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Home , 61 Virginis, Fomalhaut C, HD 210277

Finding terrestrial planets in the habitable zones of nearby

Part II Astrophysics Essay

Simon Hodgkin & Mark Wyatt (on sabbatical) Terrestrial? 4 Winn et al. 2011 Winn et al. 2011 5 Solar system 3 K−11e 15 − −3 Uranus 4 0.5 g cm 1.0 g cm

−3 K−11d 2.0 g cm ] ] 3

Earth 10 Earth 4.0 g cm−3 K−11f GJ 1214b 50% water 8.0 K−11b 55 Cnc e g cm−3 like Radius [R Radius [R 2 Earth−

maximum iron fraction 16.0 −3 water g cm 5 C−7b K−10b rock hydrogen 1 Earth iron 55 Cnc e 0 0 1 10 100 1000 2 4 6 8 10 12 14 [MEarth] Mass [MEarth]

FIG.3.— and radii of transiting exoplanets. Open circles are previously known transiting planets. The filled circle is 55 Cnc e. The stars are Solar System planets, for comparison. Left.—Broad view, with curves showing mass-radius relations for pure hydrogen, water ice, rock (MgSiO3 perovskite) and iron, from Figure 4 of Seager et al. (2007). Right.—Focus on super-Earths, showing contours of constant mean density and a few illustrative theoretical models: a “water-world” composition with 50% water, 44% silicate mantle and 6% iron core; a nominal “Earth-like” composition withterrestrialiron/siliconratioand no volatiles (Valencia et al. 2006, Li & Sasselov, submitted); and the maximum mantle stripping limit (maximum iron fraction, minimum radius) computed by Marcus et al. (2010). Data were taken from Lissauer et al. (2011) for Kepler-11, Batalha et al. (2011) for Kepler-10b, Charbonneau et al. (2009) for GJ 1214b, and Hatzes et al. (2011) for Corot-7b. We note the mass of Corot-7b is disputed (Pont et al. 2011).

The planetary temperature at the substellar point would be rotation speed to be 2.4 ± 0.5kms−1,muchslowerthanthe −1 T!!R!/a ≈ 2800 K if the planet has a low albedo, its rotation synchronous value of 65 km s . is synchronized with its orbit and the incoming heat is rera- Hence, the interpretation of the phase modulation is un- diated locally. If instead the heat is redistributed evenly over clear. The power spectral density of the photometric data also the planet’s surface, the zero-albedo equilibrium temperature displays the low-frequency envelope characteristic of stellar activity and granulation, which complicates the interpretation is T!!R!/2a ≈ 1980 K. Atmospheres of transiting planets can be studied through of gradual variations at the of 55 Cnc e. Con- occultations and orbital phase variations (see, e.g., Knut- firming or refuting this candidate orbital phase modulation is son et al. 2007). Our analysis did not reveal occultations apriorityforfuturework. (" =48±52 ppm), but did reveal a phase modulation (" = occ pha 4.3. Orbital coplanarity 168 ± 70 ppm). However, we cannot attribute the modulation to the changing illuminated fraction of 55 Cnc e, for two rea- 55 Cnc e is the innermost planet in a system of at least five sons. Firstly, the occultation depth is smaller than the full planets. If the orbits are coplanar and sufficiently close to ◦ range of the sinusoidal modulation. Secondly, the amplitude 90 inclination, then multiple planets would transit. Transits of the modulation is too large. Reflected starlight would cause of b and c were ruled out by Fischer et al. (2008).11 How- 2 asignalnolargerthan(Rp/a) ≈ 29 ppm. The planet’s ther- ever, the nondetections do not lead to constraints on mutual ≈ 2 4 ≈ inclinations. Given the measured inclination for planet e of mal emission would produce a signal (Rp/R!) (Tp/T!) ± 28 ppm for bolometric observations, and only 5 ppm for ob- 90.0 3.8deg,theotherplanetscouldhaveorbitsperfectly servations in the MOST bandpass, even for a 2800 K planet. aligned with that of planet e and still fail to transit. One possible explanation is that the ’s planet-facing McArthur et al. (2004) reported an of 53◦ ± 6.8◦ for the outermost planet d, based on a preliminary hemisphere is fainter by a fraction "pha than the other hemi- sphere, due to star-planet interactions. The planet may in- investigation of Hubble Space Telescope . This duce a patch of enhanced magnetic activity, as is the case would imply a strong misalignment between the orbits of d for τ Boo b (Walker et al. 2008). In this case, though, the and e. However, the authors noted that the astrometric dataset planet-induced disturbance would need to be a traveling wave, spannedonlya limited arcof the planet’sorbit, and no final re- because the is not synchronized with the or- 11 Our MOST observations might have led to firmer results for planet b, bit. Fischer et al. (2008) estimated the rotation period to be since it spanned a full orbit of that planet, but unfortunately no useful data 42.7±2.5d,andValenti&Fischer(2005)foundtheprojected were obtained during the transit window (see Fig. 1). The MOST observation did not coincide with any transit windows for planets c-f. Habitable? 861

