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Home , Nebular hypothesis

Better Worlds and the Prospects for Elsewhere What Says

Center for Lifetime Studies April 24 2019

Fred Chromey

Professor of Astronomy, Emeritus and Former Director, Vassar College Observatory Comparative Planetology: The Basics

What sorts of should we expect to find orbiting other ?

What sorts have been found? What sorts of planets should we expect to find?

Look at:

(1) Theory of formation—

Planets form from (solid) material leftover after formation

(2) Our —

Four types of world: INNER-----“” OUTER-----“Gas” Ice Theory of Star and Planet Formation A. The :

Star + Planets form together when a cloud of gas and contracts due to Conservation of means the cloud spins faster and faster as it contracts The condensationB. Distributed sequence condensation Solid grains form everywhere where is low enough

Refractory: Oxides of W, Ti, Al (>1300 K) Rock: Silicates of Fe, Mg, Mn, K, Na : Fe, Ni, Co : Ag, Au, Hg, S, FeS

Ices: H20, CH4, NH3 (< 200 K) Gas: H, He, Ne, Ar

Planets should be made of GAS, ICE, ROCK, and Abundance of atoms in Abundance of molecules in star-forming regions, by planet-forming regions: number: Reducing Oxidizing Conditions conditions -1 909,964 H He -4 88,714 2, (H2O) -16 477 H2O CO , CO Carbon-12 326 2 CH Nitrogen-14 102 4 SiO2,MgOx,FeOx Neon-20 100 N2 or NH3 Si, Mg, Fe 11 Ne Fe, Mg Solar system worlds

ROCK – , , , + their satellites, belt , : Interior: Silicate rock + metal (mostly iron)

Atmosphere: none, or outgassed: N2, CO2, H2O, Ar

Jovian (GAS) – and

Interior: Liquid H2 and He, small amount of rock, metal Atmosphere: Primordial H2 and He, minor constituents

ICE – and TNO’s. Satellites of Saturn, , .

Interior:, H2O and other ices, rock + metal Atmosphere: none, or outgassed: N2, CH4

“NEPTUNES” – Uranus and Neptune

Interior: Liquid H2 and He, H2O and other ices, rock, metal Atmosphere: Primordial H2 and He, minor constituents Snow Line Titan D=0.4

Jupiter M D=11.1 R Earth H20 D=1.0 G

Neptune M D= 3.9 R H2O G M R H20 G

Hot Cold

Exoplanets could be other very different from the kinds in our Solar System:

Dessert and lava worlds – No water

Water worlds – Mostly H2O

Hot

Metal worlds

Super- Habitable Exoplanets?

Our prejudice is for “Earth-like” planets

Temperature: Habitable planets will have a surface temperature such that water will be liquid there.

Look for planets in a star’s “habitable zone”

Composition: Habitable planets will be made of rock and metal And some surface water(like the Earth) or, possibly, mostly water -

Look for planets made of rock and metal and/or water The habitable zone:

The set of planetary orbits around a star in which liquid water can exist on the surface of an “Earth-like world.” The answer to the Goldilocks question. from the star is very important in determining an EP’s surface temperature

But other factors also determine climate. Better models must account for these:

• EP surface reflectivity (albedo)

• EP atmosphere – surface – stellar flux interactions are CRITICAL e.g. Earth’s greenhouse effect makes surface 33°C hotter than if it were airless – 2/3 of greenhouse

warming is due to H2O.

• EP orbit details, spin, stellar variation

H2O cycle on earth: A positive feedback loop

Atmosphere H2O responsible for 2/3 of greenhouse warming

Ice/snow Ocean (High albedo)

Freezing/melting HZ boundaries depend on star’s and temperature Inner boundary of the HZ Venus – runaway greenhouse effect:

Nitrogen Ocean Atmosphere Cool Rock Surface

Humid Warm Rock Atmosphere Surface

MoltenOcean Steam Rock Atmosphere Surface Outer boundary of the HZ

There WAS liquid water on Mars

Fossil river delta d Holden crater Early (> 3.7 Gyra) Mars was habitable – (Thicker atmosphere, liquid water with low acidity & sources of N, C, S, P)

It is still unclear how long Mars was habitable.

It is even more unclear that Mars was ever inhabited But: Active methane, organic carbon in rocks

Even on Earth, the evidence of early life (>3.5 Gyra fossil cellular structures in silica) was very hard to find. HZ will change as star evolves.

Empirical HZ for the Solar System: • “Recent Venus” sets inner boundary (steam atmosphere) • “Early Mars” sets outer

boundary (CO2 atmosphere)

3D Model HZ’s use models of an ELEP to compute inner and outer boundaries. Uncertainties: clouds, other gasses in atmosphere, tectonics, spin, etc. Habitable Exoplanets?

Our prejudice is for “Earth-like” planets

Temperature: Habitable planets will have a surface temperature such that water will be liquid there.

Look for planets in a star’s “habitable zone”

Composition: Habitable planets will be made of rock and metal (like the Earth) or, possibly, mostly water -

Look for planets made of rock and metal and/or water Is a detected in a habitable zone?

2 2p P2 = ( ) r3 G( M + m)

Kepler’s Third Law of Planetary Motion

Orbit period gives orbit radius –> stellar flux –> surface temperature Is a detected exoplanet made of rock-metal-water?

Mean density = Mass/Volume

Exoplanet mass and radius –> density –> interior composition. Note uncompressed densities:

Mercury 5300 Venus 4400 Earth 4400 Mars 3800 4.0 Cold H2 + He U N

3.5 H O 2

3.0

Rock Radius 2.5 Radius of Earth 50-50 Rock + Iron

2.0

Iron 1.5

E 1.0 V

0.5 1 2 5 10 20 Mass Mass of Earth What’s been found so far?

