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Dominik, Horne, Bode: Extrasolar planet The first cool rocky/icy exoplanet 1: Artist’s impression (by Herbert Zodet, ESO) of OGLE-2005-BLG-390Lb with its (unseen) red dwarf star OGLE-2005-BLG-390L in the background. An atmosphere surrounds the icy surface. This picture is available as a computer desktop background from http://www. eso.org/outreach/press-rel/ pr-2006/phot-03-06.html. Are we really alone? The discovery of the cool, rocky and icy planet OGLE-2005-BLG-390Lb using the technique of gravitational microlensing suggests that we are not. Martin Dominik, Keith Horne and Michael Bode explain the technique and how this discovery changes our view of the universe. Abstract The recent discovery of the first cool, rocky/icy exoplanet around a mainsequence star provides the first observational hint that planets like Earth are common in the universe. It was detected using gravitational microlensing from data of three independent observing teams under the lead of PLANET (Probing Lensing Anomalies NETwork), operating a common microlensing campaign with the UK-based RoboNet project. In the coming years intense microlensing observations will determine the abundance of cool rocky/icy planets around M-dwarfs and therefore provide the first observational test of models of planet formation and migration on objects that share their evolutionary history with Earth and other planets on which life had a chance to develop. A&G • June 2006 • Vol. 47 O n 26 January 2006, three independent campaigns announced the discovery of OGLE-2005-BLG-390Lb, the first cool, rocky/icy planet orbiting a main-sequence star other than the Sun (Beaulieu et al. 2006). This discovery from gravitational microlensing, the technique of monitoring stars in the galactic bulge magnified by the bending of light due to the gravitational field of an intervening foreground star, provides the first observational hint that planets like Earth are common in the universe. Until about a decade ago, all our knowledge about planets was based on the nine planets of our solar system – an incomplete sample, to say the least. These gave us a picture of small rocky inner planets (Mercury, Venus, Earth, Mars), outer gas giants (Jupiter, Saturn, Uranus, Neptune), and the odd ice planet (Pluto). Our understanding of planet formation was forced to undergo a revision with the detection of the first planets around main-sequence stars other than the Sun (Mayor and Queloz 1995). These revealed their existence by means of Doppler shifts in the spectra of nearby stars, caused by the gravitational pull of the orbiting planets, creating a wiggle in the radial velocity of its host star. But because these planets had to be large to cause an observable Doppler shift, the planetary systems revealed were unlike the solar system; astronomers were faced with massive gas giants in close orbits around their host star. By January 2006, about 170 extrasolar planets were known, of which more than 90% were found in radial-velocity surveys whereas 6 were detected from partial occultations of their parent star while they pass in front of it (see figure 2). The method of observing the periodic displacement of a potential planet’s host star has also been suggested, but due to insufficient astrometric accuracy this technique has not provided any detections yet. First, make your planet The currently favoured scenario for planet formation is that of core-accretion, where in a first step planetary cores form from condensed material in 3.25 Dominik, Horne, Bode: Extrasolar planet orbit period (for solar-mass stars) 30 d 1 yr 10 yr 3d 100 yr FU N Doppler: 160 ET /R ob oN et, µ 104 PL AN 1000 , VLT 10 0.1 0.01 F 1 I SIM S, HAT WASP, TrE OG LE , Keck MO A, 100 MP planet mass (Earths) the protoplanetary disc around their host star. While in an inner hotter zone only grains of dust, sand and pebbles clump together, planet formation is further supported by the presence of icy snowballs in a cooler zone outside the so-called “ice boundary”. Planets forming there are likely to grow to gas giants by accreting hydrogen and helium, and to migrate inwards, with a fair fraction ending up in close orbits: these constitute the hot planets. The expected distribution of planets as a function of their mass and orbital axis depends critically on the mass of their host star. For different stellar masses between 0.2 M⊙ and 1.5 M⊙, figure 3 shows the results of simulations carried out by Ida and Lin 2005. One can distinguish at least three different classes of planets: hot planets that have been dragged into close orbits; cooler giants that formed from icy cores in the outer regions and migrated