Today in Astronomy 111: exoplanet detection

 The successful search for 1.E+10

extrasolar planets around Number of transistors 1.E+08 in new microprocessors ordinary stars doubles every  Radial-velocity planet 24 months 1.E+06 detection

 Exoplanetary transits and 1.E+04 With Kepler planet eclipses candidates Number of extrasolar  Measurement of mass, 1.E+02 planets doubles every 29 months radius and surface 1.E+00 temperature of exoplanets 1970 1980 1990 2000 2010  Exoplanetary atmospheres Year Exponential progress: Moore’s law compared to exoplanet discovery. (Data from Intel, AMD, IBM, Zilog, Motorola, Sun Microsystems, Kepler and exoplanet.eu.)

6 December 2011 Astronomy 111, Fall 2011 1 The bad news

Exam #2 takes place here next Tuesday.  To the test bring only a writing instrument, a calculator, and one 8.5”×11” sheet on which you have written all the formulas and constants that you want to have at hand. • No computers, no access to internet or to electronic notes or stored constants in calculator.  The best way to study is to work problems like those in homework and recitation, understand the solutions and reviews we distributed, refer to the lecture notes when you get stuck, and make up your cheat sheet as you go along.  Try the Practice Exam on the web site. Under realistic conditions, of course. 6 December 2011 Astronomy 111, Fall 2011 2 The successful search for extrasolar planets

By which we mean, mature planets around normal stars. The first four extrasolar planets were observed around neutron stars (Wolszczan & Frail 1992, Backer et al. 1993.)

In the early 1990s, groups in San Francisco and Geneva were gearing up for long searches for giant planets around normal stars by Doppler-velocity techniques.  The idea: detect the relatively small, but periodic, Dopper shift in the spectrum of a star due to its orbital motion around the center of mass of a star-planet system.  The new part of the idea: simultaneously to use thousands of stellar spectral lines in a broad wavelength range, always viewed through a cell filled with a gas (iodine) which has many spectral lines but which is rare in stars.

6 December 2011 Astronomy 111, Fall 2011 3 Example: orbital speeds of Sun and Jupiter

Recall, from our discussion of the two-body system on 29 September 2011, applied to Jupiter and the Sun:

MM J 30 Reduced mass: µ ==×≅1.897 10 gm ( MJ ) MM+  J Orbital speeds, dictated by momentum conservation:

µ GM -1 v = = 13.044 km sec J Mr J µ GM MJ vv=  = = 0.012 km sec-1  MrJ M  The best planet-search spectrometers can measure stellar − Doppler velocities as small as 0.0002 km sec-1 = 20 cm sec1 . 6 December 2011 Astronomy 111, Fall 2011 4 The successful search for extrasolar planets (continued)  The observers thought they were going to detect Jupiters like this, so they were prepared to do observations over the course of many years. (Recall that Jupiter’s orbital period is 11.9 years.)  So it was much to their surprise that they detected their first planets in a matter of days:

• 51 Peg b: msini = 0.46MJ, P = 4.2 days (Mayor & Queloz 1995, Marcy & Butler 1995)

• 70 Virginis b: msini = 6.5MJ, P = 117 days (Marcy & Butler 1996)

• 47 Ursae Majoris b: msini = 2.5MJ, P = 1100 days (Butler & Marcy 1996) Jupiter-size planets, in terrestrial-planet-size orbits (or smaller)? 6 December 2011 Astronomy 111, Fall 2011 5 The successful search for extrasolar planets (continued) In 1999, after several more exoplanets had been detected, two more milestones in the search were reached:  One planet detected by radial velocity, HD 209458 b, was seen to transit: to eclipse a tiny portion of the star (Henry et al. 2000). • Thus its orbit is viewed close to edge on ( i ≈° 90 ) and its mass can be determined precisely: Deeg and Garrido 2000 mM=0.69 ± 0.05J .

6 December 2011 Astronomy 111, Fall 2011 6 The successful search for extrasolar planets (continued)  Also in 1999, the first multiple exoplanetary system was detected: υ Andromedae c and υ Andromedae d (periods 242 and 1275 days), to go with the previously- detected υ Andromedae b (4.6 days).

Top: from Butler et al. 1999 Bottom: animation by Sylvain Korzennik (CfA)

6 December 2011 Astronomy 111, Fall 2011 7 The successful search for extrasolar planets (continued)  Later (in 2005) the Spitzer Space Telescope detected the eclipse of HD 209458b (the planet) by HD 209458 (the star) – meaning that the light from the planet is directly detected at mid- infrared wavelengths Animation by Robert Hurt, SSC when not in eclipse (Deming et al. 2005).