How? Detection Methods 4048 Planets (20 Apr 2019)

•Direct images (124)

•Gravitational Microlensing (90)

•Photometric signals (2939) •Transits •Timing

•Dynamical effects (833) • •

+ About 2500 additional “candidate” planets Finding Exoplanets: Most productive method is to Monitor stars for planetary transits

Ground-based: 75+ projects

Space-based: CoRoT, Kepler, TESS, CHEOPS HD 209458 COROT – .25 m space telescope Kepler Telescope (.95 m

Launch 2009

End of primary mission May 2013

End of K2 Oct 2018

2342 confirmed EP’s TESS

Transiting Exoplanet Survey Satellite Observations began 25 July 2018 Strategy:

Four small telescopes —> Wide Field (24° x 96°) —> 200,000 Bright stars

Bright stars —> Easy follow-up

Bright stars —> Nearby stars

Nearby stars—> Red/Orange dwarves —> many planets

Currently 570 candidates (April 2019)

Limitations for transit detections

Need i = 90° Small signal (1% for Jupiter, 0.008% for Earth) Large noise ground - 0.5% space - 0.003% (CHEOPS will be .001 %) False positives (star spots, stellar companions) Long wait (3 yrs for Earth, 36 yrs for Jupiter)

Information from transit observations ? Planet radius Orbit period, Orbit major axis, inclination Guess – Planet Surface Temperature Earth as a transiting exoplanet: Transit duration and depth, in percent For stars of different spectral type (Temperature)

Type Temp (G2) 5770 12.6 hrs 0.0084 K5 5280 9.0 hrs 0.010 M0 3850 5.4 hrs 0.022 M9 2410 0.4 hrs 1.90

Radial Velocity Detections The Doppler Effect

Radial velocity time series of a star with an unseen companion Planet information from radial velocity cycle of star – Much more difficult observation than transit: favors massive planets in close orbits •Period of orbit

•Size of orbit

•Lower limit to orbital velocity

•Eccentricity of the orbit

•Lower limit* to mass of planet

* Exact mass if inclination of orbit is known Mean density = Mass/Volume

Need BOTH transit (planet volume) and RV (planet mass) detection

Earth-like EP’s in the Habitable Zone

Transiting

Radial Velocity

Proxima Cen b: 11.7 day orbit,

Flux = 0.65 solar, minimum mass 1.3 Earth

Issues with habitablity

cold – liquid H2O only in tropics or sub-stellar point tides – probably tidally locked to star flares – 30x EUV and 250x x-rays as Earth Proxima Centauri b is not Earth 2 TRAPPIST-1 – 12 pc (39 lyr) distant , at least 7 EP’s, 3-4 in HZ. Masses from, interactions, all planets are Earth or super-Earth size, all are rock/iron.

For an average star, how many Earth-like planets are in the HZ?

“Solar type” F,G,K stars: 0.1-0.3

Red dwarves, M stars: 0.3-0.7

There are 250 billion stars in our . Most are M stars.

75% of the stars near the sun are M stars. Characterizing Habitable Planets

Next 10 years will see the technology that will permit

(a) determination of some basic properties of individual EP’s 1. atmosphere present? 2. radius of non-transiting EP 3. actual surface temperature 4. atmosphere chemical composition

(b) better understanding of limits to HZ

(c) detection of biomarkers Characterizing Habitable Planets

Orbital variations in brightness can test for an atmosphere

Broad color measurements matched against SS objects could give clue about character

Absolute IR flux from non-transiting exoplanet will allow an estimate of its radius and surface temperature (brighter -> bigger)

Low resolution IR spectrum gives the EP surface temperature, if the EP atmosphere is transparent. (bluer -> hotter)

Higher resolution spectra will give EP atmosphere composition Planet absorbs visible light, but emits Infrared light.

Amount of IR emitted —> size of planet

Variation of IR emitted —> presence of atmosphere

Annual Reviews Low resolution IR spectrum gives the EP surface temperature, if the EP atmosphere is transparent. (bluer -> hotter) Higher resolution spectra will give EP atmosphere composition

Note nitrous

oxide + O2

Emitted spectra of Venus, Earth and Mars Annual Reviews Life itself? Bio-signatures in an exoplanet’s atmosphere or surface

Potential chemical biomarkers are:

O2, O3, CH4, N2O (nitrous oxide), CH3Cl(Methyl chloride)

Most secure atmospheric biomarkers are:

O2 and CH4, in the same atmosphere N2O and a reducing gas, in the same atmosphere

Surface vegetation red edge at 700 nm Terrestrial Oxygen Cycle 10,000 yr time-scale

Bio-signatures on a transiting exoplanet’s surface

Vegetation red edge at 700 nm Outlook

Many more candidates in HZ

More secure determination of candidates’ mean density (rock vs water vs ?) Radii of non-transits from IR brightness

First crude attempts at characterization with JWST, ELT’s: colors, surface temps, some atmospheric characterization, with a chance at biomarkers.

Extensive campaigns on nearby HZEP’s around red stars Alpha Cen C b (M5 V) TRAPPIST-1 d,e,f (M9 V) Space missions Gaia – in orbit (Proper motion) TESS – in orbit (Transits) CHEOPS –Launch soon 2019 (Transit of RV detected systems) JWST – Launch 2022 (imaging in infrared) ARIEL – Launch 2024 (Exoplanet atmospheres)

Ground-based facilities LSST – 2020 wide angle survey GMT –2023 24.5-m aperture E-ELT – 2024 39-m aperture TMT –2025 30-m aperture