Colors of a second earth: from exoplanets to astrobiology

Transcription

1 Colors of a second earth: from exoplanets to astrobiology Yasushi Suto Department of Physics and Research Center for the Early Universe, the University of Tokyo Introductory astronomy Sejong University, Seoul 10:30-12:00 on Monday, September 5, 2016

2 Nightfall: We didn t know anything A short novel by Issac Asimov (illustration: Alisa Haba) n No night except the total eclipse due to an inner planet every 2049 years on the Illustration planet Lagash by Alisa Haba n People realized the true world for the first time

3 Issac Asimov: Nightfall n Light! he screamed. Aton, somewhere, was crying, whimpering horribly like a terribly frightened child. Stars -- all the Stars -- we didn't know at all. We didn't know anything.

4 History of exoplanet discovery Number of planets discovered per year In 1995, Mayor and Queloz made us realize that we didn t know anything 51 Peg b 1977 planets 1257 planetary systems 490 multiple planet systems Year of discovery As of November 11,

5 How to find planets? n Radial velocity n Periodic modulation of the velocity of star due to the presence of planets n Transit Ground-based observation (2000) n Periodic dimming of the stellar light due to the occultation of planets in front of the star n Direct imaging n Separate the light from the star and planets

6 Kepler mission (March 6, 2009 launch) Photometric survey of transiting planets Searching for terrestrial/habitable planets

7 Are we alone? a Pale Blue Dot? or pale blue dots?

8 Sciences with exoplanets n the final question: Are we alone? n origin of the earth n origin of the Solar System n habitable planets origin of life n signature of extra-terrestrial life? extra-terrestrial intelligence? Where are they? E.Fermi (1950)

9 Solar planets imaged by Voyager 1 (February 4, 1990) n Earth imaged at distance of 40 AU away n A Pale Blue Dot coined by Carl Segan

10 A Pale Blue Dot Voyager 1

11 Earth and Moon from Saturn (2013) n Viewed from Cassini on July 20, 2013 n about 20,000 happy Americans are waving their hand towards Cassini, but how can we know that?

12 Can we detect signatures of life on our Earth?

13 Search for signatures of life on Earth with Galileo mission! (1990) n Launched in May, 1986 n Earth observed on December 8, 1990 n Conclusion: it is likely that life exists on Earth! n Abundant O 2 n Red-edge of vegetation n CH 4 abundance out of thermal equilibrium n Artificial pulsed radio signal Sagan, Thompson, Carlson, Gurnett & Hord: Nature 365(1993)715

14 Conventional bio-signatures Woolf & Smith (2002) Earthshine (visible) data + model n O 2 n A-band@0.76µm n B-band@0.69µm n H 2 O n 0.72, 0.82, 0.94µm wavelength [Å] n O 3 n Chappuis n Hartley Kasting et al. arxiv: Exoplanet characterization and the search for life

15 Sagan et al. (1993): spectrum of atmosphere Strong O 2

16 Red edge of (exo)plants: a possible biosignature in exoplanets n Red-edge n Significant increase of reflectivity of leaves on Earth (terrestrial planets) for λ>7000å n Widely used in the remote-sensing of our Earth Chlorophyll Chlorophyll Reflection spectrum of leaves Seager, Ford & Turner astro-ph/

17 Vesto Melvin Slipher ( ) n Discovered redshifts of galaxies and thus cosmic expansion via the Doppler method Observations of Mars in 1924 made at the Lowell Observatory: II spectrum observations of Mars PASP 36(1924)261 Astrobiology indeed in 1924!

18 Sagan et al. (1993): colors of the earth Red-edge of the vegetation on the earth detected by the Galileo mission Peru/Colombia Reflectivity Chile Argentina Wavelength [µm]

19 Sagan et al. (1993): radio observation Detection of pulsed radio signals unlikely of natural/astronomical origin

20 Arecibo message (1974) n Frank Drake sent a radio message from Arecibo radio observatory on November 16, 1974 towards globular cluster M13 (25,000 light-year away) n The message, if decoded properly, should look like this. 1 to 10 in binary Atomic numbers of H, C, N, O, P that form DNA in binary Formulas for the sugars and bases in the nucleotides of DNA Double helix of DNA Human and the human population on earth The solar system Arecibo radio telescope

21 Proxima Centauri b

22 Proxima Centauri n Alpha Centauri n The closest triple star system to our Sun n α Cen A, α Cen B, α Cen C (=Proxima Centauri) n Proxima Centauri n Distance =1.295 pc (4.25 light year) from us n M5.5 V Red dwarf n T=(3042±117)K, L=0.0017L n R=(0.141±0.007)R, M=(0.123±0.006)M n Spin period = 83.5 days n Age = 4.85 G years