6 December 2011 Astronomy 111, Fall 2011 8 The successful search for extrasolar planets (continued) 2008 saw two historic firsts:  an image of a planet orbiting the bright, nearby star Fomalhaut, which truncates the remains of the disk from which it formed. This planet’s orbit was correctly predicted two years earlier (by Alice Quillen, on the basis of the disk truncation) – the first time since Le Verrier and Galle (1846, Neptune) that anyone has Paul Kalas, UC Berkeley/STScI/NASA accomplished such a feat.

6 December 2011 Astronomy 111, Fall 2011 9 The successful search for extrasolar planets (continued)  and an image of three planets in orbit around another 0.49 HR 8799 IRAS

bright nearby star, HR 8799. 0.39 Another reported in 2010. 0.29  HR 8799 was previously Total

subtracted flux subtracted density (Jy) 0.19 known to have two debris - 35 K belts (Chen et al. 2006), 0.09 Spitzer-IRS 152 K which turn out to lie inside Photosphere -0.01 and outside the orbits of the 5 15 25 35 45 55 65 planets. The resemblance to Wavelength (μm) our Solar system’s giant planets, asteroid belt and C. Marois and B. Macintosh, Keck Observatory Kuiper belt is striking.

6 December 2011 Astronomy 111, Fall 2011 10 The successful search for extrasolar planets (continued) And this year, 2011, has seen, among many other results, the first detection of a planet in orbit around a binary star, Kepler 16AB b…  The stars are K and M type in a 41-day period; the planet is Saturn-like in a 229-day period (Doyle et al. 2011).

So the stars and the planet’s orbital distance are about right for Tatooine, but the planet’s mass is way too large.

(Lucasfilm )

6 December 2011 Astronomy 111, Fall 2011 11 The successful search for extrasolar planets (continued) …and the first detection – via imaging – of an infant giant planet orbiting in the gap of a transitional disk, LkCa 15.  Planet (blue) seems to be accompanied by a streamer of material (red) which shows up at longer wavelengths.

Andrews et al. 2011 Kraus & Ireland 2011

6 December 2011 Astronomy 111, Fall 2011 12 Today’s exoplanets

Today there are 708 objects listed as exoplanets:  181 by radial velocity and transits. • You can take 177 of them to the bank, as we’re 100% sure they’re of planetary mass.  471 others by radial velocity alone. Something like 5-10% of these may prove to be brown dwarfs eventually. • e.g. the earliest discovery on the list, HD 114762 b.  These 652 planets live in 534 planetary systems: there are 78 multiple-planet RV or RV+T systems so far. • 24 have three or more planets. • The most populous systems have six confirmed planets each: HD 10180, Kepler 11.

6 December 2011 Astronomy 111, Fall 2011 13 Exosolar systems with three or more planets

47 UMa 55 Cnc 61 Vir GJ 581 GJ 876 HD 10180 HD 125612 HD 136352 HD 181433 HD 20794 HD 31527 HD 37124 HD 39194 HD 40307 HD 69830 HIP 14810 HIP 57274 HR 8799 Kepler-11 Kepler-18 Kepler-9 KOI-730 µ Ara υ And Sun

0.01 0.1 1 10 100 Orbital semimajor axis (AU)

6 December 2011 Astronomy 111, Fall 2011 14 Today’s exoplanets (continued)

 14 others, in 9 systems, have been discovered by timing pulsars (neutron stars) or pulsating giant stars.  42 more have been found by direct imaging (29) or gravitational micro-lensing (13). Many of those imaged might be brown dwarfs. The official list currently includes only transit events which have been confirmed by RV measurements to be planets.  But the planetary-transit satellites, COROT (ESA) and Kepler (NASA), have produced a huge number of candidates for RV followup, the vast majority of which are expected to turn out to be planets.  Kepler’s current list of 2326 candidates includes candidate planets as small as RR = 0.6⊕ . (!!) 6 December 2011 Astronomy 111, Fall 2011 15

Radial-velocity planet detection and msini measurement …in which one makes a series of accurate measurements of tiny periodic variations in the Doppler shift of the host star. For help in visualizing how this works, click here. Steps: measure P and K.  Measure the period P of the Doppler shift’s variation. This determines the length of the semimajor axis of the planet- star separation via Kepler’s third law: From exoplanets.org

GM( + m) GM a3= PP 22≅ 22 44ππ

6 December 2011 Astronomy 111, Fall 2011 16 Radial-velocity planet detection and msini measurement (continued)  Measure also the amplitude K (maximum Doppler shift with respect to the average) of the star’s radial velocity: KV= sin i

Observer’s view Side view

Orbital axis

m  Line of VKr = 90 − i sight Vt Vt V Orbit Orbit i

6 December 2011 Astronomy 111, Fall 2011 17 Radial-velocity planet detection and msini measurement (continued)  If the orbital eccentricity is close to zero, the radial velocity varies sinusoidally with time.  If the orbital eccentricity is not close to zero, the radial- velocity curve looks lopsided compared to a sine wave. In this case, fitting the velocities of an elliptical orbit to the radial-velocity curve allows one to determine the eccentricity and the position of periastron, as well as Vi sin as a function of time.  Either way, this allows a determination of the planet mass m. For low eccentricity, V a aM aM m=== M MV V ⇒=msin i K v GM G G

6 December 2011 Astronomy 111, Fall 2011 18 Radial-velocity planet detection and msini measurement (continued)  The mass of the star is usually known, from its age and spectral type.  The orientation usually is not known, unless: • the star and planet are both seen in images, and the transverse component of motion is seen too; or • the star has a debris disk that is resolved in images: the planet will usually be in the same plane; or • the star and planet eclipse each other ( i ≈° 90 ).

