An Update On Exoplanets

Philip Hughes

Department of Astronomy

University of Michigan

phughes@umich.edu

http://dept.astro.lsa.umich.edu/~phughes/

Recommended

https://eyes.nasa.gov/eyes-on-exoplanets.html

Exoplanet Basics

Extrasolar (Exo-) Planets➢ Metrodorus of Chios (forerunner of Epicurus)

wrote “It would be strange if a single ear of corn grew in a large plain or were there only one world in the infinite”

➢ Giordano Bruno burned at stake in 1600 for speculation about other inhabited worlds

➢ Bernard de Fontenelle speculated about Venusians, Jupiterians and planets about distant stars in 1686

History – A Century of Errors

➢ 1916: Barnard's star announced; “rapid” motion –only 6 light years away

➢ Peter van de Kamp at Swarthmore gets 2000 images from 1938 to 1962; these reveal a “wobble” due to a Jupiter-like planet

➢ v.d.K. publishes ever more refined results through 1982 – culmination of his life's work

History – A Century of Errors➢ From 1972 onwards, v.d.K.'s results receive

scrutiny by Gatewood, Eichhorn, Hershey, Harrington and Fredrick

➢ All fail to find a “wobble” except Hershey, using the same Sproul Obs. telescope – and he finds a “wobble” for all stars explored!! Telescope optics at fault!!

➢ v.d.K. dies in 1995 with issue unresolved; but few now believe in a planet around Barnard's star

History – The Revolution

➢ Gatewood (1996) makes tentative claim to finding “wobble” in Lalande 21185; still unconfirmed

➢ Dave Latham (1989) HD 114762 – brown dwarf?

➢ Pulsar planets (1992) – planets?

➢ 1994/5 – the radial velocity method triumphs:➢ Michel Mayor/Didier Queloz (Geneva Obs.) announce

planet about 51 Peg

➢ Geoff Marcy/Paul Butler (San Fransisco State) confirm this

➢ Both groups go on to dominate the early field of exoplanet discovery

The Tally To October 10, 2016

3533 exoplanets

2650 systems

595 multiple

systems

There are also

>pulsar

planets

>free floating

planets

>candidate

planets

(136 planets 2/6/05)

Why The Success?

➢ Significant improvements in spectrometers (for examining the spectra of star light)

➢ Better sensors for recording that light

➢ Better software for analyzing the data

Accelerated, more intensive, searches

–success breeds success

How? I.

➢ Two bodies orbit a common center of mass: if one object is invisible, you might still see the motion of the other

How? II.

➢ Depending on orientation in space, the observable object might move towards and away from you

➢ Sun moves at 13 m/s due to Jupiter – can now measure to a few m/s

How? III.

➢ This can be detected through the Doppler effect:

Footnote 1: Electrons In Atoms

Footnote 2: Spectra

How? IV.➢ The tiny motion of a star due a companion planet

requires “color” changes to be measured with exquisite precision

➢ In fact, you can't measure the color changes with enough accuracy

➢ We need markers on the spectrum

➢ We have markers; the absorption lines:

Orbits & Masses See binary applet

Period from variation in spectrum

Mass of star from spectral type

Kepler’s 3rd Law ⇒ planet orbit size

Speed of star from variation in spectrum

Size of star’s orbit about CoM = speed x period

Center of Mass concept ⇒ planet mass

Beware inclination! Masses are lower limits 12 MJ < MBD < 1/12 M

⊙

Early Years➢ Jupiter mass planets, close to their parent star,

were a great surprise

➢ Ever increasing sensitivity has revealed Earth-mass planets

687 Planets Found This Way

➢ From beyond the Solar System, the Sun outshines Jupiter by a billion, and the Earth by 10 billion

➢ We have the sensitivity, but....➢ compare viewing a firefly next to a search-light

➢ It's better in the infra-red: only a factor of a million!

➢ We have also found exoplanets using...

2692 By Transits (Eclipses)

➢ e.g., HD209458 detected by 1.7% dip in light from parent star

➢ 1. at just the time predicted from spectral discovery;

➢ 2. which is just the dip expected from a planet with 0.63 Jupiter masses, bloated by 60% due to heat from star

(Artist's impression.)

Kepler Mission

Launched by NASA, 2009

Photometer monitors brightness of >145,000 stars

Periodic dimming reveals planet(s)

Revolutionized field of study

51 By Microlensing, 72 By Direct Imaging!!

➢ GQ Lupi b – brown dwarf?

➢ 2M1207 b – orbits BD

➢ AB Pic b – BD?

More Detection Methods

If an exoplanet has a strong magnetic field, how might that be used to detect the planet?

Radio emission from aurorae!

LOFAR

Puffy Planets How do we get an exoplanet’s density?

Mass from radial velocity method

Radius from transit method

Kepler 7b: 0.5 MJ

“Puffy planets” or “hot Saturns” get heated by their parent star and/or some internal source

Maybe wind through magnetosphere produces internal electric current

Mega-Earths

Kepler 10 c

Mass: 15-19 ME

Radius: ~ 2.4 RE

Density 6000-8000 kg/m3

Rocky, few volatiles (ices), dense because of pressure – like Jupiter’s core, but without the surrounding H & He

Hot Jupiters

WASP 18 b:

Orbit is decaying due to tidal effects – in less than a million years

If tidal effects cause the Moon to move away from the Earth, why do they cause the objects to move together here?

