residual light, and ­typically alerting about 800 events the community will be alerted. per season. The search for microlensing events Two teams are responsible for this through exoplanets (and exo- monitoring task: OGLE (Optical In a second step, follow-up obser- moons) is performed in a two-step Gravitational Lens Experiment) vations are taken from the alert

Figure 18: Search areas of the galactic bulge region scheduled for an OGLE-IV survey campaign. The observation cadence is colour coded: red (up to 30 frames/night), yellow (up to 10 frames/night), green (up to 3 frames/night), blue (1 frame/night), cyan (about 1 frame/2 nights). (Credit: Jan Skowron, OGLE Team)

SPATIUM 45 14

200216_Spatium_45_2020_(001_016).indd 14 16.05.20 11:11 events with high frequency (sev- Conclusions even detect exomoons around exo­ eral times per hour) and high ac- planets around distant stars. The curacy using the following net- possible detection of exomoons re- works: PLANET (Probing Lens quires extremely high photo­ Anomaly NETwork), equipped Gravitational Microlensing is a metric measuring precision of the with five telescopes on the South- powerful method for the detection order of milli-magnitudes and ern Hemisphere; microFUN (Mi- of extrasolar planets. One of its sampling rates of ideally one frame crolensing Follow-Up Network), strengths is its possibility to deter- every few minutes, applied to using a main telescope (in Chile) mine the abundance of exoplanets: many 100 million stars. In paral- plus many supplement telescopes Cassan et al. (2012) have shown lel, numerical simulations of around Earth, including amateur that on average every star in the many three-lens configurations 0.25–1.8 metre telescopes; MiND- Milky Way is surrounded by at and robust modelling codes need STEp (Microlensing Network for least one planet of Neptun mass. to be developed. It sounds very the Detection of Small Terrestrial Another strength is the sensitivity ambitious, but all this is within Exoplanets), consisting of a Danish to low masses, i.e., Earth-like exo­ reach of current or near future 1.5 metre telescope (in La Silla) planets. In the future gravitational possibilities. plus one Monet-North and one microlensing has the potential to Monet-South; RoboNet (a micro- lensing search for cool planets), be- ing part of the LCO (Las Cumbres Observatory Global Network), op- erating 1 metre and 2 metre robotic telescopes worldwide. Further reading/literature: Spatium No 32, Meylan, G., Mirages Schneider, P. / Kochanek, C. / in the Universe, November 2013 Wambsganss, J., Gravitational Lens- Spatium No 41, Mordasini, Ch., ing: Strong, Weak and Micro. Saas- ­Extrasolar Planets, May 2018 Fee Advanced Course, 33. Springer 2006, ISBN 3-540-30309-X Beaulieu, J.-P. et al., Discovery of a cool planet of 5.5 Earth masses Udalski, A. et al., OGLE-IV: Fourth through gravitational microlensing. Phase of the Optical Gravitational Nature letters 439, 437 (26 January Lensing Experiment. Acta Astronom- 2006), doi:10.1038/nature04441 ica, 65 (2015), 1–38 Benn, C. R., The moon and the Wambsganss, J., Discovering galactic ­origin of life, In: Earth, Moon and planets by gravitational microlensing: Planets 85/86, 61 (2001) magnification patterns and light curves, MNRAS 284, 172–188 (1997) Cassan, A. et al. One or more bound planets per Milky Way star from mi- Wambsganss, J., Gravitational Lensing crolensing observations. Nature 481, in Astronomy, Living Reviews in 167 (2012) Relativity, 1 (1998) Dodelson, S., Gravitational Lensing, Wambsganss, J., Gravitational lensing: Cambridge UP 2017, numerical simulations with a hierar- ISBN 978-1-107-12976-4 chical tree code. Journal of Computa- tional and Applied Mathematics 109 Liebig, Ch. / Wambsganss, J., (1999), 353–372 ­Detectability of extrasolar moons as gravitational microlenses, A&A 520, Wambsganss, J., Gravitational Micro- A68 (2010), lensing, In: Gravitational Lensing: doi:10.1051/0004-6361/200913844 Strong, Weak and Micro. Saas-Fee Lectures, Springer-Verlag. Saas-Fee Schneider, P. / Ehlers, J. / Falco, E. E., Advanced Courses. 33 (2006), 453– Gravitational Lenses, Astronomy & 540, doi:10.1007/3-540-30309-X_4 Astrophysics Library, Springer 1992, ISBN 0-387-97070-3, 3-540-97070-3 https://microlensing-source.org/

SPATIUM 45 15

200216_Spatium_45_2020_(001_016).indd 15 16.05.20 11:11 SPATIUM

The Author

versity and at the Max Planck In- Prof. Wambsganss was awarded stitute for Astrophysics in Garch- the Fellowship of the “Studienstif- ing. From 1994 to 1999 he became tung des deutschen Volkes” (1981– member of the staff at the Astro- 1987), the Postgraduate Fellowship physikalisches Institut in Potsdam, of DAAD (1987–1988), the Post- where he achieved his Habilitation doc-Fellowship Position of Prince- in 1999. In 2000 and 2001 he was ton University (1990–1992) and a visiting Professor at the University Senior Research Fellowship of of Melbourne, while being Asso- Australian Research Council ciate Professor (C3) at Potsdam (2000–2001). University from 1999 until 2004. Since 2004 Prof. Wambsganss is His research concerns the search Full Professor (C4) at the Heidel- for extrasolar planets, quasars, gal- berg University and Director of the axy clusters and gamma-ray bursts, Astronomisches Rechen-Institut strong gravitational lensing (mul- (ARI). From 2005 to 2015 he was tiple quasars, giant luminous arcs Director of the Zentrum für As- and microlensing), extragalactic Prof. Dr. Joachim Wambsganss tronomie der Universität Heidel- astrophysics and cosmology (dark studied physics and astronomy at berg (ZAH). Intermediately, he matter, Hubble constant), the the Ruprecht Karl University in was Bohdan Paczynski Visiting ­simulation, modelling, observation Heidelberg in 1981–1983 and at the Professor at Princeton University and analysis of astronomical data Ludwig Maximilian University between 2008 and 2009, and in (e-science, virtual observatory, (LMU) in Munich in 1983–1987. 2013 Schroedinger Visiting Pro- data storage, open data and open Between 1987 and 1988 he was vis- fessor at the Pauli Center for access). iting student at Princeton Univer- ­Theoretical Studies of ETH and sity. In 1990 he achieved his PhD Zurich University. Since October in Astronomy at the LMU/MPA 2017, Joachim Wambsganss is Di- Garching. From 1990 to 1994 he rector for Astrophysics & Cosmo­ was a Postdoc at Princeton Uni- logy at ISSI.

200216_Spatium_45_2020_(001_016).indd 16 16.05.20 11:11