6 December 2011 Astronomy 111, Fall 2011 19 Exoplanetary transits: sini and R measurements

During a transit, the planet passes in front of the star, dimming the system at visible wavelengths.  Because i must be very close to 90 degrees (i.e. sini → 1) for this to happen, observation of transits and radial velocities allow unambiguous determination of the planetary mass m, not just the lower limit mi sin .  The duration of the transit enables a measurement

of the diameter of the star and /or the precise orbit inclination. Flux  The depth of the flux “dip,” and time it takes the it to turn

off or on, offer a measurement t t t t of the diameter of the planet. a b c d Time

6 December 2011 Astronomy 111, Fall 2011 20 Exoplanetary transits: sini and R measurements (continued)  Much can be made of the details of the shapes of the 1 light curve at the onset (ingress) and end (egress)

of the transit, to make out 0.5 details in the atmosphere of star and planet, such as

the uniformity of the 0 brightness of each. Brightess of star, units in of its total flux − 2 − 1 0 1 2

• This will be one of the Time, in units of Rs/v first subjects taken up Uniform brightness star (this calculation) Realistic (limb-darkened) star in AST 142, in the Straight line context of binary stars.

6 December 2011 Astronomy 111, Fall 2011 21

Exoplanetary transits: sini and R measurements (continued)  The “corners” are usually labelled as indicated at right.

 During transit, the star and Flux planet move at speeds v1 and v2 in opposite directions perpendicular to the line of sight, where v and v are 1 2 t1t2 t3 t4 Time the two speeds measured from Ingress Egress the orbital periods and radial- velocity amplitudes. Thus

2Rplanet=+−( vvtt 1 2)( 2 1 )

2Rstar=+−( vvtt 1 2)( 3 1 )

6 December 2011 Astronomy 111, Fall 2011 22 Exoplanetary eclipses and the temperatures of exoplanets At mid-infrared wavelengths, the difference between transit, eclipse and points in between enables one to isolate Eclipse flux from the planet, and even to “map” the emission from the planet’s surface. Transit  Example: HD 189733b is brightest near, but not exactly at, eclipse. Flux of star alone  That’s planetary blackbody emission, at HD 189733 at λ = 8μm; Knutson et mid-infrared wavelengths. al. 2007.

6 December 2011 Astronomy 111, Fall 2011 23 Exoplanetary eclipses and T (continued)

Thus the temperature over the planet’s surface can be worked out from the planet brightness through the orbit. Animations by Tim Pyle (SSC)  Example: in HD T = 1212 K T = 189733b, the 973 K warmest spot is offset from the substellar point, in the direction of the planet’s synchronous rotation. 6 December 2011 Astronomy 111, Fall 2011 24 Transits, eclipses and exoplanetary atmospheres

Near-infrared spectra of transits and eclipses with the Hubble Space A B C D Telescope’s late, lamented NICMOS instrument have revealed molecular species familiar from the atmospheres of Jupiter and Saturn. Here, the example of HD 189733b: (A+C+D)/3 – B:  Transit: The difference of spectra in and out of minimum reveals H2O CH4 absorption by water and methane in the starlight transmitted by the planet’s atmosphere (Swain, Vasisht & Tinetti 2008).

6 December 2011 Astronomy 111, Fall 2011 25 Transits, eclipses and exoplanetary atmospheres (continued)

 Eclipse: This time the difference reveals the A B C D day-side emission

spectrum by the (A+C+D)/3 – B: planet, again revealing water which this time is joined by CO and

CO2, components not prominent in the Solar system’s giant planets (Swain et al. 2009).

6 December 2011 Astronomy 111, Fall 2011 26 Exoplanet detection

By any method, the detection of exoplanets is challenging.  One must observe each system frequently for at least an orbital period to be sure of periodicity.  For either the radial-velocity or transit method to work, one needs a spectrometer or camera for which the calibration is extremely steady for longer than the orbital period one is trying to determine. The planetary “signal” is a tiny fraction of the star’s signal.  For eclipse detection to work, one needs a sensitive mid- infrared observatory for which the calibration is extremely steady over orbital periods.  The star needs to cooperate: it can’t be too active or too heavily spotted. RV&T thus very difficult in young stars. 6 December 2011 Astronomy 111, Fall 2011 27