As of June 2012 http://xkcd.com/1071/ 1222 MACPerryman Detecting Exoplanets

• Pulsar Timing • • Transits • TTV • Reflected Light • Direct Imaging • Microlensing • Astrometry

Figure 4. Examples of radial velocity measurements: HD 210277 (top) and HD 168443 (bottom), from Marcy et al (1999), obtained with the HIRES spectrometer on the Keck telescope. The solid curves show the best-fit Keplerian models. The non-sinusoidal variations result from the eccentric orbits, and the derived M sin i values are 1.28 and 4.01MJ respectively. The fit for HD 168443 is improved further by a linear velocity trend, suggestive of an additional, nearby, long-period stellar or brown dwarf companion (courtesy of Geoffrey Marcy).

In summary, imaging of Earth-mass extra-solar planets from large ground-based telescopes equipped with adaptive optics and operating in interferometric combination, and observations in the infrared using space interferometers, are receiving considerable attention. While the commitment is impressive, dedicated space missions are probably 10–15 or more away. At the start of this section it was noted that extra-solar planetary imaging generally refers to the detection of a reflection point-source image of the planet, rather than to resolution of the extra-solar planet surface. Ground- or space-based (or lunar) interferometric arrays of 10–100 km baseline could start to tackle resolved planetary imaging (Labeyrie 1996). Bender and Stebbins (1996) undertook a partial design of a separated spacecraft interferometer which Detecting Exoplanets Annu. Rev. Astro. Astrophys. 2007.45:397-439. Downloaded from www.annualreviews.org 19:32 2007 July 27 ARI ANRV320-AA45-10 by Cambridge University on 11/27/12. For personal use only.

a HD 69830 HARPS b HD 69830 HARPS

• Pulsar Timing ) –1 i 5 i 5

• Radial Velocity 0

0 –5 0 Udry 402 Radial velocity (m s

• Transits 4 P = 8.67 days 2 m sin i = 10.2 M⊕ 0

–5 O–C –2

· –4

• TTV Santos 53,300 53,350 53,400 5 ii ) –1 )

–1 ii • Reflected Light 5

0 0 • Direct Imaging P = 31.6 days –5 Radial velocity (m s (m velocity Radial

m sin i = 11.8 M⊕ s (m velocity Radial –5 4 2 • Microlensing 0

O–C –2 5 iii –4 53,650 53,700 53,750

4

• Astrometry )

–1 iii 0 2

0 P = 197 days m sin i = 18.1 M ⊕ –2 –5

Radial velocity (m s (m velocity Radial –4 0 0.5 1 53,000 53,200 53,400 53,600 53,800

Orbital phase JD-2400000 (days)

Figure 2 HARPS radial velocities of the star HD 69830 hosting a system of three Neptune-mass planets. The best three-Keplerian model of the system is superimposed to the data, in a phase-folded manner (a) or for given intervals of time (b). Run-averaged velocities after removal of the effect of the two shorter-period planets 1 are shown in (biii ). The measured dispersion around the solution then becomes of the order of 20–30 cm− . (From Lovis et al. 2006.) Detecting Exoplanets

Extra-solar planets 1235 • Pulsar Timing • Radial Velocity • Transits • TTV • Reflected Light • Direct Imaging • Microlensing • Astrometry

Figure 7. The first detected transit of an extra-solar planet, HD 209458 (from Charbonneau et al 2000). The figure shows the measured relative intensity versus time. Measurement noise increases The firstto thedetected right due to increasing transit atmospheric of an air mass.extra-solar From the detailed planet, shape of the transit,HD some of 209458bthe physical(from characteristics Charbonneau of the planet can et be inferred al 2000). (courtesy of David Charbonneau).

