Colors of a second : from to Yasushi Suto Department of Physics and Research Center for the Early Universe, the University of Tokyo

Introductory astronomy class @ Sejong University, Seoul 10:30-12:00 on Monday, September 5, 2016 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 Illustrationplanet “Lagash” by Alisa Haba

n People realized the true world for the first time Issac Asimov: Nightfall

n “Light !” he screamed. Aton, somewhere, was crying, whimpering horribly like a terribly frightened child. “ -- all the Stars -- we didn't know at all. We didn't know anything.” History of discovery

In 1995, Mayor and Queloz made us realize that “we didn’t know anything”

1977 planets 1257 planetary systems 51 Peg b → 490 multiple planet systems

Number of planets discovered per year of planets Number Year of discovery As of November 11, 2015 http://exoplanet.eu/ How to find planets ?

n Radial velocity n Periodic modulation of the velocity of due to the presence of planets

n Transit

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 Ground-based observation (2000) star and planets Kepler mission (March 6, 2009 launch) Photometric survey of transiting planets Searching for terrestrial/habitable planets

http://kepler.nasa.gov/ Are we alone ?

a Pale Blue Dot ? or pale blue dots ? 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) 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 A Pale Blue Dot

Voyager 1 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 ? Can we detect signatures of life on our Earth ? 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 O2 n Red-edge of vegetation Sagan, Thompson, n CH4 abundance out of Carlson, Gurnett & Hord: thermal equilibrium Nature 365(1993)715

n Artificial pulsed radio signal Conventional bio-signatures

Earthshine (visible) n O data + model 2 n [email protected]µm n [email protected]µm

Woolf & Smith n H2O (2002) n 0.72, 0.82, 0.94µm

n O3 n Chappuis band @(0.5-0.7)µm n Hartley band wavelength [Å] @(0.2-0.3)µm Kasting et al. arXiv:0911.2936 “Exoplanet characterization and the search for life” Sagan et al. (1993): spectrum of atmosphere

Strong O2 absorption ＠A-band(0.76µm) Red edge of (exo)plants: a possible in exoplanets

n Red-edge Reflection n Significant increase spectrum of reflectivity of of leaves leaves on Earth (terrestrial planets) for λ>7000Å Chlorophyll n Widely used in the Chlorophyll remote-sensing of our Earth Seager, Ford & Turner astro-ph/0210277 Vesto Melvin Slipher (1875-1969) 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 ! Sagan et al. (1993): colors of the earth Red-edge of the vegetation on the earth detected by the Galileo mission

Peru/Colombia

Chile Argentina Reflectivity

Wavelength [µm] Sagan et al. (1993): radio observation Detection of pulsed radio signals unlikely of natural/astronomical origin Arecibo message (1974)

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

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 Proxima Centauri

http://www.eso.org/public/usa/news/eso1629/ A candidate in a temperate orbit around Proxima Centauri n G.Anglada-Escude et al. Nature 25 August 2016 issue, 536(2016)437 n

n Orbital period 11.186 (11.184-11.187) days

n Mp sin i =1.27 (1.10-1.46) Mearth n Eccentricity < 0.35

n Semi-major axis = 0.0485 (0.0434-0.0526) AU

n Equilibrium temperature = 234 (220-240) K

http://www.eso.org/public/usa/news/eso1629/ Pale red dot: a habitable planet ?

http://www.eso.org/public/usa/news/eso1629/ Periodograms of the independent Doppler signal

G Anglada-Escudé et al. Nature 536, 437–440 (2016) Combined periodograms of the Doppler signal

G Anglada-Escudé et al. Nature 536, 437–440 (2016) Radial velocity data

G Anglada-Escudé et al. Nature 536 (2016) 437 Simulated surface temperature (3:2 resonance case)

http://www.eso.org/public/usa/news/eso1629/ Simulated surface temperature (synchronous case)

http://www.eso.org/public/usa/news/eso1629/ http://breakthroughinitiatives.org/Initiative n A program founded on July 20, 2015 by a Russian internet investor Yuri Milne to search for extraterrestrial intelligence

n to discover signs of extraterrestrial civilizations through radio and transmissions

n Breakthrough Message to study the ethics of sending messages into deep space

n to develop a proof-of- concept light sail fleet capable of making the journey to Alpha Centauri Breakthrough Starshot http://breakthroughinitiatives.org/Initiative/3 n StarChip

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

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 StarChip Light sail accelerated by ground lasers From Earth to Proxima Centauri b

http://www.eso.org/public/usa/news/eso1629/ Proxima Centauri b http://www.eso.org/public/usa/news/eso1629/ Simulated Earth observed at 10pc away Habitable zone around host stars

Kasting, Kopparapu, Raminez & Harman: arXiv:1312.1328 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 5.7+2.2-1.7 % (orbital period of 200-400days)

Petigura, Howard & Marcy: arXiv:1311.6806 Starshade project： direct imaging of a second earth

