Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stecf.org/coordination/esa_eso/extrasolar/report.pdf Äàòà èçìåíåíèÿ: Thu Sep 4 17:58:31 2008 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:43:32 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: space sail


Rep ort by the ESA­ESO Working Group on Extra-Solar Planets
4 March 2005

Summary
Various techniques are being used to search for extra-solar planetary signatures, including accurate measurement of radial velocity and positional (astrometric) displacements, gravitational microlensing, and photometric transits. Planned space experiments promise a considerable increase in the detections and statistical knowledge arising especially from transit and astrometric measurements over the years 2005­15, with some hundreds of terrestrial-type planets expected from transit measurements, and many thousands of Jupiter-mass planets expected from astrometric measurements. Beyond 2015, very ambitious space (Darwin/TPF) and ground (OWL) experiments are targeting direct detection of nearby Earth-mass planets in the habitable zone and the measurement of their spectral characteristics. Beyond these, `Life Finder' (aiming to produce confirmatory evidence of the presence of life) and `Earth Imager' (some massive interferometric array providing resolved images of a distant Earth) appear as distant visions. This report, to ESA and ESO, summarises the direction of exo-planet research that can be expected over the next 10 years or so, identifies the roles of the ma jor facilities of the two organisations in the field, and concludes with some recommendations which may assist development of the field. The report has been compiled by the Working Group members and experts (page iii) over the period June­December 2004.


Intro duction & Background Following an agreement to cooperate on science planning issues, the executives of the European Southern Observatory (ESO) and the European Space Agency (ESA) Science Programme and representatives of their science advisory structures have met to share information and to identify potential synergies within their future pro jects. The agreement arose from their joint founding membership of EIROforum (http://www.eiroforum.org) and a recognition that, as pan-European organisations, they served essentially the same scientific community. At a meeting at ESO in Garching during September 2003, it was agreed to establish a number of working groups that would be tasked to explore these synergies in important areas of mutual interest and to make recommendations to both organisations. The chair and co-chair of each group were to be chosen by the executives but thereafter, the groups would be free to select their membership and to act independently of the sponsoring organisations. The first working group to be established was on the topic of Extra-Solar Planet research, both detection and physical study, over a period extending from now until around 2015. The group worked on its report from June until December 2004 and reported its conclusions and recommendations to a second ESA-ESO meeting, held at ESA HQ in Paris in February 2005.

Terms of Reference and Comp osition The goals set for the working group were to provide: · A survey of the field: this will comprise: (a) a review of the methods used or envisaged for extra-solar planet detection and study; (b) a survey of the associated instrumentation world-wide (operational, planned, or proposed, onground and in space); (c) for each, a summary of the potential targets, accuracy and sensitivity limits, and scientific capabilities and limitations. · An examination of the role of ESO and ESA facilities: this will: (a) identify areas in which current and planned ESA and ESO facilities will contribute; (b) analyse the expected scientific returns and risks of each; (c) identify areas of potential scientific overlap, and thus assess the extent to which the facilities complement or compete; (d) identify open areas which merit attention by one or both organisations (for example, follow-up observations by ESO to maximise the return from other ma jor facilities); (e) conclude on the scientific case for the very large facilities planned or proposed.

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The working group membership was established by the chair and co-chair: the report is not a result of consultation with the community as a whole. The experts contributed considerable information for the report, but the conclusions and recommendations are the responsibility of the members.

Chair: Co-Chair: Members:

Michael Perryman Olivier Hainaut Dainis Dravins Alain L´ eger Andreas Quirrenbach Heike Rauer

ESA ESO Lund IAS Leiden DLR ESO­ECF ESA­ECF OHP Marseilles ESA ESA ESO LAOG Grenoble Tel Aviv Obs de Paris-Meudon ev Gen` e Heidelberg COROT Eddington Darwin OWL Planet Finder Transits Genie Radial velocity Microlensing

ECF support: Florian Kerber Bob Fosbury Experts: Fran¸ cois Bouchy Fabio Favata Malcolm Fridlund Roberto Gilmozzi Anne-Marie Lagrange Tsevi Mazeh Daniel Rouan Stephane Udry Joachim Wambsganss

Catherine Cesarsky (ESO) March 2005

´ Alvaro Gim´ enez Canete (ESA) ~

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Contents
1 Survey of the Field 1.1 1.2 1.3 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Search for Earth-Mass Planets and Habitability . . . . . . . . . . Present Limits: Ground and Space . . . . . . . . . . . . . . . . . . . 1 1 2 5 9 9 9

2 The Perio d 2005­2015 2.1 Ground Observations: 2005­2015 . . . . . . . . . . . . . . . . . . . . 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 2.1.6 2.1.7 2.2 Radial Velocity Searches . . . . . . . . . . . . . . . . . . . . .