detection of the HD 209458 transits by Charbonneau et al (2000), is monitoring some 24 000 stars in a 5.7◦ square field in the of Auriga; ASP (Arizona Search for Planets) uses a 20 cm aperture in a similar manner; and ASAS (All-Sky Automated Survey) has as its goal the photometric monitoring of 107 stars brighter than 14 mag over the entire sky, making more than 100 3-min exposures per∼ night. Such searches should soon extend the detection of transits to later spectral types (cooler, less massive K and M stars) than the -like (F- and G-type) stars favoured in the radial velocity surveys, in which the transit effect should be more pronounced due to the smaller stellar size. Observations of more than 34 000 stars in the 47 Tucanae, uniformally sampled over nine days by the Hubble Space Telescope in July 1999, may result in several tens of transit detections if such planets exist in globular clusters (Gilliland 1999), although preliminary analysis for 27 000 stars has revealed no convincing planet candidates (Brown et al 2000). Between 15 and 20 detections would have been expected if the occurrence rate in 47 Tuc were the same as indicated by radial velocity searches in the solar neighbourhood; the discrepancy suggests that at least one of the processes of formation, migration or survival of close-in planets may be significantly altered in the cluster environment. 2 Equation (1) indicates that Lp/L a− . For planets very close to the central star, a modulation due to the (Doppler shifted)∗ ∝ reflected light intensity could therefore be expected, even if the planet cannot be imaged as such (Bromley 1992, Charbonneau et al 1998, Seager and Sasselov 1998, Seager et al 2000). This may still occur even if the orbital plane is somewhat inclined to the line of site, with a signature dependent upon inclination (no modulation would be observed for face-on systems). The close-in planetary system τ Bootis has a 0.046 AU, 4 4 5 = Mp sin i 3.89MJ, and hence Lp/L 10− , some 10 –10 times higher than for the case of a = system, although τ Boo and∗ its∼ planet are separated by, at most, 0.003 as. The system HAT-P-7b: Welsh et al. 2010 L146 WELSH ET AL. Vol. 713

orbital phase 0.96 0.98 1.00 1.02 1.04

1.000

0.998

0.996

normalized flux 0.994

0.992

1.000150

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normalized flux 1.000000

0.999950 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 orbital phase Figure 1. Upper: detrended and phase-folded light curve at one-minute cadence along with the ELC fit. For scale, the horizontal bars show the vertical size of the lower panel. Lower: light curve averaged in 5 minute and 75 minute bins. The double-humped shape is due to the ellipsoidal variations of the star plus light from the planet. (A color version of this figure is available in the online journal.) maximum does not occur just outside occultation as one would ablackbodyapproximationevaluatedatawavelengthof6000Å, expect for simple “reflection” from the planet. Two maxima and a hybrid method7 where model atmospheres are used to occur 0.15–0.20 away in phase, a result of the “ellipsoidal determine the intensities at the normal for each tile and a variation” of the star’s light, as discussed in Section 4. parameterized limb darkening law is used for other angles; The absolute timing is still preliminary in this early version the model is then filtered through the Kepler spectral response of the data calibration pipeline, but relative timing precision is function (spanning roughly 4250–8950 Å, peaking at 5890 Å reliable (though without the modest BJD correction, which is with a mean wavelength of 6400 Å; see Koch et al. 2010 and unimportant for our investigation over the 33.5 time series). Van Cleve & Caldwell 2009). The two methods give essentially So we do not report the value for the of transit T0, though the same results, with the exception of a higher albedo from of course it is a parameter in the model fitting. Also, the period the hybrid models. Interestingly, the blackbody models yielded measured is based only on these 33.5 days, so the precision is significantly lower chi-square values, so we quote the blackbody not as high as would be if other epochs many cycles away were model values in this work. This needs to be kept in mind included. when interpreting the temperature and albedo estimates given in Section 4.Weuseatwo-parameterlogarithmiclimbdarkening 3. MODELING law and adopt an eccentricity of zero consistent with the radial velocities and phase of occultation. We assume that the planet is We employ the ELC code of Orosz & Hauschildt (2000)to tidally locked in synchronous rotation. For more details on using model the transit, occultation, and phase-varying light from the ELC to model data, see Wittenmyer et al. (2005). planet and star. The code simultaneously fits the photometry Following the prescription of Wilson (1990), the light from along with several observational parameters: the radial velocity the planet is modeled as the sum of an isothermal component K,andthemassandradiusofthestar.TheK amplitude was (with temperature Tp that essentially adds a constant flux at all taken from Winn et al. (2009), and the mass and radius from phases outside of occultation), and a “reflection” component the asteroseismology analysis of the Kepler data (Christensen- on the day hemisphere. The local temperature is given by Dalsgaard et al. 2010). These parameters are not fixed; rather 4 4 F T T [1+Abol ∗ ], which comes from assuming the fluxes = p × Fp the models are started and steered toward them via a chi-square and temperature are coupled by the Stefan–Boltzmann law. The penalty for deviations. Markov chain Monte Carlo and genetic bolometric albedo Abol,alsoknownasthe“heatalbedo,”is algorithms were used to search parameter space, find the global the ratio of re-radiated to incident energy, and should not be chi-square minimum, and determine confidence intervals of the confused with the Bond albedo. For stars, a radiative atmosphere fitted parameters. has A 1(localenergyconservation)whileforconvective ´ bol The analytic model of Gimenez (2006)forasphericalstarand atmospheres= a value of 0.5 is appropriate (half the energy is planet is not sufficient to model the phase variations. Therefore, ELC is used in its full numerical mode: the star and planet are tiled in a fine grid, and the intensity and velocity from each tile 7 ELC can also fully employ stellar atmosphere intensities, where no is summed to give the light curve and radial velocities. Limb and parameterized limb darkening is used. But preliminary tests gave significantly worse fits unless the stellar temperature was allowed to be several thousand gravity darkening are included, and the gravitational distortions degrees hotter. The cause seems to be related to the very wide Kepler bandpass are modeled assuming a standard Roche potential. We employed and the lack of freedom to adjust the limb darkening. Detecting Exoplanets106 Winn