inwards; and cool rocky and icy planets. For the region in which radial-velocity surveys are sensitive, these simulations are in remarkable agreement with the observed distribution of semi-major axes of orbits and planet masses. While gas giants are predicted to be much less abundant for less massive stars (also in agreement with the observational data from radial-velocity surveys), a strong population of cool, rocky/icy planets is predicted for any mass of host star, which includes the Earth and other planets with the potential to support life. However, different techniques are required to study planets of this evolutionary type. The range of orbital semi-major axes between 0.1 and 10 AU appears to be a good match with the sensitivity region of gravitational microlensing, and this technique currently is the only one capable of detecting planets with mass as low as that of Earth. OGLE-2005-BLG-390Lb Kepler transits: 6+3 0.01 astrometry: 0 0.1 1 orbit radius (AU) microlensing: 3 10 100 2: Distribution of known extrasolar planets with mass and orbital axis or period (for a solar-mass host star) as well as the detection limits for various techniques: radial velocity/Doppler wobble (black) for velocities of 3 m s–1 and 1 m s–1; astrometry (green) for ground-based (Keck, VLTI) and space-based observations (SIM); eclipsing transits (blue) from the ground (WASP, TrES, HAT) or space (Kepler); and microlensing (red) ground-based (PLANET/RoboNet, OGLE, MOA, µFUN) or space-based (MPF) campaigns. The planets of our solar system are marked by yellow circles. While the larger velocity and the larger range of inclinations for which an eclipse can occur favour closer orbits for the radial-velocity and the transit technique, respectively, the astrometric signal increases with orbit size. For all these techniques, a few orbits need to be observed, so the duration of the campaign limits the orbit size for which planets can be detected. This restriction does not apply for microlensing, making it the only technique capable of detecting planets more distant than ~8 AU from their parent star within a reasonable time. Microlensing is most sensitive to orbital radii around the Einstein radius rE = DL θE ~ 2–4 AU, where the detection limits are of statistical nature. The only principal limitation arises from the finite size of the source star washing out the signal for planets below a critical mass, which is around the mass of the Earth for ground-based campaigns, while the higher photometric accuracy that can be achieved from space would even allow the detection of martianmass planets. The unique capability of microlensing for detecting low-mass planets at several AU, and the fact that it is currently the only technique that can detect Earth-mass planets, are reflected by the unique position of OGLE-2005-BLG-390Lb in discovery space among all known exoplanets. Gravitational microlensing The phenomenon of bending of light by the gravitational field of stars, now commonly known as “gravitational microlensing”, was noted by Einstein as early as 1912, but he did not publish the corresponding “little calculation” in full until much later (Einstein 1936) after intense persuasion by Czech engineer and amateur scientist R W Mandl. For those interested in the historical details, there is a delightful article by Renn, Sauer and Stachel (Renn et al. 1997). According to General Relativity (Einstein 1915), a point-like (spherically symmetric and non-rotating) object with mass M bends a light ray passing at a distance ξ by an angle α̂ = 2RS/ξ (for ξ ≫ RS), where RS = (2GM)/c2 denotes the Schwarzschild radius; G is the universal gravitational constant, and c is the vacuum speed of light. For this gravitational lens, in contrast to the usual shaped lenses made of material with larger refractive index than the surroundings, the deflection angle decreases with distance from the centre. The deflection law implies that for an observed background star at distance DS, per3.26 fectly aligned with an intervening foreground star with mass M at distance DL, a ring-like image of the size of the angular Einstein radius ___________ √ DS – DL θE = 2RS _______ DL – DS (1) is created. This ring breaks up into two distorted images of the source star, separated by ~2θE, if the intervening “lens” star is slightly displaced from the line-of-sight. For source stars in the galactic bulge, typical distances are DS ~ 8.5 kpc and DL ~ 6.5 kpc, for which θE = 600 (M / M⊙)1/2 µas (2) This means that current optical telescopes cannot resolve these images. However, the distortion of the images results in an observable magnification of the source star given by 2 u_____ + 2 A(u) = ________ u√u2 + 4 (3) for the lens