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A Super- Transiting a Naked-Eye

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Citation Winn, Joshua N. et al. “A SUPER-EARTH TRANSITING A NAKED-EYE STAR.” The Astrophysical Journal 737.1 (2011): L18.

As Published http://dx.doi.org/10.1088/2041-8205/737/1/l18

Publisher IOP Publishing

Version Author's final manuscript

Citable link http://hdl.handle.net/1721.1/71127

Terms of Use Creative Commons Attribution-Noncommercial-Share Alike 3.0

Detailed Terms http://creativecommons.org/licenses/by-nc-sa/3.0/ arXiv:1104.5230v3 [astro-ph.EP] 7 Jul 2011 aa n httetu eidadmnmmms r 0 are minimum and radial-veloci period true the the in that had aliasing and e to data, Cnc due 55 that mischaracterized argued re- been More (2010) Fabrycky (2008). & al. Dawson et cently, Fischer by confirmed were parameters asfr5 n f28dad14 and d 2.8 of e Cnc 55 for mass innermost its of period short “e”. and designated mass planet, low unusually the and t lnt Nvke l 03,teeitneo nMdwarf M 10 an of of existence distance the a of 2003), at two al. between companion et resonance Cnc (Novak 3:1 55 planets why the reasons its are other attention the many attracted Among as has e Sun. host (Lissauer the to Kepler-11 and known 2011), 2011), et al. are al. et Fischer (Lovis other 10180 2005, few HD Wisdom planets: a 2004, Marcy Only al. 1997, 2008). al. et et al. 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NTHONY 010 MOST INTRODUCTION 1 ,2011 6, J , lntr ytm lnt n aelts omto,int formation, satellites: and planets — systems planetary AYMIE R aelt,aCnda pc gnymis- Agency Space Canadian a satellite, ⊙ 2,6 3 h lntsms,rdu,adma est r 8 are density mean and radius, mass, planet’s the , .J M J. F. R , U(urure l 2006), al. et (Mugrauer AU UE-AT RNIIGANKDEESTAR NAKED-EYE A TRANSITING SUPER-EARTH A .M M. AINER M ⊕ OFFAT epciey Those respectively. , ATTHEWS K USCHNIG 8 J , ASON e A02139 MA ge, 2 ot Institute ronto R , a ihthe with ia, n Space and s 6 EBEKAH D , MOST nzstrasse nada .R F. ,Cam- ., y Hal- ty, t,50 nto, p etr,i press in Letters, ApJ ,CA z, . IMITAR 4d 74 Succ. ABSTRACT OWE ty pc eecp.Tetast cu ihtepro 07 d) (0.74 period the with occur transits The telescope. space t .D I. 9 S , S stigations. ASSELOV AWSON LAVEK htmti eod uigottast o lnt ( b planets for transits out 14 ruling record, photometric di 2010). planet’s Winn the e.g., (see, about orbit details and be- many atmosphere, system, mensions, reveal exoplanetary can an transits of cause tran importance of the occurrence The enhances 25%. sits to 13% from probability, transit eoye l 21)dtce rni f5 n ihthe with properties. Cnc system the 55 of of analysis complementary transit a a Telescope detected Space (2011) Spitzer al. et popu- Demory growing radii. but we and small 4 the Section measured of with In context super- the of planet. in lation the e of Cnc 55 density place and estimate yielding radius, analysis, Sec- mass, curve and the light data, the the presents presents 3 2 D tion Section by (2010). predicted Fabrycky characteristics & the son with signal transit a ing trans e. detect and planet to d) smallest insufficient (260 was the f precision of the planets and for d), transits (5200 out d rule to enough complete n 8 and h ot tatcAoayrsle ntels ffietrack- fine of loss the through in passages resulted Anomaly during Atlantic hits South the ray few cosmic a when for except interruptions 07-22, February 2011 from were continuous observations nearly The photometry. CCD ground-based tional nm). broadband (350-700 custom spectrum Its termi- visible min. the the 101 above covers of filter km period 820 orbital an orbit with polar nator Sun-synchronous 2004). o al. a short- et in Matthews photometry of is 2003, al. capable optical et (Walker photometer, stars ultraprecise bright CCD long-duration and telescope cadence, w cm equipped 15 microsatellite Canadian a a STars), of cillations e hs ftepae,btwt ulrne(168 range full a with but planet, the of phase ted ice ta.(08 erhdfrtast nter11- their in transits for searched (2008) al. et Fischer increased an be would period shorter the of implication One hl hsmnsrp a ne eiw elandthat learned we review, under was manuscript this While reveal- Cnc 55 of photometry space-based present we Here, eosre 5Ccwith Cnc 55 observed We eue h ietIaigmd,smlrt conven- to similar mode, Imaging Direct the used We g5 n ( Cnc 55 ng . )adc(43d.Hwvr h iecvrg a not was coverage time the However, d). (44.3 c and d) 7 ro 5Ccei rgtrta hto n other any of that than brighter is e Cnc 55 of ar d ftast,w eetdasnsia signal sinusoidal a detected we transits, of ide 3 trsms n aist e0 be to radius and mass star’s e okio opsto upeetdb a by supplemented composition rock-iron a s R M ssga a osrihfradinterpretation straightforward no has signal is D , 3 UCINSKI D , ⊕ ANIEL hteepce uainaddphfrthe for depth and duration expected the th . IANA V . 10 F 6 = 63 D ABRYCKY W , ros—sas niiul(5Cnc) (55 individual stars: — eriors RAGOMIR ± . ) ae ntoweso nearly of weeks two on based 0), ERNER rae asadconsequently and mass greater 0 2. . 35 erfrterae ota okfor work that to reader the refer We . OBSERVATIONS ⋆ M 4,5 5 .W W. D , ⊕ M , MOST 2 , AVID ATTHEW . EISS 00 .G B. ± Mcoaiblt Os- & (Microvariability 6 . 963 0 . .H J. 14 UENTHER − + 0 0 R . . OLMAN 029 051 ⊕ and , M 7 ± ⊙ , 3 , MOST for s P aw- ith its = - - f 2 Winn et al. 2011