Transit Searches . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Reflected Light . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Microlensing Searches . . . . . . . . . . . . . . . . . . . . . . 17

Astrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Direct Detection . . . . . . . . . . . . . . . . . . . . . . . . . 22 Other Searches . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Space Observations: 2005­2015 . . . . . . . . . . . . . . . . . . . . . 28 2.2.1 2.2.2 2.2.3 2.2.4 Space Transit Measurements: COROT, Kepler and Eddington 28 Space Astrometry Missions: Gaia and SIM . . . . . . . . . . . 30 Space-Based Microlensing: MPF . . . . . . . . . . . . . . . . . 32 Other Space Missions: JWST, Spitzer, SOFIA . . . . . . . . . 34

2.3

Summary of Prospects 2005­2015 . . . . . . . . . . . . . . . . . . . . 38 40

3 The Perio d 2015­2025 3.1

Ground Observations: 2015­2025 . . . . . . . . . . . . . . . . . . . . 40 3.1.1 3.1.2 OWL/ELT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Observations at an Antarctic Site . . . . . . . . . . . . . . . . 44

3.2

Space Observations: 2015­2025 . . . . . . . . . . . . . . . . . . . . . 48 3.2.1 Darwin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 iv


3.2.2 3.2.3 3.3 3.4 3.5

The Darwin Ground-Based Precursor: GENIE . . . . . . . . . 51 Terrestrial Planet Finder (TPF) . . . . . . . . . . . . . . . . . 52

ESA Themes: 2015­2025 . . . . . . . . . . . . . . . . . . . . . . . . . 53 Other Concepts and Future Plans . . . . . . . . . . . . . . . . . . . . 56 Summary of Prospects: 2015­2025 . . . . . . . . . . . . . . . . . . . . 57 59

4 The Role of ESO and ESA Facilities 4.1 4.2

The Expected Direction of Research . . . . . . . . . . . . . . . . . . . 59 Follow-Up Observations . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.2.1 4.2.2 4.2.3 High-Mass Planets . . . . . . . . . . . . . . . . . . . . . . . . 60 Low-Mass Planets . . . . . . . . . . . . . . . . . . . . . . . . . 62 Summary of Follow-Up Facilities Required . . . . . . . . . . . 63

4.3 4.4

Statistics of Exo-Planets: Implications for Darwin/OWL . . . . . . . 64 Astrophysical Characterisation of Host Stars . . . . . . . . . . . . . . 65 4.4.1 A Dedicated Spectral Survey . . . . . . . . . . . . . . . . . . . 65

4.5 4.6 4.7

Potential Overlap and Competition . . . . . . . . . . . . . . . . . . . 66 Open Areas: Survey Mission Beyond Kepler/Eddington . . . . . . . . 68 Other Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.7.1 4.7.2 4.7.3 Fundamental Physical Data . . . . . . . . . . . . . . . . . . . 68 Fundamental Planetary Data . . . . . . . . . . . . . . . . . . 69 Amateur Networks . . . . . . . . . . . . . . . . . . . . . . . . 69 72 75 75 77 79

5 Recommendations App endices A Space Precursors: Interferometers, Coronographs and Ap o dizers B Beyond 2025: Life Finder and Planet Imager C ESO 1997 Working Group on Extra-Solar Planets v