and source star being separated by u θE. With µ denoting (the absolute value of) their relative proper motion, the source moves by θE relative to the lens within tE = θE /µ and for uniform motion, one finds ___________ √ ( ) t – t0 2 u(t) = u02 + _____ t E (4) where u0 θE is the closest angular approach, occurring at epoch t0. A typical proper motion µ ~ 15 µas d–1, implies a microlensing event timescale tE ~ 40 (M / M⊙)1/2 days (5) i.e. around a month for lens stars between 0.3 M⊙ and 1 M⊙. Einstein concluded that “there is no great chance of observing this phenomenon”. In order to maximize the chances, it appears most promising to monitor dense resolved fields of stars providing as many target stars as possible, such as the Magellanic Clouds or the galactic bulge. This chance was quantified by Petrou (1981) and Paczyński (1986) for stars in the Large Magellanic Cloud and by Kiraga and Paczyński (1994) for stars in the galactic bulge. They found that, in each case, only about one in a million stars is A&G • June 2006 • Vol. 47 Dominik, Horne, Bode: Extrasolar planet 4 (a): The PLANET (Probing Lensing Anomalies NETwork) telescopes. Their distribution in longitude allows round-the-clock coverage during the galactic bulge observing season from May to September each year. (b): Locations of RoboNet sites. Boyden (1.5 m) SAAO (1.0 m) (a) 103 102 101 1 103 102 101 Mp (Earth mass) 1 Perth (0.6 m) Canopus (1.0 m) 103 102 101 1 103 102 La Silla Danish (1.54 m) 101 1 (b) 103 102 101 1 10–1 1 a (AU) 101 3: Distribution of planets around host stars with different mass resulting from a simulation based on core-accretion models (Ida and Lin 2005). The migration of planets has artificially been halted at 0.04 AU. Separated by population gaps, one can easily distinguish hot planets, cooler gas giants and cool rocky/icy planets. While planets below ~10 M⊕ between 0.1 and 10 AU are predicted to be common for all masses of the host star, gas giants become rare for the lower masses resembling M-dwarfs. (Courtesy of S Ida) brightened by more than 30% at a given time – no great chance, indeed. But unlike in 1936, technological advances in the development of computers and digital cameras have made surveys involving frequent monitoring of tens of millions of stars feasible. Consequently, the first microlensing surveys directed at the Magellanic Clouds and the galactic bulge went into operation from 1990, and the first event was reported in 1993 by the MACHO (MAssive Compact Halo Object) collaboration (Alcock et al. 1993). Nowadays, the OGLE (Optical Gravitational Lensing Experiment) collaboration, using the 1.3 m Warsaw Telescope at Las Campanas Observatory (Chile), and the MOA (Microlensing Observation in Astrophysics) collaboration, using their 1.8 m telescope at Mt John Observatory (New Zealand), monitor more than 100 million stars daily. Online data analysis allows them to issue public alerts on about 1000 microlensing events per year A&G • June 2006 • Vol. 47 while they are in progress (Udalski 2003). These events are designated with the name of the survey campaign, the year, “BLG” for the galactic bulge and a running number, so that, for example, the 147th event alerted by OGLE in 2003 is named OGLE-2003-BLG-147. Mao and Paczyński (1991) showed that planets orbiting the lens star can reveal their existence by causing an observable deviation to the symmetric light curve that would be created by a single isolated lens star. While a deviation of the desired amplitude is less likely for less massive planets, only the finite size of the source star limits it. With 1% photometric accuracy achievable from ground-based telescopes, planets with masses as small as that of Earth can be detected, and even planets with the mass of Mars become detectable with a space-based mission. With q denoting the planet-to-star mass ratio, the duration of the planetary signal is of the order of the time __ √ q t E, during which the source moves by the angular Einstein radius of the planet, or the time t⋆ = θ⋆ / θE, in which it moves by its own angular radius, whichever is the larger. This equates to a range from a few hours to a few days, which in any case is much smaller than the orbital period, so that a snapshot of the planet is obtained. The only parameters characterizing the planet that affect the light curve therefore are the mass ratio q and a separation parameter d, where d θE is the instantaneous angular separation from its parent star. While this means that very little information about the orbit is provided, in particular its inclination, eccentricity and orbital phase are unknown, it allows the detection of planets in wide orbits which is otherwise prevented because the