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Exoplanets - Moving from Discovery to Understanding Douglas Caldwell SETI Institute, [email protected] C. th A Brief History of 4 2050? BCE time

• Where have we been? – Early conjecture – First discoveries • Where are we now? – Large-scale surveys – Statistics of populations – Atmospheres of large hot • What is next? – Improved populations for small planets – Targets for characterization – Atmospheres of small planets – Biomarkers – …?

AAPT 2021 Winter Meeting 2 C. th Exoplanets: Conjecture to Science 4 1584 1698 BC time

• Epicurus (4th Century BCE): – “There are infinite worlds both like and unlike this world of ours…”

• Giordano Bruno: On the Infinite and Worlds, 1584 – Universe is infinite, homogenous, and filled with innumerable celestial bodies

• Christiaan Huygens: Cosmotheoros, 1698 – has no atmosphere based on occultation observations – has a period similar to and little seasonal variation due to low obliquity – Observations can provide no direct answer to question of habitability, or even existence of planets around fixed

S. J. Dick, Plurality of Worlds, Cambridge University Press: Cambridge, 1982 M. D. Lemonick, Mirror Earth, Walker & Company: New York, 2012

AAPT 2021 Winter Meeting 3 C. th Exoplanets: Conjecture to Science 4 1584 1698 1755 1796 1952 BC time

• : Universal Natural History & Theory of the Heavens, 1755 – Stars (other ) orbit around a common center, other have stars orbiting their common center – Stars have worlds and the worlds are inhabited, or likely will be in time

• Pierre Simon de Laplace: “,” 1796 – Mathematical & physical description of formation

• Otto Struve (UC Berkely): in The Observatory (vol 72, p. 199), 1952 – Described detection of exoplanets – Predicted “hot-” – Predicted transiting planets

S. J. Dick, Plurality of Worlds, Cambridge University Press: Cambridge, 1982 M. D. Lemonick, Mirror Earth, Walker & Company: New York, 2012

AAPT 2021 Winter Meeting 4 th C. 4 1584 1698 1755 1796 1952 19921995 BC time • First Exoplanet discoveries

Pulsar Planets: 2 planets seen with masses 3.4 & 51 Peg-b: hot- around a -like ,

2.8 Mearth, periods of 66, 98 days 0.47 Mjup, period = 4.23 days Wolszczan, A., Frail, D. A. 1992. A planetary Mayor, Michael; Queloz, Didier, 1995. "A system around the millisecond pulsar PSR1257 + Jupiter-mass companion to a solar-type star". 12. Nature 355, 145 Nature. 378, 355–359

NASA/JPL-Caltech/R. Hurt (SSC) ESO/M. Kornmesser/Nick Risinger

AAPT 2021 Winter Meeting 5 th C. 4 1584 1698 1755 1796 1952 19921995 1999 2015 BC time • Detection Techniques Radial Velocity: Transits:

Microlensing: Direct Imaging: Orbital motion of 51 Eri b detected between two H- band observations taken with the Gemini Imager in December 2014 and September 2015. (credit: Christian Marois &

AAPT 2021 Winter Meeting the GPIES team) 6 NASA, ESA, and A. Feild (STScI) th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015 BC time • Kepler Operations: May 13, 2009 – May 12, 2013

• Kepler is optimized for finding terrestrial planets ( 0.5 to 10 MEarth) in the “habitable zone” of Sun-like stars • Continuously and simultaneously monitor >100,000 stars from a thermally stable orbit away from the Earth-Moon system

• Photometric precision of < 20 ppm in 6.5 hours on mV=12 Sun-like stars • 0.95 m Schmidt telescope feeds 42 CCDs in the 95 megapixel focal plane Mod 3

AAPT 2021 Winter Meeting 8 7 th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 201320152017 2021 BC time

• Where are we now? Kepler has shown that small planets are common in the and about half of Sun-like stars have a rocky planet in their habitable zones.

