Better WorldsExoplanets and the Prospects for Life Elsewhere

What Astronomy Says

Center for Lifetime Studies

April 24 2019

Fred Chromey

Professor of Astronomy, Emeritus andFormer Director, Vassar College Observatory

Comparative Planetology: The Basics

What sorts of planets should we expect to find orbiting other stars?

What sorts have been found?

What sorts of planets should we expect to find?

Look at:

(1) Theory of planet formation—

Planets form from (solid) material

leftover after star formation

(2) Our solar system—

Four types of world:INNER-----“Rock”OUTER-----“Gas”

IceNeptunes

A. The Nebular Hypothesis:

Theory of Star and Planet Formation

Star + Planets form together when a cloud of gas and dust contracts due to gravity

Conservation of angular momentum means the cloud spins faster and faster as it contracts

Solid grains form everywhere where temperature is low enough

Refractory: Oxides of W, Ti, Al (>1300 K)Rock: Silicates of Fe, Mg, Mn, K, NaIron: Fe, Ni, CoVolatiles: Ag, Au, Hg, S, FeSIces: H20, CH4, NH3 (< 200 K)Gas: H, He, Ne, Ar

The condensation sequenceB. Distributed condensation

Hydrogen-1 909,964Helium-4 88,714Oxygen-16 477Carbon-12 326Nitrogen-14 102Neon-20 100Si, Mg, Fe 11

Abundance of atoms in star-forming regions, by number:

ReducingConditions

H2, He

H2O

CH4

N2 or NH3

Ne

Fe, Mg

Oxidizingconditions

(H2O)

CO2, CO

SiO2,MgOx,FeOx

Abundance of molecules in planet-forming regions:

Planets should be made of

GAS, ICE, ROCK, and METAL

Solar system worlds

ROCK – Mercury, Venus, Earth, Mars + their satellites, belt asteroidsEuropa, Io:Interior: Silicate rock + metal (mostly iron)Atmosphere: none, or outgassed: N2, CO2, H2O, Ar

Jovian (GAS) – Jupiter and SaturnInterior: Liquid H2 and He, small amount of rock, metalAtmosphere: Primordial H2 and He, minor constituents

ICE – Pluto and TNO’s. Satellites of Saturn, Uranus, Neptune. Interior:, H2O and other ices, rock + metal Atmosphere: none, or outgassed: N2, CH4

“NEPTUNES” – Uranus and NeptuneInterior: Liquid H2 and He, H2O and other ices, rock, metalAtmosphere: Primordial H2 and He, minor constituents

EarthD=1.0

M

R

H2O

G

JupiterD=11.1

TitanD=0.4

M

R

H20

G

NeptuneD= 3.9

M

R

H20

G

Snow Line

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 Jupiters

Metal worlds

Super-Earths

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 metalAnd 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.

Flux 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

OceanIce/snow(High albedo)

Freezing/melting

H2O responsible for 2/3 of greenhouse warming

HZ boundaries depend on star’s luminosity and temperature

Inner boundary of the HZ

Venus – runaway greenhouse effect:

OceanNitrogenAtmosphere

HumidAtmosphere

OceanSteam Atmosphere

Cool Rock Surface

Warm RockSurface

Molten Rock Surface

Outer boundary of the HZ

There WAS liquid water on Mars

Fossil river deltaHolden crater

d

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

P2 =2p( )

2

G M +m( )r3

Kepler’s Third Law of Planetary Motion

Is a detected Exoplanet in a habitable zone?

Orbit period gives orbit radius –> stellar flux –> surface temperature

Mean density = Mass/Volume

Exoplanet mass and radius –> density –> interior composition.

Is a detected exoplanet made of rock-metal-water?

Note uncompressed densities:

Mercury 5300 Venus 4400 Earth 4400Mars 3800

Cold H + He2

2H O

Rock

Iron

50-50 Rock + Iron

RadiusRadius of Earth

MassMass of Earth

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5 1 2 5 10 20

U

N

EV

What’s been found so far?

How?

Detection Methods4048 Planets (20 Apr 2019)

•Direct images (124)

•Gravitational Microlensing (90)

•Photometric signals (2939)•Transits•Timing

•Dynamical effects (833)•Astrometry •Radial velocity

+ 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 missionMay 2013

End of K2 Oct 2018

2342 confirmed EP’s

TESS

Transiting Exoplanet Survey SatelliteObservations 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 radiusOrbit period, Orbit major axis, inclinationGuess – Planet Surface Temperature

Earth as a transiting exoplanet:Transit duration and depth, in percentFor stars of different spectral type (Temperature)

12.6 hrs 0.00849.0 hrs 0.0105.4 hrs 0.0220.4 hrs 1.90

TypeSun (G2)K5M0M9

Temp5770528038502410

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

•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,

Issues with habitablitycold – liquid H2O only in tropics or sub-stellar pointtides – probably tidally locked to starflares – 30x EUV and 250x x-rays as Earth

Flux = 0.65 solar, minimum mass 1.3 Earth

Proxima Centauri bis 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 galaxy. 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’s1. atmosphere present?2. radius of non-transiting EP3. actual surface temperature4. 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

Annual Reviews

Planet absorbs visible light, but emitsInfrared light.

Amount of IR emitted —> size of planet

Variation of IR emitted —> presence of atmosphere

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

Emitted spectra of Venus, Earth and Mars

Higher resolution spectra will give EP atmosphere composition

Note nitrous oxide + O2

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 atmosphereN2O 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 starsAlpha 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 facilitiesLSST – 2020 wide angle surveyGMT –2023 24.5-m apertureE-ELT – 2024 39-m apertureTMT –2025 30-m aperture