• Pulsar Timing Receding limb

• Radial Velocity Approaching limb • Transits • TTV • Reflected Light • Direct Imaging • Microlensing Figure 3. The Rossiter-McLaughlin effect. Shown are three planet trajectories that produce identical light curves, but have different orienta- • Astrometry tions relative to the stellar spin axis and hence produce different Rossiter- McLaughlin signals. In the bottom panels, the dotted lines show a model of the effect in the absence of limb darkening; the solid lines show a model that includes limb darkening. Adapted from Gaudi & Winn (2007).

Measurements of the projected spin-orbit angle λ have been published for 5 systems (Queloz et al. 2000, Winn et al. 2005, 2006, 2007; Wolf et al. 2007; Narita et al. 2007). In all cases λ is consistent with zero at the 1-2σ level, with accuracies ranging from 1–30 degrees. This has led to the working hypothesis that the migration of hot generally preserves spin-orbit alignment. Importantly, star-planet tidal interactions (which are responsible for spin synchronization and orbital circularization of the planet) do not confound the interpretation because the expected timescale for tidal reorientation is generally 1010 yr or longer, as estimated using the equilibrium-tide theory of Hut (1981). Observations of the Rossiter-McLaughlin effect provide only a lower limit on the stellar obliquity, the angle between the orbital and rotation axes. The RM waveform is sensitive the angle between the sky projections of those axes, and the inclinations with respect to the sky must be determined with other data. The orbital inclination is often known to within ≈1 deg from the transit light curve, but the stellar inclination is usually unknown. The single excep- tion thus far is HD 189733. That star is chromospherically active and exhibits quasiperiodic flux variations (presumably due to star spots) from which the ro- tation period can be measured (Henry & Winn 2007). The combination of a measured rotation period, stellar radius, and projected rotation rate (which can be measured from either the amplitude of the RM effect or from the observed spectral-line broadening) places a constraint on the stellar inclination, which is consistent with being edge-on. More work remains to be done on confronting the specific predictions of planet-planet scattering and Kozai cycles with the data. The case of HD 147506 (also known as HAT-P-2) is especially interesting because the planet has a highly eccentric orbit (e = 0.5), naturally raising the possibility of planet-planet scat- Atmospheres: transit spectroscopy

12 D. K. Sing, F. Pont, et al. • STIS (blue) and ACS Sing et al. 2011 (red) transmission spectra for HD189733b.

• The right Y-axis is labeled in units of estimated atmospheric scale heights, assuming T=1340 K (H=0.0004 Rpl/Rstar).