1.003 Tracking lost Tracking lost Pixel shift 1.002 1.001 1.000 0.999

Relative flux Inf. conj. of b 0.998 Transits of e 0.997 0 2 4 6 8 10 12 14 Time [HJD − 2,455,600]

1.0004 1.0002 1.0000

0.9998 0.9996 0.9994 0.9992 Folded with predicted ephemeris 1.0004

Relative flux 1.0002 1.0000

0.9998 0.9996 0.9994 0.9992 After subtracting out−of−transit variation −0.4 −0.2 0.0 0.2 0.4 Phase

FIG. 1.— MOST photometry of 55 Cnc. Upper.—The time series, after decorrelation (small gray dots) and after further correction with the running averaged background method (large open circles, 0.25 d averages). Vertical bars mark the predicted transit times of planet e, and the inferior conjunction of planet b (which was missed during a failure of fine tracking). Middle.—Phased light curve, folded with P = 0.736540 d and Tc [HJD] = 2,453,094.6924 (Dawson & Fabrycky 2010) and averaged into 2 min phase bins. The solid curve is the best-fitting model. Bottom.—Same, but with the best-fitting model of the out-of-transit variation has been subtracted from the data. ing. Individual exposures lasted 0.5 s but were downloaded reason, an additional correction was performed with the “run- in stacks of 40 for the first 0.6 d, and stacks of 80 for the ning averaged background” method of Rucinski et al. (2004). remaining 14.4 d. The data were divided into 5 time intervals, each spanning ap- Aperture photometry was performed on the Direct Imag- proximately 32 MOST orbits (2.3 d). Within each interval, the ing subraster of the Science CCD. Data affected by cosmic data were folded with the satellite’s and boxcar- rays, image motion, or other problems were identified and re- smoothed, giving a reconstruction of the stray-light waveform moved. To improve the homogeneity of the data, we omitted during that time interval. This waveform was then subtracted data from the first 0.6 d (which had a different effective expo- from the observed magnitudes. sure time) and the final 2.1 d (which suffered from a tracking The upper panel of Figure 1 shows the final time series, loss followed by a major shift in image registration). The final and the lower two panels show the data after folding with the time series has 18,373 data points and a time sampling of 43 s Dawson & Fabrycky (2010) ephemeris. A dip is observed at outside of the interruptions. nearly zero phase, where the transit signal would be expected. Further processing was needed to remove the familiar pe- In addition, a gradual rise in flux is observed away from zero riodic artifacts in the time series due to scattered Earthshine. phase, which is evident in the middle panel, and which has First, the observed magnitude of 55 Cnc was fitted to a linear been subtracted in the lower panel based on the model de- function of the background level, X position, and Y position, scribed in § 3. and then this function was subtracted from the observed mag- We emphasize that the signal shown in Figure 1 is not the nitudes. The Fourier spectrum still had significant peaks at outcome of a period search: the data were phased with the the 14.26 c d−1 orbital frequency of the satellite and its har- predicted ephemeris. Nevertheless, when a period search is monics, as well as sidelobes at ±1cd−1 away from those performed the strongest signal is at 0.74 d, as shown in Fig- frequencies (arising from the modulation of the stray light by ure 2. The signal has the predicted period, and the observed the Earth’s albedo pattern as viewed by the satellite). For this is bracketed by the two predicted epochs of Dawson & Fabrycky (2010). It is 37 min later than the circular-orbit Transits of 55 Cnc e 3 prediction and 21 min earlier than the eccentric-orbit predic- sion 0.1, and the difference u1 − u2 (which has a negligible tion. Furthermore the depth and duration of the signal con- effect) was held fixed at the theoretical value. form with expectations (see § 3). With close matches to four The likelihood was taken to be exp(−χ2/2) with the usual predicted parameters (period, phase, depth, and duration) we sum-of-squares definition of χ2. The1σ uncertainty in each consider the existence of transits to be established. data point was taken to be the root-mean-square (rms) out- of-transit flux multiplied by a factor β intended to take into account the time-correlated noise. The factor β is the ratio between the standard deviation of residuals binned to 15 min, P = 2.81703 d 0.73654 d and the standard deviation one would expect based on the un- 150 binned data assuming white noise (see, e.g., Pont et al. 2006, Carter & Winn 2009). The rms and β values were 101 ppm and 1.3, respectively. Table 1 gives the results. 