Proxima b: The Alien World Next Door - Is Anyone Home? Edward Guinan Biruni Observatory Dept. Astrophysics & Planetary Science th 40 Anniversary Workshop Villanova University 12 October, 2017 [email protected] Talking Points i. Planet Hunting: Exoplanets ii. Living with a Red Dwarf Program iii. Alpha Cen ABC -nearest Star System iv. Proxima Cen – the red dwarf star v. Proxima b Nearest Exoplanet vi. Can it support Life? vii. Planned Observations / Missions Planet Hunting: Finding Exoplanets A brief summary For citizen science projects: www.planethunters.org Early Thoughts on Extrasolar Planets and Life Thousands of years ago, Greek philosophers speculated… “There are infinite worlds both like and unlike this world of ours...We must believe that in all worlds there are living creatures and planets and other things we see in this world.” Epicurius c. 300 B.C First Planet Detected 51 Pegasi – November 1995 Mayer & Queloz / Marcy & Butler Credit: Charbonneau Many Exoplanets (400+) have been detected by the Spectroscopic Doppler Motion Technique (now can measure motions as low as 1 m/s (3.6 km/h = 2.3 mph)) Exoplanet Transit Eclipses Rp/Rs ~ [Depth of Eclipse] 1/2 Transit Eclipse Depths for Jupiter, Neptune and Earth for the Sun 0.01% (Earth-Sun) 0.15% (Neptune-Sun) 1.2% (Jupiter-Sun) Kepler Mission See: kepler.nasa.gov Has so far discovered 6000+ Confirmed & Candidate Exoplanets The Search for Planets Outside Our Solar System Exoplanet Census May 2017 Exoplanet Census (May-2017) Confirmed exoplanets: 3483+ (Doppler / Transit) 490+ Multi-planet Systems [April 2017] Exoplanet Candidates: 7900+ orbiting 2600+ stars (Mostly from the Kepler Mission) [May 2017] Other unconfirmed (mostly from CoRot)Exoplanets ~186+ Potentially Habitable Exoplanets: 51 (April 2017) Estimated Planets in the Galaxy ~ 50 -100 Billion! Most expected to be hosted by red dwarf stars Nomad (Free-floating planets) ~ 25 - 50 Billion Known planets with life: 1 so far. http://phl.upr.edu/projects/habitable-exoplanets-catalog Habitable Zones dG Outer HZ Edge 2AU dK Outer HZ Edge dM 1AU 0.8AU Inner HZ Edge Earth-equiv Pos. Outer HZ Edge 0.5AU 0.2AU Inner HZ Edge Earth-equiv Pos. 0.3AU 1AU Earth-equiv Pos. 0.14AU Inner HZ Edge 0.1AU * Earth not drawn to scale Liquid Water Habitable Zones for mid-dM, -dK and -dG star Note that the HZs of dM-stars are located <0.3 AU from the host star. [1 AU = 150 million km] Living with a Red Dwarf www.astronomy.villanov a.edu/livingwithareddwa rf/opener.htm Program supported by grants from NASA (HST & Chandra) & NSF/RUI. The original “Living With a Red Dwarf” Program logo (not NASA approved) Physical properties of dM0-8 Stars compared to the Sun. Living with a Red Dwarf Do Red Dwarfs make “Friendly” Host Stars for Life-Bearing Planets? Photometry of stars with ages: Rotation, star spots, cycles: Age-Rotation Relations. Study effects of X-UV radiation (photoionization / photo-dissociation) on hosted planets’ water and atmospheres (volatiles). X-UV radiation & winds can erode (eliminate) planet atmospheres. As part of the Living with a Red Dwarf Program we have been collecting data on the properties of nearby stars (within 30 Ly; ~245 dM stars) that factor into their suitability as hosts of Potentially Habitable Planets. These factors include X-ray+ UV emissions, rotation / starspots, flare rates and ages. Rotation-Age-Log (Lx) Relations (Ages from Clusters, MGs, Wide Binaries, Space Motions) Proxima Cen: Prot = 83.4 +/-1.2 d (from 11-yrs of time-series Photometry. Age: 5.4+/-0.4 Gyr (age from alpha Cen A) Alpha Centauri Star System 4.3 LY (~25.4 trillion km) 1.0 LY = 5.9x1012 miles (=5.9 trillion miles) = 9,625 billion km The alpha Centauri star system – compared to the Sun (depicted to scale) Alpha Cen A & B is an eccentric 79.9-yr binary system- Spectroscopic Doppler studies indicate no Planets (upper mass limit of >2.0 Mj). Many theoretical studies indicate that planets are unlikely due to binary star tidal interactions…But … alpha Cen Bb ex-Exoplanet Earth-size planet reported in 2012 P = 3.236 d A = 0.04 AU K = 0.51+/- 0.04 m/s Min. Planet mass (Mp): 1.13 Mearth T = 1800- 2200 K (~1500 – 1900C) Dunusque et al. Nature 2012 Too Hot for LIFE even If it existed. (recent studies do not Support the existence of this planet) Proxima Centauri-The Nearest Star: --- Red Dwarf Distance= 4.25 LY; Mass = 0.123 Mo, R = 0.14 Ro; T = 3050 K; L = 0.0015 Lsun, HZ = 0.03 - 0.08 AU Anglada-Escude’ et al. (2016) report the an Earth-size HZ planet. 