orbital period would exceed the lifetime of a project – and/or its investigator! The largest sensitivity is reached when the planet’s angular separation from its host star is around 3.27 Dominik, Horne, Bode: Extrasolar planet the angular Einstein radius θE, which for a lens star at DL ~ 6.5 kpc with a mass between 0.3 M⊙ and 1 M⊙ gives a projected distance rE ~ 2–4 AU. It is a lucky coincidence that the gravitational radius RS of stars (a few km) combines with typical distances within the Milky Way (few kpc) to result in a distance that is comparable to reasonable planetary orbits (few AU). The daily sampling of the microlensing surveys appears not to be sufficient for characterizing planetary anomalies in the light curves. Public alerts on events as observed and online provision of photometric data, however, allows other teams to carry out follow-up observations. The PLANET (Probing Lensing Anomalies NETwork) collaboration uses a network of 1 m-class optical telescopes (shown in figure 4a) distributed in longitude around the southern hemisphere in order to allow round-theclock coverage of galactic bulge microlensing events during the observing season from May to September each year. Having started a pilot campaign in 1995, PLANET now involves 33 collaborators affiliated with 19 institutions in 10 countries (France, UK, Denmark, Germany, Austria, Chile, Australia, New Zealand, USA, South Africa), and is led by Jean-Philippe Beaulieu (Institut d’Astrophysique de Paris, France) and Martin Dominik (University of St Andrews, United Kingdom). Its objective is to achieve a photometric accuracy of 1–2% and a standard sampling interval of between 1.5 and 2.5 hours, which can be decreased to a few minutes for ongoing anomalies (Dominik et al. 2002). Since 2005, PLANET has been operating a common microlensing campaign with RoboNet-1.0 (http://www.astro.livjm.ac.uk/ RoboNet). Led by astronomers from Liverpool John Moores University on behalf of a consortium of 10 UK universities including St Andrews, RoboNet-1.0 is a PPARC-funded prototype global network of the largest fully robotic (2 m) telescopes built to date (figure 4b). It comprises the Liverpool Telescope (http://telescope.livjm.ac.uk; sited on La Palma, Canary Islands, Spain) which is a national facility of the UK for time-domain astronomy (figure 5), plus time acquired for the project from the Faulkes Telescope North (Maui, Hawaii, USA) and Faulkes Telescope South (Siding Spring, Australia). All three telescopes were designed and built in Merseyside by Telescope Technologies Ltd, taking advantage of expertise within the Astrophysics Research Institute of LJMU, and are controlled from the ARI’s Telescope Management Centre. From October 2005, ownership of the Faulkes Telescopes passed from the Faulkes Telescope Corporation to the Las Cumbres Observatory. Besides searching for planets through microlensing (Bergdorf et al. 2006), RoboNet also follows up gammaray bursts as its other core science programme – science which is again critically dependent on rapid and flexible response. 3.28 5: The Liverpool Telescope in its fully open enclosure at the Roque de los Muchachos Observatory on La Palma (Canary Islands), 2400 m above sea level. (Dr R Smith, Liverpool John Moores University) Virtual astronomers With the smaller field-of-view of the telescopes used by PLANET/RoboNet (compared to the OGLE and MOA survey telescopes), the smaller diameters of some of these, and the aim of highprecision nearly continuous coverage, it is not possible to monitor all events that the surveys pinpoint. Some alerts are immediately disqualified by being too faint or in too crowded fields. We aim to select our targets and their sampling rate so that the detection efficiency for planetary companions is maximized. In order to achieve this goal, the current properties of all ongoing events are monitored and the most promising targets selected following a prioritization algorithm. Except for the Perth 0.6 m telescope, which is automated, the observing priorities are passed to an observer at each of the PLANET telescopes who then follows the instructions. For the RoboNet telescopes, however, the observing requests are submitted to the sophisticated eSTAR software (http://www.estar. org.uk), which was developed in a collaboration between Exeter University and LJMU. This new technology allows a network of telescopes to make detections and to respond much faster than any human. User agents working as “virtual astronomers” collect, analyse and interpret data 24 hours a day, every day of the year, alerting us only when a discovery is made. Regularly updated light curves of monitored events as well as their parameters are available