Exoplanets. II Sensitivity to Exoplanets Extrasolar Planet Detectability What Have We Found? 4122 planets in 3063 systems as of 10/25/19 (http://exoplanet.eu/ ) 4084 confirmed planets (through 10/24/19) 3312 unconfirmed candidates (http://exoplanetarchive.ipac.caltech.org)
Detected by • radial velocity or astrometry (about 20%) • Transiting planets (about ¾) • Microlensing: • Imaging: • Timing:
First Extrasolar Planet: 51 Pegasi b
Radial velocity/ Doppler Shift Reported 6 October 1995 Center of the Solar System Upsilon Andromedae
Periods: 4.6, 241, 3848 days Doppler Wobble: Gliese 876
The three planets of Gl 876:
masses = 2.5 MJ, 0.8 MJ, and 7.5 M⊕ Gliese 876
M4V star 3 planets, including the least massive known (0.75 M⊕) Direct Imaging Example: HR 8799
Jupiter is at 5 au Why young planets are bright
Source: A. Burrows Beta Pictoris ~9 au from star
M: ~ 7 MJ P: ~20 yrs T: ~1600K Transit Example - Ground Transit Example - Kepler The Search for Earth-like Planets
• 0.95m telescope • Launched March 2009 • Observe 105 square degrees of the sky • Observe ~ 100,000 stars • Observe continuously for 4.5 years (got 3.5) -6 • Photometric accuracy: ~10 (0.1 R⊕)
Kepler Kepler Focal Plane Kepler Field Kepler Field TESS • Transiting ExtraSolar planet Survey • To date: 29 confirmed planets 1183 candidate planets Kepler / TESS Timing • Hot Jupiters: P2 (years) = a3(AU)
Microlensing Example
Extrasolar Planets Orbits Orbital Eccentricity Extrasolar Planets Orbits Extrasolar Planets Orbits Extrasolar Planets Masses Planetary Mass Distribution Extrasolar Planets
Planets are preferentially found around metal-rich stars - mostly younger than the Sun. Metallicities updated
Extrasolar Planetary Systems
55 Cancri (G5V): 5 planets
• 1 MU 0.4 au • 1 MJ 0.15 au • 1 Ms 0.25 au • 0.5 MJ 0.8 au • 4 MJ 5 au
Tatooine TRAPPIST 1
Discovered by the Transiting Planets and Planetesimals Small Telescope (TRAPPIST)
2 Planets reported May 2016 5 more planets reported 22 February 2017 (from the Spitzer Space Telescope)
TRAPPIST 1 • A small star – about the size of Jupiter • An ultra-cool dwarf (M8) – 2560 K (Sun is 5800 K) TRAPPIST 1 Transit observations
TRAPPIST 1
Inferred planetary parameters: – Density [ Mass/([4π/3]radius3) ] -> composition – Incident flux -> surface temperature TRAPPIST 1 Planets in Context TRAPPIST 1 System
Star: M8V, Teff ~ 2500K, 0.08 M¤, Size of Jupiter 40 light years from Earth TRAPPIST 1 planetary atmosphere modeling Starting with a solar-system like planetary atmosphere: • 1b: hotter than Venus • 1c, 1d: energy input more than Sun on Earth: could be Venus-like. • 1e: most likely to have surface liquid water All could be Venus-like due to enhanced UV
H2O dissociation Reference: Lincowski, A.P. et al., 2018 Nov. 1, Astrophysical Journal TRAPPIST 1 Caveats
Three planets are in the habitable zone • “In the habitable zone” does not mean inhabited • “In the habitable zone” does not necessarily mean habitable • Planets are probably tidally-locked
Barnard’s Star Redux Planet reported November 2018 (Ribas et al.) Technique: radial velocities
• Mass > 3.2 M⊕ • Separation: 0.40 au • Near the frost line • Period: 233 d • e: 0.3 Exoplanet Summary • No Solar System-like systems found – Many systems more compact than SS • 10% Dominated by hot Jupiters – Densities consistent with gas giants – Sodium and Hydrogen have been detected • Water Worlds • Super-Earths • Many eccentric orbits • Biases are important Exoplanet Comparison How do you make a hot Jupiter? Existing picture of SS formation needs some changes • Nebular theory predictions formation of other SS – Suggests more planets form around metal-rich stars. – Jovian planets should be far from star in circular orbits • Revision – Jovian planets formed far from star in circular orbits – Subsequently migrated inward Planetary Migration
Occurs in the presence of protoplanetary disk • Planet moving through disk creates density waves • Waves exert gravitational force on planet • Planet loses orbital energy, moves toward star • Some stars show evidence of consuming planets.
• What about Jupiter? The Planetary Shuffle
• Gravitational encounters → eccentric orbits – Two Jovian planets get close: 1 ejected, one spirals inward, elliptical orbit – Small planetesimals ejected (to Oort cloud); Jovian planet loses orbital energy • Happened in our SS • Resonances – Lead to eccentric orbits – Can yield migration or ejection
Habitability
• Does habitable imply inhabited? Consequence for Drake’s Equation • Earth-like planets unlikely in systems with a hot Jupiter (10%)
• Add term ft (terrestrial planets) = 0.9
N = N* fs fGHZ fp ft nh fl fJ f fEu fm fi fc L/T N = 28,800 • .