100 3.2. Signal-injection tests To further investigate the effects of the correlated noise and stray-light removal algorithms on the fitted transit parameters, we injected and recovered fake transit signals. Beginning with 50 the aperture photometry, we subtracted the best-fitting tran- sit model and added a fake signal with a different period and Signal Residue [ppm] transit time. The fake signal had the same transit depth, du- ration (in phase units), ǫpha and ǫocc as the best-fitting model. 0 Then, we processed the data just as was done with the un- 0.5 1.0 1.5 2.0 doctored data. This was repeated for 103 randomly chosen −1 Frequency (cycles ) periods within 50% of the true period. The recovered values of the transit and occultation depths FIG. 2.— Box-fitting Least Squares frequency spectrum of the MOST had a scatter of 47 ppm, in excess of the statistical error of data. The spectrum was computed with the method of Kovács et al. (2002). 15 ppm, and were systematically smaller by 1.9% than the Positive peaks indicate detections of candidate transit signals with durations consistent with a near-equatorial transit of 55 Cnc. injected depths. The fitted orbital phase modulations had a scatter of 68 ppm, in excess of the statistical error of 15 ppm, and the amplitudes were 6.4% smaller than the injected val- ues. Table 1 reports the values after correcting for these biases 3. ANALYSIS and increased dispersions. 3.1. Light curve fitting 4. DISCUSSION A transitmodelwas fitted to the lightcurvebased on the for- mulas of Mandel& Agol (2002),andthe Monte Carlo Markov 4.1. Comparison to theoretical models Chain (MCMC) code of Holman et al. (2006) and Winn et al. Both the mass and radius of 55 Cnc e are known to within (2007). The orbit was assumed to be circular, and the stellar 10%, providing a valuable example with which to test theo- limb-darkening law was assumed to be quadratic. To model retical models of super-Earth structure. To provide a broad the out-of-transit variation seen in the middle panel of Fig- view, the left panel of Figure 3 shows the masses and radii of ure 1, we added a term the transiting , along with theoretical curves taken from Seager et al. (2007) for “mathematicians’ planets” com- ǫpha Fpha = (1 − cos2πφ), (1) posed of pure hydrogen, water, rock (MgSiO3 perovskite) and 2 iron. The right panel focuses on the super-Earths and shows where φ is the orbital phase relative to midtransit. For com- the contours of constant mean density, along with some theo- pleteness the model also included an occultation at φ =0.5, al- retical curves based on more detailed models. though occultations were not detected. The model parameters 55 Cnc e falls between the rock and water lines, suggest- were the planet-to-star radius ratio Rp/R⋆, star-to-orbit radius ing it is neither a gaseous planet, nor is it simply a scaled-up ratio R⋆/a, i, time of midtransit Tc, ampli- terrestrial planet. Although the mean density of 55 Cnc e is tude of the orbital phase modulation ǫpha, occultation depth similar to that of Earth, the greater compression of the interior ǫocc, flux normalization (taken to be the flux just outside of of 55 Cnc e implies that it has a different composition. The transit), and limb-darkening coefficients u1 and u2. uncompressed density of 55 Cnc e would be smaller than that Uniform priors were adopted for Rp/R⋆, cosi, Tc, ǫpha, of Earth, implying that any rock and iron must be accompa- ǫocc and the flux normalization. We used Gaussian priors nied by water, gas, or other light elements. on the stellar radius and mass, R⋆ =0.943 ± 0.010 R⊙ and The right panel of Figure 3 also shows that the known +0.051 super-Earths span a factor of 20 in mean density, implying M⋆ =0.963−0.029 M⊙, which together act as a prior on R⋆/a. The radius prior is based on the interferometrically measured a correspondingly large range of possibilities for composi- stellar radius (von Braun et al. 2011), and the mass prior is tion and internal structure. A striking contrast exists between based on the analysis of the stellar spectroscopic propertiesby 55 Cnc e and Kepler-11e, which have similar masses but den- Takeda et al. (2007). Priors on the limb-darkeningcoefficients sities differing by a factor of 10. were based on theoretical values u1 =0.657 and u2 =0.115, obtained by integrating a Kurucz model with effective tem- 4.2. Atmosphere and orbital phase modulation perature 5327 K and logg =4.48 over the MOST bandpass. Any atmosphere around 55 Cnc e would be strongly heated, The sum u1 + u2 was subject to a Gaussian prior with disper- as the planet is located less than 4 R⋆ from its host star. 4 Winn et al. 2011