23 Proxima Centauri

24 A terrestrial planet candidate in a temperate orbit around Proxima Centauri n G.Anglada-Escude et al. Nature 25 August 2016 issue, 536(2016)437 n Proxima Centauri b n Orbital period ( ) days n M p sin i =1.27 ( ) M earth n Eccentricity < 0.35 n Semi-major axis = ( ) AU n Equilibrium temperature = 234 ( ) K

25 Pale red dot: a habitable planet?

26 Periodograms of the independent Doppler signal G Anglada-Escudé et al. Nature 536, (2016)

27 Combined periodograms of the Doppler signal G Anglada-Escudé et al. Nature 536, (2016)

28 Radial velocity data G Anglada-Escudé et al. Nature 536 (2016) 437

29 Simulated surface temperature (3:2 resonance case)

30 Simulated surface temperature (synchronous case)

31 Breakthrough Initiatives n A program founded on July 20, 2015 by a Russian internet investor Yuri Milne to search for extraterrestrial intelligence n Breakthrough Listen to discover signs of extraterrestrial civilizations through radio and laser transmissions n Breakthrough Message to study the ethics of sending messages into deep space n Breakthrough Starshot to develop a proof-ofconcept light sail spacecraft fleet capable of making the journey to Alpha Centauri

32 n StarChip Breakthrough Starshot n A cm-sized nano-spacecraft of several grams n With camera, computer, communication laser, plutonium power source, and light sail n A 4m 4m light sail for each spacecraft is accelerated by the focused ground-based lasers n 0.2c in 10 minutes n A fleet of 1000 StarChips to Proxima Centauri in 20 years n Technology not available, maybe in 20 years

33 StarChip

34 Light sail accelerated by ground lasers

35 From Earth to Proxima Centauri b

36 Proxima Centauri b

37 Simulated Earth observed at 10pc away

38 Habitable zone around host stars Kasting, Kopparapu, Raminez & Harman: arxiv:

39 Occurrence of earth-size habitable planets around Sun-like stars n Planets with (1-2) Earth radius around GKstars n Kepler Transit planets corrected for selection effect n 11±4 % (1-4 times the Solar flux on the earth) n % (orbital period of days) Petigura, Howard & Marcy: arxiv:

40 Starshade project: direct imaging of a second earth Space telescope + occulting satellite at 50,000km away! (Princeton Univ. + JPL/Caltech)

41 Expected daily change of the reflected light of the earth Ford, Seager & Turner: Nature 412 (2001) 885 n Assume that the earth s reflected light is completely separated from the Sun s flux! n Periodic change of 10% level due to different reflectivity of land, ocean, forest n Cloud is the most uncertain factor: weather forecast!

42 Colors of a Second Earth: estimating the fractional areas of ocean, land and vegetation of Earth-like exoplanets ApJ. 715(2010)866, arxiv: Colors of a Second Earth. II: Effects of Clouds on Photometric Characterization of Earth-like Exoplanets ApJ. 738(2011)184, arxiv: n Yuka Fujii, H.Kawahara, A.Taruya, Y.Suto (Dept. of Phys., Univ. of Tokyo), S.Fukuda, T.Nakajima (Univ. of Tokyo, Center of climate system research), Edwin Turner (Princeton Univ.)

43 Colors of a second earth n Beyond a pale blue dot n Impossible to spatially resolve the surface of a second earth n Color should change due to the rotation n A second earth = a dot

44 Earth at Night

45 Night in Korea and Japan

46 annual vegetation global map by the earth observing satellite Terra

47 Colors of our earth vegetation soil(b) soil(a) cloud snow ocean

48 A pale blue dot? Not really Simulated photometric light-curves of Earth Reflectivity Eurasia n Adopted Earth data in March Africa [μm] [μm] [μm] America [μm] time[hour] n Spin inclination = 0 (vernal equinox) n cloudless Fujii et al. (2010)

49 Estimating fractional areas of surface components from colors of a second earth n 2 week observation of a cloudless Earth at 10 pc away n Reasonably well reproduced n possible to identify vegetation! Fujii et al. (2010)

50 Surface latitude map estimated from real satellite data with cloud model ocean vegetation cloud soil snow Fujii et al. (2011)

51 Old M-star Young M-star G-star F-star The color of plants on other worlds N.Kiang, Scientific American (2008)

52 Summary: A pale blue dot? Not really! n Future direct imaging of daily change of colors of another earth is challenging, but would reveal the presence of ocean, land, cloud, and/or even vegetation on their surface n Detection of a second Earth may not be a mere fairly tale nor a science fiction any more n Detection of oxygen, water vapor, and even the red-edge may be a promising path towards astrobiology