Finding terrestrial planets in the habitable zones of nearby stars Part II Astrophysics Essay Simon Hodgkin & Mark Wyatt (on sabbatical) Terrestrial? 4 Winn et al. 2011 Winn et al. 2011 5 Exoplanets 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 Venus 55 Cnc e 0 0 1 10 100 1000 2 4 6 8 10 12 14 Mass [MEarth] Mass [MEarth] FIG.3.—Masses 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 orbital period 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 star’s planet-facing McArthur et al. (2004) reported an orbital inclination 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 astrometry. 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 stellar rotation 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 • Radial Velocity • 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 years 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 ANRV320-AA45-10 ARI 27 July 2007 19:32 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 –5 402 Udry 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 sin i = 11.8 M⊕ Radial velocity (m s –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 –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 constellation 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 Sun-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 globular cluster 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).