5 Exoplanets K−11e 15 −3 −3 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

Radius [R Radius [R 2 Earth−like

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 with terrestrial iron/silicon ratio and 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 ± 0.5km s−1, much slower than the −1 T⋆pR⋆/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⋆pR⋆/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 a priority for future work. (ǫ =48±52 ppm), but did reveal a phase modulation (ǫ = 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 a signal no larger than (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 3.8 deg, the other planets could have orbits perfectly 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 ǫpha 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-induceddisturbance 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, and Valenti & Fischer (2005)found the projected were obtained during the transit window (see Fig. 1). The MOST observation did not coincide with any transit windows for planets c-f. Transits of 55 Cnc e 5 sults have been announced. Additional astrometric measure- the atmosphere through occultation spectroscopy. Apart from ments and analysis are warranted. Kepler-10b, for which phase modulation was also tentatively detected (Batalha et al. 2011), these effects have not yet been seen for super-Earths. Transit timing constraints on the system’s architecture will 6 not be easily obtained, given the shallow transit and the small 55 Cnc e amplitudes of the predicted signals. Even planet b, the nearest 8 planet to e, is expected to perturb e’s transit epoch by less than Kepler−10b 1 s over the course of its 14 d period. The most readily de- 10 Corot−7b tectable effect may be the Römer delay due to planet d, which should cause a sinusoidal variation in planet e’s transit epoch 12 Kepler−4b with peak-to-trough amplitude of 24 s and period 5191 d. On the other hand, follow-up observations of the star itself 14 will continue to be rewarding. Already the and angu-

Visual magnitude Kepler−11 16 lar diameter of the star have been measured, the stellar vari- GJ 1214b ability has been tracked for 11 (Fischer et al. 2008), and 18 there is potential for the detection of p-mode oscillations that 100 1000 10000 would help define the stellar properties (see, e.g., Gilliland et Transit depth [ppm] al. 2011, Nutzman et al. 2011). The brightness of the star has already enabled the discovery of 4 other planets in the system, FIG. 4.— Stellar brightness and transit depths. The V band magni- and continued monitoring has a greater potential to reveal ad- tudes and transit depths of the transiting planets with known masses and radii. ditional bodies than is the case for fainter stars. < Super-Earths (Mp ∼ 10 M⊕) are labeled. Finally, there is some pleasure in being able to point to a naked-eye star and know the mass and radius of one of its planets.