11.1 mag Proxima Centauri Red Dwarf Program Star since 2005 M5.5 V star) Compared to the Sun and Jupiter From our study Proxima is ~5.4+/-0.5 Gyr old and has a rotational period of 83-d & ~15% spot covered, high levels of magnetic activity: ~1 major flare/30 hrs. Proxima Centauri b Announced Aug. 25, 2016 Nature Letter doi:10.1038/nature19106 A terrestrial planet candidate in a temperate orbit around Proxima Centauri Guillem Anglada-Escude’ et al. 2016 Period Analysis of spectroscopic radial velocity observations of Proxima from Anglada-Escude’ et al. 2016. A definite period of 11.186 days is found arising from reflex motion of 1.4 m/s (~5 kph = ~3 mph) from the planet Proxima b Proxima b Keplerian fit Proxima b Period (d) 11.186 Doppler amplitude: 1.38 m/s [5 km/hr / 3 mph] Derived quantities Orbital semi-major axis, a: 0.049 AU Minimum mass, mpsini (M⊕) 1.27 (1.10–1.46) Equilibrium temperature: 234K (-39 C) With a Greenhouse effect of 50 C: T = +11 C (similar to Earths mean temperature of 15 C) Irradiance compared with Earth 65% Geometric probability of transit about 1.5% Transit depth (Earth-like density) about 0.5% [From Anglada-Escude’ et al. 2016] HZ~ 0.04-0.09 AU Sept. 2016 http: //phl.upr.edu/projects/habitable-exoplanets-catalog Loss of water from X- Comparison of the Spectral X-Near-IR UV Stellar radiation Irradiance of Proxima b and Earth (from Ribas et al. 2016) X-UV Radiation Dissociates water: Ly-alpha H2O -> 2 H + O 1216 A FUV radiation ionize H -> H+ + e- Stellar winds drag H+ ions away (if no protective geomagnetic field) via ion pick-up mechanisms. If atmosphere is lost, X-ray EUV FUV NUV UV- VIS - NIR later outgassing can lead to a CO2 rich atmosphere High-energy spectral irradiance received by Prox b and the Earth. The integrated values are calculated per unit wavelength. From Ribas et al. (2016) Proxima b X-ray EUV ~250 x 50-30 X Earth FUV 15-30 X High-energy spectral irradiance received by Prox b and the Earth. The values are calculated per unit wavelength. From Ribas et al. (2016) Effect of the intense solar wind of a young, active solar-type star on the Magnetosphere of a nearby tidally locked planet (X-UV/ Wind Flux data from Sun in Time program) To star-> Weaker Winds from an older Sun Strong Wind from young star From Griessmeier, Guinan et al. 2004, AA, 425,Griessmeier 753. et al. 2004 during From Lammer et al. 2008 , Space Sci. Rev –Atmospheric Escape and Evolution of Terrestrial Planets and Satellites Proxima b: Climate for Synchronous Rotation Possible Outcomes for Proxima b Star’s properties are well known but initial conditions of planet -such water inventories and geomagnetic fields are not constrained. As shown by Ribas et al. (2016), Prox b has endured very high radiation levels especially when the star was young. This makes outcomes uncertain. Proxima Cen b: Possible Outcomes 1. Low initial water inventories / weak geomagnetic fields: atmosphere /water Outcome: like Mercury (dry barren planet) or Mars (cold, dry /ice planet) if Water not resupplied. 2. Weak geomagnetic fields; low – moderate initial water inventory. Young M5 V star’s strong X-UV radiation & wind – produce photolysis's H2O–> 2H + O. Two outcomes: a thick (>100 Bar) O2 rich atmosphere or if all H & O lost; later outgassing of CO2 –> all (or most?) water lost -a Venus-like planet. 3. Abundant initial water (or water resupplied by comets) with a strong protective geomagnetic field: Water remains or is resupplied by comets – could result in a Water World (like Earth) or Ice world (depending on outgassing). 4. Planet forms further from the host star and migrates inward. This could avoid / diminish the higher luminosity and strong X-UV effects of the young host star. Water - Ice planet if it has sufficient water / strong geomagnetic fields Possible Evolutionary Outcomes for Proxima b Venus -Type Planet Thick CO2 Atmosphere Dense Iron Planet Beyond Proxima Cen b: Investigating the next nearest Potentially Habitable Exoplanets: Kapteyn b (13 Ly) & Wolf 1061 c (14 Ly) – Assessing their Suitability for Life Edward Guinan Scott Engle Villanova University Ignasi Ribas I.C.E. Barcelona, Spain Nearest Potentially Habitable Planets Property Earth-Sun Proxima b Kapteyn b Wolf 1061 c [1.0 AU] [0.049 AU] [0.17 AU] [0.083 AU] Mass (ME) 1.00 >1.27 >4.80 >4.25 Radius (RE) 1.00 1.1 ~1.7 ~1.6 S/SE 1.00 0.65 0.40 0.60 Teq(K) 255 [288] 227 [277] 213 [263] 223 [273] <fx> 0.67 c.g.s 163 5.4 ~30 [243 x Earth] [8.1 x Earth] [45 x Earth] <f Ly-alph> 7.0 130 12 ~50 (FUV) [18.6 x Earth] [1.7 x Earth] [7.1 x Earth] S/SE = instellation relative to Earth-Sun at 1.0 AU <f> mean irradiances [flux units: c.g.s = ergs/s/cm2] X-ray and Ly-alpha Irradiances on Earth: 0.67 and 7.1 ergs/s/cm2 TRAPPIST-1 Planetary System 39 Light years away contains 2 Earth size planets In the liquid water habitable zone.