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1
1.1

Survey of the Field
Intro duction

The field of exo-planet research has exploded dramatically since the discovery of the first such systems in 1995. Underlying this huge interest three main themes of exo-planet research can be identified: (a) characterising and understanding the planetary populations in our Galaxy; (b) understanding the formation and evolution of planetary systems (e.g., accretion, migration, interaction, mass-radius relation, albedo, distribution, host star properties, etc.); (c) the search for and study of biological markers in exo-planets, with resolved imaging and the search for intelligent life as `ultimate' and much more distant goals. Detection methods for extra-solar planets can be broadly classified into those based on: (i) dynamical effects (radial velocity, astrometry, or timing in the case of the pulsar planets); (ii) microlensing (astrometric or photometric); (iii) photometric signals (transits and reflected light); (iv) direct imaging from ground or space in the optical or infrared; and (v) miscellaneous effects (such as magnetic superflares, or radio emission). Each have their strengths, and advances in each field will bring specific and often complementary discovery and diagnostic capabilities. Detections are a pre-requisite for the subsequent steps of detailed physical-chemical characterisation demanded by the emerging discipline of exo-planetology. As of December 2004, 135 extra-solar planets have been discovered from their radial velocity signature, comprising 119 systems of which 12 are double and 2 are triple. One of these planets has also been observed to transit the parent star. Four additional confirmed planets have been discovered through transit detections using data from OGLE (and confirmed through radial velocity measurements), and one, TrES-1, using a small 10-cm ground-based telescope. One further, seemingly reliable, planet candidate has been detected through its microlensing signature. The planets detected to date (apart from those surrounding radio pulsars, which are not considered further in this report) are primarily `massive' planets, of order 1 MJ , but extending down to perhaps 0.05 MJ (around 15 M ) for three short-period systems, although the inclination (and hence true mass) of two of these is unknown1 . Detection methods considered to date are summarised in Figure 1, which also gives an indication of the lower mass limits which are likely to be reached in the foreseeable future for each method. More information and ongoing pro jects are given in Jean Schneider's www page: http://www.obspm.fr/encycl/searches.html. An earlier ESO Working Group on the `Detection of Extra-Solar Planets' submitted a report with detailed recommendations in 1997 (Paresce et al., 1997). A summary and status of these recommendations is attached as Appendix C.
1

the following notation is used: MJ = Jupiter mass; M



= Earth mass 0.003 MJ .

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Figure 1: Detection methods for extra-solar planets, updated from Perryman (2000). The lower
extent of the lines indicates, roughly, the detectable masses that are in principle within reach of present measurements (solid lines), and those that might be expected within the next 10­20 years (dashed). The (logarithmic) mass scale is shown at left. The miscellaneous signatures to the upper right are less well quantified in mass terms. Solid arrows indicate (original) detections according to approximate mass, while open arrows indicate further measurements of previously-detected systems. `?' indicates uncertain or unconfirmed detections. The figure takes no account of the numbers of planets that may be detectable by each method.

1.2

The Search for Earth-Mass Planets and Habitability

The search for planets around stars in general, and Earth-mass planets in particular, is motivated by efforts to understand their frequency of occurrence (as a function of mass, semi-ma jor axis, eccentricity, etc.) and their formation mechanism and, by analogy, to gain an improved understanding of the formation of our own Solar System. Search accuracies will progressively improve to the point that the detection of telluric planets in the `habitable zone' will become feasible, and there is presently no reason to assume that such planets will not exist in large numbers. Improvements in spectroscopic abundance determinations, whether from Earth or space, and developments of atmospheric modelling, will lead to searches for planets which are progressively habitable, inhabited by micro-organisms, and ultimately by intelligent life (these may or may not prove fruitful). Search strategies will be assisted by improved understanding of the conditions required for development of life on Earth. 2