4.4. Potential for follow-up observations We thank Laura McKnight and Andrew Howard for Figure 4 shows the stellar brightness and transit depth for thought-provoking conversations. Many people provided each of the known transiting planets. 55 Cnc is a uniquely helpful feedback on this work, including Simon Albrecht, bright host star, towering above the other super-Earth hosts Sarah Ballard, Rory Barnes, Jacob Bean, Heather Knutson, and nearly 2 mag brighter than any other transit star. How- David Latham, Barbara McArthur, Frederic Pont, Daniel ever, Figure 4 also shows that the transit depth for 55 Cnc e Rouan, Sara Seager, and the anonymous referee. We are is among the smallest known. This combination of factors grateful to Kaspar von Braun for communicating the results causes the follow-up landscape for 55 Cnc e to differ from of his team’s CHARA measurement prior to publication. that of other planets. J.M., D.G., A.M., and S.R. thank NSERC (Canada) for The shallow depth will make certain follow-up observa- financial support. T.K. is supported by a contract to the tions challenging despite the abundance of photons. To re- Canadian Space Agency. R.K. and W.W. were supported solve the transit ingress and egress, and thereby improve es- by the Austrian Science Fund. D.D. is supported by a timates of the planet’s orbital inclination and absolute dimen- FQRNT scholarship. R.D. is supported by a National Science sions, it will be necessary to improve the signal-to-noise ratio Foundation Graduate Research Fellowship, and D.C.F. by in the phased light curve by observing more transits or us- NASA Hubble Fellowship HF-51272.01-A. M.H. and J.W. ing a larger-aperture telescope. More data are also needed to were supported by NASA Origins award NNX09AB33G. check on the candidate orbital phase modulation, and study

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TABLE 1 SYSTEM PARAMETERS FOR 55 Cnc e

Parameter Value

Transit epoch [HJD] 2,455,607.05562 ± 0.00087 2 Transit depth, (Rp/R⋆) [ppm] 380 ± 52 Transit duration, first to fourth contact [d] 0.0658 ± 0.0013 Transit ingress or egress duration [d] 0.00134 ± 0.00011 Planet-to-star radius ratio, Rp/R⋆ 0.0195 ± 0.0013 Transit impact parameter 0.00 ± 0.24 Orbital inclination, i [deg] 90.0 ± 3.8 Fractional stellar radius, R⋆/a 0.2769 ± 0.0043 Fractional planetary radius, Rp/a 0.00539 ± 0.00038 Orbital distance, a [AU] 0.01583 ± 0.00020 Amplitude of orbital phase modulation, ǫpha [ppm] 168 ± 70 Occultation depth, ǫocc [ppm] 48 ± 52 Planetary mass [M⊕] 8.63 ± 0.35 Planetary radius [R⊕] 2.00 ± 0.14 −3 ± 1.5 Planetary mean density [g cm ] 5.9 1.1 −2 ± 3.5 Planetary [m s ] 21.1 2.7

NOTE. — These parameters were determined by fitting the MOST light curve as described in the text, in combination with external constraints on the orbital ± ± −1 +0.051 period P = 0.7365400 0.0000030 d and stellar reflex velocity K⋆ = 6.1 0.2ms (Dawson & Fabrycky 2010), M⋆ = 0.963−0.029 M⊙ (Takeda et al. 2007), and stellar radius R⋆ = 0.943 ± 0.010 R⊙ (von Braun et al. 2011). We further assumed the to be zero, and the limb-darkening law to be quadratic with coefficients u1 and u2 such that u1 − u2 = 0.542 and u1 + u2 = 0.772 ± 0.100.

Walker, G. A. H., et al. 2008, A&A, 482, 691 Winn, J. N. 2010, in Exoplanets, ed. S. Seager (Tucson: University of Winn, J. N., Holman, M. J., & Roussanova, A. 2007, ApJ, 657, 1098 Arizona Press) [arXiv:1001.2010] Wisdom, J. 2005, Bulletin of the American Astronomical Society, 37, 525