Finding terrestrial planets in the habitable zones of nearby stars

Transcription

1 Finding terrestrial planets in the habitable zones of nearby stars Part II Astrophysics Essay Simon Hodgkin & Mark Wyatt (on sabbatical)

2 Terrestrial? 15 Exoplanets Solar system g cm 3 Winn et al. 211 K 11e Uranus 1. g cm 3 Radius [R Earth ] 1 5 hydrogen 55 Cnc e water rock iron Mass [M Earth ] Radius [R Earth ] Earth Venus K 11f K 11b K 1b K 11d GJ 1214b C 7b 5% water 55 Cnc e Earth like 2. g cm 3 maximum iron fraction Mass [M Earth ] 4. g cm 3 8. g cm g cm 3 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 (MgSiO 3 perovskite) and iron, from Figure 4 of Seager et al. (27). Right. Focus on super-earths, showing contours of constant mean density and a few illustrative theoretical models: a water-world composition with 5% water, 44% silicate mantle and 6% iron core; a nominal Earth-like composition with terrestrial iron/silicon ratio and no volatiles (Valencia et al. 26, Li & Sasselov, submitted); and the maximum mantle stripping limit (maximum iron fraction, minimum radius) computed by Marcus et al. (21). Data were taken from Lissauer et al. (211) for Kepler-11, Batalha et al. (211) for Kepler-1b, Charbonneau et al. (29) for GJ 1214b, and Hatzes et al. (211) for Corot-7b. We note the mass of Corot-7b is disputed (Pont et al. 211).

3 Habitable?

4 861 As of June 212

5 Detecting Exoplanets Pulsar Timing Radial Velocity Transits TTV Reflected Light Direct Imaging Microlensing Astrometry Figure 4. Examples of radial velocity measurements: HD (top) and HD (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 4.1M J respectively. The fit for HD is improved further by a linear velocity trend, suggestive of an additional, nearby, long-period stellar or brown dwarf companion (courtesy of Geoffrey Marcy).

6 Detecting Exoplanets Pulsar Timing Radial Velocity Transits TTV Reflected Light Direct Imaging Microlensing Astrometry a Radial velocity (m s 1 ) HD 6983 i ii iii P = 8.67 days m sin i = 1.2 M P = 31.6 days m sin i = 11.8 M P = 197 days m sin i = 18.1 M.5 1 HARPS b Radial velocity (m s 1 ) O C Radial velocity (m s 1 ) O C Radial velocity (m s 1 ) i HD ,3 53,35 53,4 ii 53,65 53,7 53,75 iii HARPS 53, 53,2 53,4 53,6 53,8 Orbital phase JD-24 (days) re 2

7 Detecting Exoplanets Pulsar Timing Radial Velocity Transits TTV Reflected Light Direct Imaging Microlensing Astrometry The first detected transit of an extra-solar planet, HD 29458b (from Charbonneau et al 2).

8 HAT-P-7b: Welsh et al. 21 orbital phase normalized flux normalized flux orbital phase

9 Detecting Exoplanets Pulsar Timing Radial Velocity Transits TTV Reflected Light Direct Imaging Approaching limb Receding limb Microlensing Astrometry Figure 3. The Rossiter-McLaughlin effect. Shown are three planet trajectories that produce identical light curves, but have different orientations 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 (27).

10 Atmospheres: transit spectroscopy Sing et al. 211 STIS (blue) and ACS (red) transmission spectra for HD189733b. The right Y-axis is labeled in units of estimated atmospheric scale heights, assuming T=134 K (H=.4 Rpl/Rstar). The prediction from ACS Rayleigh scattering (134±15 K red solid and dashed lines) is also shown, as is a haze-free model atmosphere for HD b from Fortney et al. (21)

11 Detecting Exoplanets Pulsar Timing Radial Velocity Transits TTV Reflected Light Direct Imaging Microlensing Astrometry

12 Detecting Exoplanets Pulsar Timing Radial Velocity Transits TTV Reflected Light Oppenheimer & Hinkley 29, ARA&A.6 microns Fomalhaut b planet H band Direct Imaging 5" " Microlensing Astrometry Figure 2 Images of (left) the ring of debris around Fomalhaut and (right) HR 4796A (left: Hubble image STScl-PRC8-39a; Kalas et al. 28, courtesty of NASA, ESA, and P. Kalas of University of California, Berkeley; right: Schneider et al. 1999, courtesy of B. Smith, G. Schneider, and NASA). Currie et al. 212: Fomalhaut b is very plausibly a planet identified from direct imaging even if current images of it do not, strictly speaking, show thermal emission from a directly imaged planet.

13 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: Gliese 581

14 Detecting Exoplanets Pulsar Timing Radial Velocity Transits TTV 15 Reflected Light Declination (milli-seconds of arc) Direct Imaging Microlensing Astrometry Right ascension (milli-seconds of arc)

15 red symbols : transit discoveries black symbols : RV discoveries

16 red symbols : transit discoveries black symbols : RV discoveries

17 Detecting Exoplanets Wright & Gaudi, 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 15K. The frost line of the Solar System is around 4.2 AU.

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19 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).

20 The Drake Equation 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 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 galaxy 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 fi = the fraction of habitable planets on which life evolves = 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

21 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.