on the PLANET web pages (http://planet.iap.fr). Moreover, further information about ongoing identified or suspected anomalies is provided there. Major updates are also provided as emails to anybody who has subscribed to our PLANET anomaly alert mailing list. Because microlensing relies on the chance alignment of unobserved foreground lens stars with observed background source stars, there is very little choice in the type and distance of the former; indeed, an indication of the nature of the foreground star is only obtainable during the course of the event by measuring the event timescale tE. The mass density and kinematics of the galaxy dictates that the lens stars, and therefore the planets orbiting them, lie at distances between 4.5 and 9 kpc. With the probability for a lens star___ to cause a microlensing event increasing with √M , about 50% of the events are caused by M-dwarfs, whereas only 20% involve G-stars (like the Sun) or earlier types. For a source star located in Baade’s window (l,b) = (1°, –3.9°), the probability density for the mass of the lens star is shown in figure 6. The absence of planetary signals in 42 events well-covered by PLANET observations from 1997 until 1999, gave the first significant upper limits on the abundance of gas giants around M-dwarfs: it was found that less than 1/3 of the stars that gave rise to the monitored events host Jupiter-mass planets at orbital radii between 1.5 and 4 AU (Albrow et al. 2001). But it took until 2003 for the first planet to be discovered through microlensing, orbiting the lens star that caused event OGLE-2003-BLG-235, also known as MOA-2003-BLG-53 (Bond et al. 2004). With a mass about 1.5 times that of Jupiter, the planet created a strong signal in the form of spikes a week apart, which could be detected by the OGLE and MOA surveys. A further Jupiterlike planet, about three times as massive, was detected in April 2005 in event OGLE-2005BLG-071, using data from OGLE, µFUN, MOA and PLANET/RoboNet (Udalski et al. 2005). The event OGLE-2005-BLG-390 was reported by the OGLE Early-Warning System (EWS) on A&G • June 2006 • Vol. 47 Dominik, Horne, Bode: Extrasolar planet 1 100% galactic bulge (Baade’s window) (l,b) = (1°, –39°) DS = 8.5 kpc Γ–1 dΓ/d(lg M /M�) 75% 50% 0.5 25% M 0 0.01 K G F A B O 0% 0.1 1 M /M� 10 6: Probability density for the mass of the lens causing a microlensing event towards the galactic bulge on a source star located in Baade’s window (l,b) = (1°, –3.9°) at DS = 8.5 kpc, as given by its differential contribution to the event rate Γ. It is assumed that the lens is a main-sequence star, where the spectral type and approximate colour are indicated, or a brown dwarf. The bold line shows the cumulative distribution. About 50% of the events are caused by M-dwarfs, while just 20% involve G-stars or earlier types. 11 July 2005, and was subsequently monitored with the telescopes constituting the PLANET/ RoboNet network. Figure 7 shows the field around OGLE-2005-BLG-390 as observed with the Danish 1.54 m telescope by PLANET. From the reddening-corrected luminosity I0 = 14.25 and colour indices (V–I)0 = 0.85 and (V–K)0 = 1.9, the source star appears to be a G2–4 giant with angular radius θ⋆ = (5.25 ± 0.73) µas, corresponding to R⋆ = (9.6±1.3) R⊙ at DS = 8.5 kpc. The light curve, involving data from six different observing sites, is shown in figure 8. It first followed the characteristic brightening expected for a single isolated lens star and a point-like source star, and reached a peak magnification of about 3 on 31 July. On 10 August, however, an anomalous rise of 0.15 mag was observed by PLANET/RoboNet with the Danish 1.54 m telescope at ESO La Silla (this was the night starting on 9 August in Chile, but the early hours of 10 August in the UK). An OGLE point from this night showed the same trend. By monitoring the second half of this anomaly, which lasted about a day, with the Perth, West Australia, 0.6 m telescope, and because we had previously obtained a dense coverage of the peak region of the event, we were able to conclude that the lens star has a low-mass planet in orbit. It inherited its name OGLE-2005-BLG-390Lb because it is a planetary secondary (“b”) to the lens star (“L”) that caused the 390th microlensing event towards the galactic bulge (BLG) alerted by OGLE. The MOA collaboration was able to identify the source star on its frames and confirmed the observed deviation. From the light curve, we inferred an event timescale 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-2005A&G • June 2006 • Vol. 47 BLG-390 one of the shorter events. For this bright and large source star, its size significantly affects the light curve during the planetary deviation, which allowed us to determine t⋆ = 0.28 d, in which the source moves by its own angular radius θ⋆. This yields a proper motion µ = θ⋆/ t⋆ ~ 20 µas d–1 = 7.6 mas yr–1 and the angular Einstein radius θE = µtE ~ 210 µas. The two extractable parameters characterizing the planet are the mass ratio q = 7.6 × 10–5 and d = 1.610, where d θE is the current angular separation from its parent star. 