Very broadly, the search for potentially habitable planets is being concentrated around Sun-like stars (spectral type and age), focussing on Earth-mass planets, in low-eccentricity orbits at about 1 AU representing the `continuously habitable zone' (the habitable zone is the distance range from the parent star over which liquid water is likely to be present; the continuously habitable zone is the region throughout which liquid water should have been present over a significant fraction of the star's main-sequence lifetime). Further details and potential spectral diagnostics of life are given in this section. Such considerations may imply that the fraction of habitable planets is small, but they should add to the knowledge of where to look. Assessment of the suitability of a planet for supporting life, or habitability, is based on our knowledge of life on Earth. With the general consensus among biologists that carbon-based life requires water for its self-sustaining chemical reactions, the search for habitable planets has therefore focused on identifying environments in which liquid water is stable over billions of years. Earth's habitability over early geological time scales is complex, but its atmosphere is thought to have experienced an evolution in the greenhouse blanket of CO2 and H2 O to accommodate the 30% increase in the Sun's luminosity over the last 4.6 billion years in order to sustain the presence of liquid water evident from geological records. In the future, the Sun will increase to roughly three times its present luminosity by the time it leaves the main sequence, in about 5 Gyr. The habitable zone is consequently presently defined by the range of distances from a star where liquid water can exist on the planet's surface. This is primarily controlled by the star-planet separation, but is affected by factors such as planet rotation combined with atmospheric convection. For Earth-like planets orbiting main-sequence stars, the inner edge is bounded by water loss and the runaway greenhouse effect, as exemplified by the CO2 -rich atmosphere and resulting temperature of Venus. The outer boundary is determined by CO2 condensation and runaway glaciation, but it may be extended outwards by factors such as internal heat sources including long-lived radionuclides (U235 , U238 , K40 etc., as on Earth), tidal heating due to gravitational interactions (as in the case of Jupiter's moon Io), and pressureinduced far-infrared opacity of H2 , since even for effective temperatures as low as 30 K, atmospheric basal temperatures can exceed the melting point of water. These considerations result, for a 1 M star, in an inner habitability boundary at about 0.7 AU and an outer boundary at around 1.5 AU or beyond. The habitable zone evolves outwards with time because of the increasing luminosity of the Sun with age, resulting in a narrower width of the continuously habitable zone over 4 Gyr of around 0.95­1.15 AU. Positive feedback due to the greenhouse effect and planetary albedo variations, and negative feedback due to the link between atmospheric CO2 level and surface temperature may limit these boundaries further. Migration of the habitable zone to much larger distances, 5­50 AU, during the short period of post-main-sequence evolution corresponding to the sub-giant and red giant phases, has been considered. Within the 1 AU habitability zone, Earth `class' planets can be considered as 3


those with masses between about 0.5­10 M or, equivalently assuming Earth density, radii between 0.8­2.2 R . Planets below this mass in the habitable zone are likely to lose their life-supporting atmospheres because of their low gravity and lack of plate tectonics, while more massive systems are unlikely to be habitable because they can attract a H-He atmosphere and become gas giants. Habitability is also likely to be governed by the range of stellar types for which life has enough time to evolve, i.e. stars not more massive than spectral type A. However, even F stars have narrower continuously habitable zones because they evolve more strongly (and rapidly), while planets orbiting in the habitable zones of late K and M stars become trapped in synchronous rotation due to tidal damping, which may preclude life apart from close to the light-shadow line. Mid- to early-K and G stars may therefore be optimal for the development of life. Owen (1980) argued that large-scale biological activity on a telluric planet necessarily produces a large quantity of O2 . Photosynthesis builds organic molecules from CO2 and H2 O, with the help of H+ ions which can be provided from different sources. In the case of oxygenic bacteria on Earth, H+ ions are provided by the photodissociation of H2 O, in which case oxygen is produced as a by-product. However, this is not the case for anoxygenic bacteria, and thus O2 is to be considered as a possible but not a necessary by-product of life (for this signature of biological activity, as well as for any other, a key issue is that of false positives, i.e. cases where the signature is detected but there is no actual life on the planet, while the case of false negatives, when there is some life on the planet but the signature is absent, is significantly less `serious'). Indeed, Earth's atmosphere was O2 -free until about 2 billion years ago, suppressed for more than 1.5 billion years after life originated. Owen (1980) noted the possibility, quantified by Schneider (1994) based on transit measurements, of using the 760-nm band of oxygen as a spectroscopic tracer of life on another planet since, being highly reactive with reducing rocks and volcanic gases, it would disappear in a short time in the absence of a continuous production mechanism. Plate tectonics and volcanic activity provide a sink for free O2 , and are the result of internal planet heating by radioactive uranium and of silicate fluidity, both of which are expected to be generic whenever the mass of the planet is sufficient and when liquid water is present. For small enough planet masses, volcanic activity disappears some time after planet formation, as do the associated oxygen sinks. O3 is itself a tracer of O2 and, with a prominent spectral signature at 9.6 µm in the infrared where the planet/star contrast is significantly stronger than in the optical (1.4 â 10-7 rather than 2 â 10-10 for the Earth/Sun case), should be easier to detect than the visible wavelength lines. These considerations are motivating the development of infrared space interferometers for the study of bands such as H2 O at 6­8 µm, CH4 at 7.7 µm, O3 at 9.6 µm, CO2 at 15 µm and H2 O at 18 µm. Higher resolution studies might reveal the presence of CH4 , its presence on Earth resulting from a balance between anaerobic decomposition of organic matter and its interaction with atmospheric oxygen; its highly disequilibrium co-existence with O2 could be strong evidence for the existence of life.