Final Kepler catalog (“DR25”) has 2327 planets The average number of HZ planets with radii 0.5– Current Planet Count: 4307 confirmed planets*: 1.5 Re per Solar-like star is 0.38 - 0.6. The nearest Transit: 3296 Radial Velocity: 821 HZ planet around G and K stars is ~6 pc and Imaging: 51 Microlensing: 106 there are likely 4 within 10 pc (Bryson et al. 2020)

Figure. 13 Bryson et al. 2020

AAPT 2021 Winter Meeting Earth 8 *Data from NASA Exoplanet Archive: https://exoplanetarchive.ipac.caltech.edu (planets/star) th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 2013201520172018 BC time • Finding Nearby Exoplanets to Characterize Solar neighborhood surveys: • ground-based M-dwarf surveys (MEarth, TRAPPIST, SPECULOOS,…) • K2 has found over 400 exoplanets • Transiting Exoplanet Survey Satellite (TESS) launched April 18, 2018 K2 completed 18 campaigns

TRAPPIST-1b,c,d: 3 planets orbiting a

very cool (2550K) small (0.08 MSun) star 40 light years away (Gillon, 2016 Nature, 533, p221-224), four more seen with Spitzer (Gillon, 2017 Nature 542) and K2 AAPT 2021 Winter Meeting 9 th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015201720182021 BC time • Finding Nearby Exoplanets: TESS TESS completed its primary mission in June 2020 • observed >200,000 stars over the whole sky • observed for 27 days • some observed for up to 1 year at N & S poles • more than 25 million targets observed in full-frame images

Image Credit: NASA/MIT/TESS and AAPT 2021 Winter Meeting Ethan Kruse (USRA) South North10 th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015201720182021 BC time

Characterizing Planets: Mass and Radius - , 135 800

ApJ ReproducedDressingfrom al.et 2015

11 th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015201720182021 BC time • Finding Nearby Exoplanets: TESS TESS has found >2100 planet candidates (12/11/2020) • 82 confirmed planets • 56 with measured masses

Earth AAPT 2021 Winter Meeting 12 *Data from NASA Exoplanet Archive: https://exoplanetarchive.ipac.caltech.edu th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015201720182021 BC time • Finding Nearby Exoplanets: TESS With shorter observing campaigns, K2 (90 days) and TESS (27 days) find shorter-period planets => planets receive more energy unless their star is cooler Earth

marker size represents Earth planet size

AAPT 2021 Winter Meeting 13 *Data from NASA Exoplanet Archive: https://exoplanetarchive.ipac.caltech.edu th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015201720182021 BC time • Characterizing Atmospheres: Spectroscopy & JWST Transit spectroscopy: observe star light through Reflection spectroscopy: observe star light a planet’s atmosphere during transit reflected off planet • Atmosphere scale height: H = kT/µg • Very low contrast (10-10) • Very small signal => big mirror, relatively poor • Need to mask direct starlight spectral resolution • Can use occultation of hot planets • need many transits to build up signal => short period, hot planets are easiest ) -10 1 Planet /Star Flux (10 Flux/Star Planet Seager & Bains, 2015, Science & Bains, 2015, Seager Advances, AAPT 2021 Winter Meeting Image credit: A. Field, STScI /Batalha PSU 14 th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015201720182021 2030 2050 BC time • Finding planets in the Habitable Zone to understanding Habitable Environments and finding Living Worlds Simulated spectrum of Earth twin at 10 pc observed with 10-m aperture

Key Science Questions from the 30-Year Roadmap, AURA, ExoPAG Reports: • - Search hundreds of Solar-type stars to yield dozens of nearby, potentially habitable Earth-size planets (exo-). • - Directly measure , water vapor, and other molecules in their atmospheres. • - Understand the diversity of terrestrial planets

“A 10-m class LUVOIR telescope…could achieve R=70 visible wavelength spectra at SNR=5 for earth-twins at … 10 pc for integration times of order 100 hours.” Credit: Robinson, Stapelfeldt, & Marley 2016, PASP 128 AAPT 2021 Winter Meeting 15 th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015201720182021 2030 2050 BC time • Finding planets in the Habitable Zone to understanding Habitable Environments and finding Living Worlds

Model disk-averaged, broad-band of Earth exhibits 20% variability in reflectivity.