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 were able to calculate probability densities based on the likelihood that a certain value would produce the observed event with its given parameters, assuming mass functions for lens and source stars in the galactic bulge and disc as well as their spatial mass density and velocity distribution for a double-exponential disc and a tilted barred bulge (Dominik 2006). The results of this analysis, shown in figure 9, yield the masses of lens star and planet as M ~ 0.22 M⊙ (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 found 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. We have also found a 95% probability for the source being a mainsequence star rather than a stellar remnant, and an 80% probability that the lens resides in the 7: True-colour image of the field around OGLE2005-BLG-390 resulting from a combination of images taken by PLANET in three different filters (BVI) at the Danish 1.54 m telescope at ESO La Silla (Chile). (Courtesy of the PLANET collaboration) galactic bulge as compared to a 20% probability that it belongs to the galactic disc. OGLE-2005-BLG-390Lb appears not to be massive enough to accrete enough gas to grow into a gas giant planet like Jupiter or Saturn. Instead, its expected surface temperature of 50–70 K points to an icy nature, making it a more massive version of Pluto or resembling the cores of the ice giants Uranus or Neptune, rather than the inner rocky planets like Venus or Earth. An artist’s impression of the planet, created by ESO, is shown in figure 1. With lens and source separating at a rate µ = 7.6 mas yr–1, cutting-edge instruments could in principle allow the resolution of the lens star from the source star and measurement of its magnitude in the foreseeable future. Such measurements would result in a reliable (20% uncertainty) determination of its mass (and thereby that of planet OGLE-2005-BLG390Lb). The biggest challenge here is with the contrast between lens and source magnitudes, which is expected to range between 7 and 8.5 for the K-band, while for shorter wavelengths it becomes even larger, between 9.5 and 12 for V. The detection of a planet with five Earth masses by microlensing opens a new window on the discovery of planets. For the first time, a rocky/icy planet that is in an orbit resembling that of the Earth around the Sun has been detected. This gives the first experimental hint that planets of this type, which includes all planets that provide conditions suitable for life to develop, are common. How common are these planets? Radial-velocity surveys indicate that the fractional abundance of giant planets around Mdwarfs is about 10 times smaller than around F- or G-stars, indicating fJ ~ 1–2%, which seems 3.29 Dominik, Horne, Bode: Extrasolar planet 1.6 3 3 magnification 2.5 1.5 OGLE 2 1.4 1 1.3 2000 3000 planetary deviation 3592 3593 2 1.5 OGLE Danish RoboNet Perth Canopus MOA 1 3560 3580 JD – 2450000 3600 8: Planetary model light curve of event OGLE-2005-BLG-390, along with data collected by PLANET/ RoboNet with the Danish 1.54 m telescope at ESO La Silla (Chile), the Perth 0.6 m (Western Australia), the Canopus 1.0 m (near Hobart, Tasmania) and the Faulkes Telescope North 2.0 m (Haleakala, Hawaii), as well as by OGLE (Las Campanas, Chile) and MOA (Mt John, New Zealand). Also shown are model light curves for an isolated single lens star without planets (orange, short-dashed), and for a binary source star (light black, long-dashed), which fail to match the observed data. 0.5 0 3 p (DL/RGC) tE = 11.0 d t� = 0.28 d –5 q = 7.6×10 25% 1 m/M⊕ 0% 10 75% 50% 1 25% 0.6 0.8 DL/RGC 1 0% 1.2 100% (c) orbital radius d = 1.610 2.5 2 1.5 1 0.5 0 75% 50% 1 0 100% (b) lens distance 2 0 0.4 p (lg a/[1 AU]) 1 0.1 (a) mass p (lg P/[1 yr]) p (lg M/Mref) 1.5 DS = (1.05 ± 0.25) RGC, (l,b ) = (359.73°, –2.36°) M/M� 1 100% 3 θ� = 75% (5.25±0.73) µas 2 50% 25% 1 a/(1 AU) 0% 10 100% (d) period 75% 50% 25% 10 P/(1 yr) 100 0% 9: Properties of the planetary system that caused the event OGLE-2005-BLG-390: mass of the planet m and its host star M, their distance DL as well as the orbital radius a and period P. The four panels show the probability densities of lg m/M⊕, lg M/M⊙, DL/RGC, lg a/(1 AU) and lg P/(1 yr), where RGC ~ 7.6 kpc denotes the distance to the galactic