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The possibility that O2 and O3 are not unambiguous identifications of Earth-like biology, but rather a result of abiotic processes, has been considered in detail by L´ eger et al. (1999) and Selsis et al. (2002). They considered various production processes such as abiotic photodissociation of CO2 and H2 O followed by the preferential escape of hydrogen from the atmosphere. In addition, cometary bombardment could bring O2 and O3 sputtered from H2 O by energetic particles, depending on the temperature, greenhouse blanketing, and presence of volcanic activity. They concluded that a simultaneous detection of significant amounts of H2 O and O3 in the atmosphere of a planet in the habitable zone presently stands as a criterion for large-scale photosynthetic activity on the planet. Such an activity on a planet illuminated by a star similar to the Sun, or cooler, is likely to be a significant indication that there is local biological activity, because this synthesis requires the storage of the energy of at least 2 photons (8 in the case on Earth) prior to the synthesis of organic molecules from H2 O and CO2 . This is likely to require delicate systems that have developed during a biological evolutionary process. The biosignature based on O3 seems to be robust because no counter example has been demonstrated. It is not the case for the biosignature based on O2 (Selsis et al., 2002), where false positives can be encountered. This puts a hierarchy between observations that can detect O2 and those that can detect O3 . Habitability may be further confined within a narrow range of [Fe/H] of the parent star (Gonzalez, 1999b). If the occurrence of gas giants decreases at lower metallicities, their shielding of inner planets in the habitable zone from frequent cometary impacts, as occurs in our Solar System, would also be diminished. At higher metallicity, asteroid and cometary debris left over from planetary formation may be more plentiful, enhancing impact probabilities. Gonzalez (1999a) has also investigated whether the anomalously small motion of the Sun with respect to the local standard of rest, both in terms of its pseudo-elliptical component within the Galactic plane, and its vertical excursion with respect to the mid-plane, may be explicable in anthropic terms. Such an orbit could provide effective shielding from high-energy ionising photons and cosmic rays from nearby supernovae, from the X-ray background by neutral hydrogen in the Galactic plane, and from temporary increases in the perturbed Oort comet impact rate.

1.3

Present Limits: Ground and Space

Figure 2 illustrates the detection domains for the radial velocity, astrometry, and transit methods as a function of achievable accuracy. It also shows the location of the exo-planets known to date, in a mass-orbital radius (period) diagram. The fundamental accuracy limits of each method are not yet firmly established, although such knowledge is necessary to predict the real performances of dedicated surveys on ground and in space. Granular flows and star spots on the surface of late-type stars place specific limits on the photometric stability, the stability 5


Figure 2: Detection domains for methods exploiting planet orbital motion, as a function of planet mass and orbital radius, assuming M = M . Lines from top left to bottom right show the locus of astrometric signatures of 1 milli-arcsec and 10 micro-arcsec at distances of 10 and 100 pc; a measurement accuracy 3­4 times better would be needed to detect a given signature. Vertical lines show limits corresponding to orbital periods of 0.2 and 12 years, relevant for Gaia (where very short and very long periods cannot be detected) although not for SIM. Lines from top right to bottom left show radial velocities corresponding to K = 10 and K = 1 m s-1 ; a measurement accuracy 3­4 times better would be needed to detect a given value of K . Horizontal lines indicate photometric detection thresholds for planetary transits, of 1% and 0.01%, corresponding roughly to Jupiter and Earth radius planets respectively (neglecting the effects of orbital inclination, which will diminish the probability of observing a transit as a increases). The positions of Earth (E), Jupiter (J), Saturn (S) and Uranus (U) are shown, as are the lower limits on the masses of known planetary systems as of December 2004 (triangles).

of the photocentric position, and the stability of spectroscopically-derived radial velocities, whether these observations are made from ground or space. A series of hydrodynamical convection models covering stellar ob jects from white dwarfs to red giants has been used to give estimates of the photometric and photocentric stellar variability in wavelength-integrated light across the HR diagram (Svensson & Ludwig, 2005). (a) Radial velocity experiment accuracies are close to the values of around 1­3 m s-1 at which atmospheric circulation and oscillations limit measurement precision, implying mass detection limits only down to 0.01­0.1 MJ (depending on orbital period); detection of an Earth in the habitable zone would require accuracies of 0.03 - 0.1 m s-1 . Observations from space will not improve these limits, and no high-precision radial velocity measurements from space have been proposed. 6