• The prediction from ACS Rayleigh scattering (1340±150 K red solid and dashed lines) is also shown, as is a haze-free model atmosphere for HD 189733b from Fortney et al. (2010)

Figure 14. STIS and ACS transmission spectra for HD189733b. Plotted blue-ward of 5600 AistheSTISG430Lmeasurements(circles)˚ with the ACS measurements from Pont et al. (2008) red-ward of 5600 A(squares).˚ The wavelength bins are indicated by the X-axis error bars and the 1- error is indicated by the Y-axis error bars. The right Y-axis is labeled in units of estimated atmospheric scale heights, assuming T=1340 K (H=0.0004 R /Rstar). The prediction from ACS Rayleigh scattering (1340 150 K red solid and dashed lines) is pl ± also shown, as is a haze-free model atmosphere for HD 189733b from Fortney et al. (2010, Fig. 7) which uses a planet-wide average T-P profile, and is normalized to the radii at infrared wavelengths. the prescriptions used for limb-darkening, as well as the oc- hydrostatic atmosphere as a function of wavelength was culted stellar spot fits of visit 2. The average transmission found in Lecavelier des Etangs et al. (2008a), spectrum we obtain (see Fig. 14) between 4200 and 5700 A˚ " P () 2⇡R is featureless, lacking the broad Na and K absorption lines, z()=Hln abs ref pl , (6) ⌧eq kTµg with a blue-ward slope similar to the ACS measurements. ✓ r ◆ Like the ACS spectrum (but unlike HD209458b) we find where "abs is the abundance of dominating absorbing no evidence for the wide pressure-broadened sodium wings species, T is the atmospheric temperature at z, H = kT/µg (e.g. Fortney et al. 2010), though there is good evidence is the atmospheric scale height, µ is the mean mass of the that the sodium line core is present both from ground based atmospheric particles, Pref is the pressure at the reference measurements (Redfield et al. 2008) as well as our G750M altitude, and () is the absorption cross section. This al- measurements (Huitson, Sing, in prep), which indicates that lows the derivation of the apparent planetary radius as a either the sodium abundance is much lower in HD189733b function of wavelength from the known cross section vari- or more likely that the optical and near-UV transmission ations. Assuming a scaling law for the cross section in the spectrum covers lower pressures and higher altitudes than ↵ form = 0(/0) , the slope of the planet radius as a in HD209458b. The high altitudes are also illustrated by function of wavelength is given by derived planetary radii for both ACS and STIS, which are dR kT both well in excess of those observed in the near-IR and at pl = ↵H = ↵ . (7) Spitzer IRAC wavelengths. For a hydrostatic atmosphere at d ln µg 1300 K, the ACS spans 2 scale heights above the 1.88µm ⇠ If the cross section as a function of wavelength is known, the and 8µm radii of Sing et al. (2009) and Agol et al. (2010) local atmospheric temperature can thus be estimated by respectively, while the G430L spans 2to6scaleheights ⇠ µg dRpl above. ↵T = . (8) k d ln For the ACS measurements, Lecavelier des Etangs et al. The featureless slope and lack of the expected sodium (2008a) showed that Rayleigh scattering with ↵ = 4 4 and potassium alkali line wings further indicates optical at- (Rayleigh cross section is = 0(/0) ) provides a tem- mospheric haze, as first detected by Pont et al. (2008) using perature of 1340 150 K, consistent with other estimates. ± HST ACS. The e↵ective transit measured altitude z of a They also concluded that the absorption from atmospheric

c 2011 RAS, MNRAS 000,1–?? Detecting Exoplanets

• Pulsar Timing • Radial Velocity • Transits • TTV • Reflected Light • Direct Imaging • Microlensing • Astrometry Detecting ANRV385-AA47-07Exoplanets ARI 15 July 2009 4:38

Oppenheimer & Hinkley 2009, ARA&A • Pulsar Timing 0.6 microns H band • Radial Velocity • Transits • TTV • Reflected Light b planet 2006 2004

• Direct Imaging 5" 1"