Space telescope + occulting satellite at 50,000km away! (Princeton Univ. + JPL/Caltech) 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 ! Colors of a Second Earth: estimating the fractional areas of ocean, land and vegetation of Earth-like exoplanets ApJ. 715(2010)866, arXiv:0911.5621 Colors of a Second Earth. II: Effects of Clouds on Photometric Characterization of Earth-like Exoplanets ApJ. 738(2011)184, arXiv:1102.3625

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.) http://www.space.com/scienceastronomy/color-changing-planets-alien-life-100513.html 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 Earth at Night 2012

http://earthobservatory.nasa.gov/Features/NightLights/page3.php Night in Korea and Japan… annual vegetation global map by the earth observing satellite Terra

http://earthobservatory.nasa.gov/GlobalMaps/ Colors of our earth

vegetation

soil（B) soil（A)

cloud snow ocean A pale blue dot ? Not really

0.08 Africa 0.7-0.8[μm] Eurasia 0.6-0.7[μm]

America 0.5-0.6[μm]

Simulated 0.4-0.5[μm] Reflectivity Reflectivity photometric 0.00

light-curves of Earth 0.0 time[hour] 24.0 n Adopted Earth data in March n Spin inclination = 0 (vernal equinox) n cloudless Fujii et al. (2010) 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） Surface latitude map estimated from real satellite data with cloud model

ocean vegetation

soil cloud snow

Fujii et al. (2011) 赤，青，黒… Old Young G-star F-star 異星の植物は何色か M-star 惑星表面を覆う植物は，遠くからで も確認できる「生命存在の証拠」だ M-star 太陽系以外の惑星で光合成生物を探すとき，何色を指標にすればよいだろう 例えば赤色矮星の惑星ならば植物は真っ黒ということも十分ありうる

. 9 キアン（.!3! ゴダード宇宙研究所）

地球外生命を探す試みは，もはや エネルギーを得る生物も存在するが， 3&や5&/ハンターの世界にとどまら 地表の多様性に富んだ生態系はすべて ない。宇宙人の到来を待つのではなく， 太陽の光に依存している。 私たちのほうが彼らを探す時代になっ た。科学技術の進歩した文明は見つか 光合成の証拠とは？ らないかもしれないが，基本的な生命 光合成の存在を示す証拠にはおも 現象の物理的・化学的な証拠ならば見 につの種類が考えられる。つは， つかる可能性がある。 光合成によって発生する酸素や，それ 太陽以外の恒星を中心星とする惑星 が変化して生じるオゾンなどの大気中 は，すでに個以上が見つかってい のガス。もうつは色で，これは緑色 る。太陽系の外にあることから「系外 のクロロフィルなど，特定の色素が惑 惑星」と呼ばれるこれらの天体に生命 星表面を覆うことを手がかりにする。 が存在するかどうかは現時点では断言 こうした色素探しの歴史は古い。 できないが，生命発見は時間の問題だ 世紀ほど前の天文学者は，火星が季節 ろう。昨年月には，ある系外惑星の によって明るく見えたり暗く見えたり 大気を通過した光のスペクトルを観測 することを植生の季節変動によるもの していた天文学者のグループが，水蒸 だろうと考えた。そして火星表面で反 気の存在を確認している。今日，地球 射された光のスペクトルを調べ，緑色 と同程度のサイズの惑星における「生 植物の存在を示す証拠を見つけ出そう 命が存在する証拠」を光のスペクトル とした。 を観測してとらえるために，各国の宇 緑色を手がかりにしたこの戦略には 宙機関が望遠鏡の開発を進めている。 生命存在の証拠の中でも，「 光合成」 恒星の型と植物の色 図は左から右へ，恒

は際立ってはっきりとした目印になる 3TUDIOS 星の型が暗→明へと並べてある。- 型星（赤色 と考えられている。他の惑星で光合成 矮星）は暗い恒星なので，地球サイズの惑星の

が行われている可能性はどのくらいか 植物は，利用可能なあらゆる光を吸収するために -ONDOLITHIC 黒く見えるかもしれない（すぐ右）。 - 型星は若

と聞かれれば，「 きわめて高い」とい 72%. いうちは紫外光のフレアを放射するので，惑星に

うのが答えだ。地球上では，光合成は 生息するすべての生物体は水中にいるはずだ（左 #(2)3

活発に行われており，ほぼすべての生 から 番目）。 私たちの太陽は ' 型星だ（ 番目）。 !.$ 太陽よりもさらに明るい& 型星を周回する惑星で

命を支える基盤となっている。深海底 “The"2/7. color of plants on other worlds” は，多量の光が降り注ぐため，その多くを反射す

の熱水噴出口における熱やメタンから る必要があるかもしれない（ 番目）。 +%..

 日経サイエンス  年  月号N.Kiang, Scientific American (2008) 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