centre, together with their cumulative distribution (in bold). Constrained by the measured event timescale tE and the proper motion µ = θ⋆/t⋆, these are based on the assumption of a double-exponential disc and a tilted, barred bulge as well as a mass function for the lens stars (Dominik 2006), while stellar typing provides a prior estimate for the distance of the source star DS = (1.05 ± 0.25) RGC. For deriving the orbital radius and the period, randomly oriented circular orbits have been assumed, where period, mass and orbital radius are related by Kepler’s third law. While the dots on the abscissa and the arrows mark the expectation value and the standard deviation, the vertical line indicates the median and the shaded region a central 68.3% confidence +5.5 +0.21 +1.5 +8.7 +0.12 interval. The latter give m = 5.5–2.7 M⊕, M = 0.22–0.11 M⊙, a = 2.6–0.6 AU, P = 9.0–2.9 yr, and DL = 0.86–0.14 RGC. to agree with the small number of detections by microlensing. A study of the detection efficiencies for the PLANET campaign suggests that terrestrial planets are about 15 times less likely to be detected than massive gas giants, so that the respective number of detections points to an abundance ratio fE / fJ ~ 10 and therefore fE ~ 20%. The current PLANET/RoboNet telescope network is expected to yield ~50–80 fJ 3.30 detections of massive gas giants and ~5–10 fE terrestrial planets within three years. This lets us expect to find a few more cool, rocky/icy planets around M-dwarfs over the coming years, allowing a measurement of the relative abundance of gas giants and rocky/icy planets, and in turn providing the first observational test of models of planet formation and migration on objects that share their evolutionary history with Earth and other planets suitable for harbouring life. The deployment of further 2 m-class robotic telescopes at two additional prospective sites in the southern hemisphere (for example, in Chile and South Africa) would significantly boost the planet detection capabilities to ~200–250 fJ giants and ~20–25 fE terrestrial planets. The enhanced number of detections would then also allow us to study the distribution of properties (mass and orbital axis) of planets made of rock and ice. ESA’s Darwin mission and NASA’s TPF (Terrestrial Planet Finder) both aim to look for signs of life on planets around nearby stars by determining the abundance of biomarkers such as oxygen, ozone, methane and nitrous oxide as well as the greenhouse gases water vapour and carbon dioxide in their atmospheres. These missions are planned to be deployed from 2015 and will cost several billion pounds. Prior to that, one would like to know whether there are a sufficient number of promising candidate planets to look at. At this time, gravitational microlensing is the only way to find out. The discovery and formation of extrasolar planets forms one of the research initiatives of the recently formed Scottish Universities Physics Alliance (SUPA), allowing Scotland to build an international profile in the interdisciplinary emerging field of astrobiology. In a cooperation between the universities of Edinburgh, St Andrews and Strathclyde, researchers aim to understand the origin and diversity of life on Earth, in the solar system, and throughout the universe. Besides theoretical studies of star and planet formation, the University of St Andrews has established a strong profile in the hunt for extrasolar planets involving different techniques. This not only includes a leading role for the planet-hunting strategy and the interpretation of the results of the PLANET/RoboNet microlensing campaign, but also involvement in the SuperWASP project (Pollacco 2005), a wide-angle search for eclipsing transits, and the study of young stellar objects and debris discs (Greaves 2005). ● References Albrow M D et al. 2001 ApJ 556 L113. Alcock C et al. 1993 Nature 365 621. Beaulieu J-P et al. 2006 Nature 439 437. Bond I A et al. 2004 ApJ 606 L155. Burgdorf M J, Bramich D M, Dominik M, Bode M F, Horne K D and Steele I A 2006 Exoplanet Detection via Microlensing with RoboNet-1.0 to appear in P&SS. Dominik M et al. 2002 P&SS 50 299. Dominik M 2006 MNRAS 367 669. Einstein A 1915 Sitzungsber. preuss. Akad. Wiss. 47 831. Einstein A 1936 Science 84 506. Greaves J S 2005 Science 307 68. Ida S, Lin D N C 2005 ApJ 626 1045. Kiraga M, Paczyński B 1994 ApJ 430 L101. Mao S, Paczyński B 1991 ApJ 374 L37. Mayor M, Queloz D 1995 Nature 378 355. Paczyński B 1986 ApJ 304 1. Petrou M 1981 PhD thesis, University of Cambridge. Pollacco D 2005 A&G 46 19. Renn J, Sauer T and Stachel J 1997 Science 275 184. Udalski A et al. 2005 ApJ 628 L109. Udalski A 2003 Acta Astronomica 53 291. A&G • June 2006 • Vol. 47