The idea of `stacking up' many radial velocity observations to average the effects of stellar oscillations is appealing, but faces several complications: (i) even if p-mode oscillation effects can be minimised, beating amongst these modes may induce large radial-velocity variations (up to 10 m s-1 peak-to-peak) over timescales of a few hours, specifically some 5­6 hours for µ Arae (Bouchy, private communication). The star will therefore need to be observed over several hours for each epoch (radial velocity point); (ii) simulations by Bouchy (private communication) show that a precision of 1ms-1 is reached in about 15­20 min, while the gain is much less rapid with increasing observation time. A precision of 0.1 m s-1 (still insufficient for the detection of the Earth around the Sun) will therefore be very expensive in terms of telescope time; (iii) a wavelength calibration precision from night-to-night is then needed at the level of the long-term precision targeted. Reference calibration to 0.1 m s-1 will require further improvements in calibration techniques. With HARPS, a precision of about 0.5 m s-1 is reached, as illustrated by asteroseismology results on µ Arae with 250 observations each. Investigations are ongoing (Udry, private communication) into the possibility of having a reference at the 0.02 m s-1 level for an instrument on OWL, while the HARPS GTO programme anyway pushing in this direction will soon help to better characterise the question. In conclusion, a very high radial velocity precision seems possible, but at a very high cost. There is a significant difference in the case of transiting candidates: now the period and phase are known, and with e 0 for short period planets, a series of accumulated measurements can be used to constrain the radial velocity semi-amplitude. With HARPS at a precision of 1 m s-1 , for short-period planets, it is expected that limits of a few Earth-masses, for P < 10 days, can be reached. If the transiting ob ject is larger, then the radial velocity effect will be larger and easier to detect. False positive detections will be the main problem. (b) Photometric (transit) limits below the Earth's atmosphere are typically a little below the 1% photometric precision, limited by variations in extinction, scintillation and background noise (depending on telescope aperture size), corresponding to masses of about 1 MJ for solar-type stars. One main challenge is to reach differential photometric accuracies of around 1 mmag over a wide field of view, in which airmass, transparency, differential refraction and seeing all vary significantly. The situation improves above the atmosphere, and a number of space experiments are planned to reach the 0.01% limits required for the detection of Earth-mass planets. HST can place much better limits on transit photometry than is possible from the ground, as exemplified by HD 209458 (see Sections 2.1.2 and 2.2.1). Simulations have been made by the COROT teams in order to estimate the transit detection threshold due to stellar activity. In the case of a very active star, the detection of an Earth (80 ppm) is not possible. In the case of a quiet star (like the Sun), it is possible if several transits are summed. In the case of COROT, 1.6 M is detected after 10­30 transits. Another complication is again false-positives, where statistical effects, stellar activity, and background binaries can all mimic transit events, and which call for independent confirmation of detections in general.

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(c) Astrometric measurements do not yet extend below the 1 milli-arcsec of Hipparcos, implying current detectability limits typically above 1­10 MJ . Even with the expected advent of narrow-field ground-based astrometry at 10 micro-arcsec (e.g. PRIMA), detections would be well short of Earth-mass planets, even within 10 pc. Above the atmosphere, astrometric accuracy limits improve significantly. The studies of Svensson & Ludwig (2005) indicate that for log g 4.4, resulting displacements are around 10-7 - 10-8 AU suggesting, for example, that this effect will not degrade the Gaia measurements, with the exception of nearby (< 100 pc) red giants. Nevertheless, that work treats only the variability caused by the evolution of stellar surface inhomogeneities driven by thermal convection (stellar granulation). At lower temporal frequencies, the variability is much higher (but not yet treatable by hydrodynamic models), caused by magnetic stellar activity, spottiness, and rotation, all of which may make substantial additional contributions to the astrometric (and photometric) variability. (d) Microlensing searches are not limited by current measurement accuracies for Earth-mass planets, which can produce relatively large amplitude photometric signals (a few tenths of a magnitude or larger), though small amplitude signals are more frequent. The limitations of this method are rather of statistical nature: even if all stars acting as microlenses have planets, only a small subset of them would show up in the microlensed lightcurve, depending on the pro jected separation and the exact geometry between relative path and planetary caust