Figure 2 • Microlensing Images of (left) the ring of debris around Fomalhaut and (right) HR 4796A (left: Hubble image STScl-PRC08-39a; Kalas et al. 2008, courtesty of NASA, ESA, and P. Kalas of University of California, Berkeley; right: Schneider et al. 1999, courtesy of B. Smith, • Astrometry G. Schneider, and NASA). Indeed, the most striking finding of the past 15 years is the diversity in the properties of the exoplanetsCurrie found. et Withal. 2012: the discovery Fomalhaut of the so-called b is very hot jupiters plausibly (Mayor “a & planet Queloz 1995),identified massive from direct planetsimaging” on roughly four-even to if ten-day current orbits images (semimajor of it axes do less not, than strictly about 0.1 speaking, AU), scientists show quickly thermal realizedemission that many solarfrom systems a directly yet to imaged be discovered planet. look nothing like our own. Furthermore, the mass ranges of these planets suggest no obvious classes based solely on mass. Most notably, the distribution of planet masses seems to rise sharply toward the lower masses, with a long, decreasing

tail into the larger masses (>15 MJ), suggesting a large population of low-mass (<5 MJ) planets yet to be discovered. Also, the distribution of exoplanet eccentricities is one of the biggest remaining mysteries in the field, and reproducing the distribution numerically or analytically has posed a significant challenge to theorists ( Juric & Tremaine 2008). The eccentricity distribution (See Udry & Santos 2007, especially their figure 6) is significantly different from the planets in our own Solar System. by Cambridge University on 11/27/12. For personal use only. Marois et al. (2008) have imaged three planets in orbit around the star HR8799 using adaptive optics and angular differential imaging (ADI; see Section 6.1; see also Marois et al. 2006a). The three planets may be in nearly circular, face-on orbits; have projected separations of 24, 38, and 68 AU (see Figure 3); and have masses that are estimated to be between 5 and 13 Jupiter masses. The Annu. Rev. Astro. Astrophys. 2009.47:253-289. Downloaded from www.annualreviews.org researchers suggest that this system is a scaled up version of our own Solar System, yet nothing like these exist in our system. Nearly concurrently, observations by Kalas et al. (2008) of the star Fomalhaut reveal a comoving companion at 119 AU. Dynamical models suggest an upper limit of the object of at most 3 Jupiter masses, however, a surprising lack of flux in IR wavelengths suggest the object detected may actually be some kind of vast circumplanetary disk. These along with the direct observations of disk structures are indicators of the further diversity that high-contrast observations are bound to yield. The transit method (Charbonneau et al. 2000, Henry et al. 2000, Brown et al. 2001, Udalski et al. 2002), combined with radial velocity follow-up measurements, can provide the radii and, thus, the densities of some planets (Figure 4). In addition to the extremely diverse range of densities these objects possess, Figure 4 indicates that the size of planets, brown dwarfs, and even the lowest

www.annualreviews.org High-Contrast Observations 259 • Debris Disks and planets

• Astronomers using Herschel have detected massive debris discs around two nearby stars hosting low-mass planets.

• The discovery suggests that debris discs may survive more easily in planetary systems without very massive planets.

• Top: 61 Virginis

• Bottom: Detecting Exoplanets

• Pulsar Timing • Radial Velocity • Transits

150 • TTV

2012.8 • Reflected Light

100 • Direct Imaging 2011.6 • Microlensing

50

• Astrometry (milli-seconds of arc) of (milli-seconds Declination

2010.4 0

0 50 100 (milli-seconds of arc)

Figure 2: The figure illustrates the principle of how the (angular) position of a star on the celestial sphere would be perceived to change with time, as a result of various physical e↵ects, when viewed at extremely high angular resolution. Far from being fixed in one direction, a typical star moves through space at a velocity of some 10–30 km per second, tracing out a linear path across the sky with time (dashed line). Superimposed on this, the star’s position appears to oscillate due to the Earth’s motion around the Sun, providing access to the star’s distance through the e↵ect of parallax (dotted line, which shows this ‘reflex’ motion over roughly four years). A planet in orbit around the star results in a yet more complex motion of the star around the star–planet system’s centre of mass (solid line).

(Ruggles, 1997). The first recorded developments emerged in Mesopotamia around 1000 BCE where, in the land between the Tigris and Euphrates rivers now occupied by southern Iraq, Assyro– Babylonian astronomers systematically observed the night skies, building on common lore already conscious of the changing daylight over the . They observed, measured, and recognised, for the first time, that certain celestial phenomena were periodic: amongst them the regular appearance of Venus, and the eighteen year cycle of lunar eclipses. Their careful records formed the basis for later developments, not only in ancient Greek and Hellenistic astronomy, but also in classical Indian and medieval Islamic astronomy. Early Greek philosophers, the Pythagoreans amongst them, played a key part in astronomy’s earliest awakening (Heath, 1932; Hoskin, 1997). They believed that the underlying regularities, or laws of nature, were discoverable by reason. As part of this philosophical school, astronomers of ancient Greece tried to understand the Universe based on principles of ‘cosmos’, or order. The revolutionary idea that the Earth might be spherical began to replace the pre-Socratic view that its surface was flat (Crowe, 1990). Plato (427–347 BCE) and his contemporaries knew that the heavens rotated night after night with constant speed, the ‘fixed’ stars preserving their relative positions as the heavens turned. Moving amongst them in a complex and unfathomable way were the seven wanderers—the Greek planetes—the Sun, the Moon, and the planets visible to the naked