16-m 8-m 4-meter

~9 days 16-m 8-meter 4-meter

Red: Exposure times to yield 5% photometric precision for Earth-twin at 10 pc using 16-m, 8-m, and 4-m telescopes Blue: Exposure times to yield 5% photometric precision for Earth-twin at 20 pc using 16-m, 8-m, and 4-m telescopes Models: Ford, Seager, & Turner 2001; Palle et al. 2008; Oakley & Cash 2009; Fuji et al. 2010 Earth/Moon Image: EPOXI, Crow, et al. 2011 AAPT 2021 Winter Meeting 16 th C. 4 1584 1698 1755 1796 1952 19921995 1999 2009 20132015201720182021 2030 2050 BC time • Summary:

• Exoplanets are common You are here – There are as many planets as stars – Exoplanet systems are common: 1841 of 4307 confirmed planets are in multis – About 50% of Solar-like stars have a rocky habitable zone planet

• Characterizing Exoplanets is hard – Measuring masses is easiest for big, short-period planets around bright stars – Observing atmospheres requires big telescopes and is easier for big, nearby planets – Finding reliable signs of life requires big telescopes, detailed models, and many observations

AAPT 2021 Winter Meeting 17 REFERENCES time

Exoplanet census and occurrence rates: - Thompson, S. E., et al. Planetary Candidates Observed by Kepler. VIII. A Fully Automated Catalog With Measured Completeness and Reliability Based on Data Release 25. ApJS, 235:38, April 2018 - Burke, C. J., et al. Occurrence Rates for the Kepler GK Dwarf Sample. ApJ, 809:8, August 2015. - C. D. Dressing and D. Charbonneau. The Occurrence Rate of Small Planets around Small Stars. ApJ, 767:95, 2013. - Bryson, S., et al. The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data. To appear in AJ, 2020. arXiv:2010.14812 - NASA Exoplanet Archive: https://exoplanetarchive.ipac.caltech.edu

Whole-Earth Spectra: - C. Sagan, et al. 1993, A search for life on Earth from the Galileo spacecraft. Nature, 365:715–721. - P. R. Christensen and J. C. Pearl. 1997, Initial data from the Mars Global Surveyor thermal emission spectrometer experiment: Observations of the Earth. J. Geophys. Res., 102:10875–10880. - M. C. Turnbull, et al. 2006, Spectrum of a Habitable World: Earthshine in the Near-. ApJ , 644:551–559. - E. Pall ́e, et al. 2009, Earth’s transmission spectrum from lunar eclipse observations. Nature, 459:814–816. - C. A. Crow, et al. 2011, Views from EPOXI: Colors in Our Solar System as an Analog for Extrasolar Planets. ApJ, 729:130 - T. E. Livengood, et al. 2011, Properties of an Earth-Like Planet Orbiting a Sun-Like Star: Earth Observed by the EPOXI Mission. , 11:907–930. - T. D. Robinson, et al. 2014, Detection of Ocean Glint and Ozone Absorption Using LCROSS Earth Observations. ApJ, 787:171.

AAPT 2021 Winter Meeting 18 REFERENCES time

Exoplanet atmosphere detection: - R. Allart, et al. 2017, Search for water vapor in the high-resolution transmission spectrum of HD 189733b in the visible. A&A, 606:A144. - G. Fu, et al. 2017, Statistical Analysis of Hubble/WFC3 Transit Spectroscopy of Extrasolar Planets. ApJ, 847: L22.

Exoplanet atmosphere modeling: - T. D. Robinson, K. R. Stapelfeldt, and M. S. Marley. 2016, Characterizing Rocky and Gaseous Exoplanets with 2 m Class Space-based Coronagraphs. PASP, 128(2):025003. - T. D. Robinson, et al. 2011, Earth as an Extrasolar Planet: Earth Model Validation Using EPOXI Earth Observations. Astrobiology, 11:393–408.

- S. J. Dick, Plurality of Worlds, Cambridge University Press: Cambridge, 1982 - M. D. Lemonick, Mirror Earth, Walker & Company: New York, 2012 - J. Wenz, The Lost Planets: Peter van de Kamp and the Vanishing Exoplanets, The MIT Press: Cambridge, MA, 2019

AAPT 2021 Winter Meeting 19