8 red symbols : transit discoveries black symbols : RV discoveries red symbols : transit discoveries black symbols : RV discoveries Detecting Exoplanets

Wright & Gaudi, 2012 http://uk.arxiv.org/abs/1210.2471 snow line distance to be asl = 2.7 AU(M∗/M⊙). Radial velocity detections (here what is actually plotted is Mp sin i) are indicated by red circles (blue for those also known to be transiting), transit detections are indicated by blue triangles if detected from the ground and as purple diamonds if detected from space, microlensing detections are indicated by green pentagons, direct detections are indicated by magenta squares, and detections from pulsar timing are indicated by yellow stars. The letters indicate the locations of the Solar System planets. The shaded regions show rough estimates of the sensitivity of various surveys using various methods, demonstrating their complementarity.

Snow Line: separation at which it is cool enough for hydrogen compounds such as water, ammonia, and methane to condense into solid ice grains. Depending on density, that temperature is estimated to be about 150K. The frost line of the Solar System is around 4.2 AU. Figure 5: The points show the masses versus semimajor axis in units of the snow line distance for the exoplanets that have been discovered by various methods as of Dec. 2011. See the Extrasolar Planets Encyclopedia (http://exoplanet.eu/) and the Exoplanet Data Explorer (http://exoplanets.org/). Here we have taken the snow line distance to be asl = 2.7AU(M∗/M ). Radial velocity detections (here what is actually plotted is Mp sin i)are indicated by red! circles (blue for those also known to be transiting), transit detections are indicated by blue triangles if detected from the ground and as purple diamonds if detected from space, microlensing detections are indicated by green pentagons, direct detections are indicated by magenta squares, and detections from pulsar timing are indicated by yellow stars. The letters indicate the locations of the Solar System planets. The shaded regions show rough estimates of the sensitivity of various surveys using various methods, demonstrating their complementarity.

29

Visualization of the planetary systems discovered by Kepler (Batalha et al.), i.e. those targets with more than one transiting object. There are 885 planet candidates in 361 systems, doubling the number of systems in the original Kepler Orrery. In this video, orbits are to scale with respect to each other, and planets are to scale with respect to each other (a different scale from the orbits). The colors are in order of semi-major axis. Two-planet systems (242 in all) have a yellow outer planet; 3-planet (85) green, 4-planet (25) light blue, 5-planet (8) dark blue, 6-planet (1, Kepler-11) purple. At the end of the video the catalog numbers appear (Kepler Object of Interest, KOI). Mankind has always felt the urge of actively doing something of extraordinary relevance. By doing so, we have caused a great deal of grief and disaster. The WETI Institute proposes to abandon our reckless anthropocentric ambition, and to strive for a more humble approach of letting the The Drake Equation universe explore us instead.

N = Ng . fp . ne . fl . fi . fc . fL

From James Kasting, How to Find a Habitable Planet, where:

N = the number of advanced, communicating civilizations in our

Ng = the number of stars in our galaxy fp = the fraction of stars that have planets ne = the number of earth-like planets per system fl = the fraction of habitable planets on which life evolves fi = the probability that life will evolve to an intelligent state fc = the probability that intelligent life will communicate over long distances fL = the fraction of a planet’s lifetime during which it supports a technological civilization

Devised in 1961 by Frank Drake at the launch of SETI Finding terrestrial planets in the habitable zones of nearby stars

• The first aim of this essay is to assess how common terrestrial-habitable planets are in the Galaxy. Consideration should be given to the known planet population, the sensitivity of ongoing surveys, what we understand about the stellar population of our Galaxy, and what constitutes a habitable planet.

• The second aim of the essay is to discuss the prospects for actually discovering and then confirming a habitable planet around a nearby star.