Moon orbits Earth slowly – Earth is source of rotational energy

WASP 18b orbits in only 1 day, star spins slowly; orbit is source of rotational energy

Kepler Candidates 2012

Our Solar System & Planet Migration

Properties Of Solar System

➢ Orbits: almost circular orbits, in a plane, with most rotations and revolutions in the same direction

➢ Terrestrial/Jovian dichotomy

➢ Rock & Ice: why the asteroids and comets?

➢ Solar system wackiness: Earth's large moon.....tilt of Uranus

Formation of Solar System

➢ Well into C20th close encounter (Sun-star) was popular (cf tides). But it can't explain:➢ orbits

➢ terrestrial/Jovian division

➢ and it's highly unlikely to have happened! (and planet systems are common)

➢ Nebula Theory (Kant, Laplace, C18th) now accepted based on its ability to explain these properties

The Interstellar Medium

The Interstellar Medium

➢ The “space” between stars is filled with gas/dust(smoke), even the densest being a good vacuum on Earth, visible because of size

➢ The densest/coolest parts are the cores of Giant Molecular Clouds (molecular because protected from UV of nearby stars)

Collapse!

➢ GMC cores collapse because the are cold (10s K) and dense – gravity wins over pressure

➢ Collapse releases gravitation potential energy kinetic energy heat

➢ Thus temperature & pressure go up.... but density goes up, so gravity always wins

Collapse contd.

➢ The original nebula had some spin: conservation of angular momentum causes the cloud to spin up as it contracts

➢ Centripetal force holds up the cloud in the 'spin plane', but not along the axis, and collisions➢ allow collapse to disk

➢ order motions in the disk

A Good Theory!

➢ Orbits: almost circular orbits, in a plane, with most rotations and revolutions in the same direction!

➢ We can test the model because ➢ we can compute the consequences of such a scenario

and confirm the steps/end result

➢ we see IR emission from hot nebulae cores where stars are forming

➢ we see gas/dust disks

A Good Theory...?

➢ Terrestrial/Jovian dichotomy

➢ OK! The solar nebula contained ➢ 74% Hydrogen

➢ 24% Helium .....both from the “Big Bang”

➢ 2% everything else, including C, N, O, Ni, Fe, etc. .....from the life cycles of earlier stars

➢ Please explain!➢ fusion takes H/He and creates “heavy” elements

➢ a supernova spews those into the ISM, where it mixes in and enriches the ISM; no terrestrial planets around the first stars!

Terrestrial/Jovian Dichotomy

➢ Nebula core is dense – collapses fast and first to form Sun. Solar environment is hot; it's cooler further out.

Growth of Planets➢ Temperature determines what can condense (gas

to solid) to form seeds of planets at a given distance from Sun.➢ only metals (Ni, Fe) close in (only need T<1600K)

➢ metals, plus rocks (minerals) further out (some condense even at T=500K)

➢ metals plus rocks, plus ices (CH4, NH3, H2O) beyond the snow (frost) line (need T<150K)

Growth of Planets contd.➢ H & He never condense

➢ Planetesimals form by accretion – a growth by collision (electrostatic then gravitational force)

➢ Inside frost line we get rock/metal “seeds”. That's only 0.6% of all the stuff there. The planetesimals are small, and it's too hot to pull in H & He

➢ Outside the frost line we get rock/metal and ices– 2% of all stuff; planetesimals are larger, and it's cool enough to pull in H & He

Growth of Planets contd.

➢ Inside frost line: small metal/rock planets form –the terrestrials

➢ Outside the frost line: large bodies form – the Jovians; the heat from accretion turning the ices to gas➢ The gas giants are thus SS in miniature

➢ Moons (rings – see later) form about them from disks of dust/gas

➢ Later, asteroids etc. can be captured to complement the moon systems

So how to we get hot Jupiters????

Planet Migration

Migration

➢ Giants can exist close to a Sun-like star – they increase in size, but their gravity is more than sufficient for them to retain their gases

➢ However, they cannot form there

➢ They must form further out and migrate inwards

➢ How does migration happen?

Migration contd.➢ Type 1

➢ Earth mass planet drives spiral wave (cf. spiral galaxies)

➢ Gain/loss of angular momentum with inner/outer wave not in balance – planet loses AM

➢ Type 2

➢ Higher mass planets clear gap

➢ Material moves inward, trying to refill gap

➢ Planet + gap move in

Migration contd.➢ Type 3

➢ Similar principles

➢ Planet interacts gravitationally with vortices in disk

➢ Planet is “scattered”

Nice Model Of Solar System

Uranus

The big question: What stops migration?

Kepler Candidates 2012

The Solar System is an anomaly!

Exoplanet Atmospheres

The Rise Of Oxygen

➢ In the Earth’s early, Oxygen-free atmosphere, simple organisms would have been anaerobic;they were probably➢ chemoautotrophs – getting energy from inorganic

compounds➢ modern Archaea in hot springs get their energy from H/S/Fe

compound reactions

➢ Photosynthesis evolved from light absorbing pigments, that eventually allowed➢ photohetero(auto)trophs – getting energy from

sunlight➢ “blue-green” algae release oxygen

Oxygen contd.➢ Oxygen is highly

reactive:➢ Oxidizes surface rock & Fe

minerals in oceans➢ Rocks > 2 billion years old

have only 1% modern oxygen levels

➢ No more than 10% current until about 1 billion years ago

➢ Then reaches current levels

➢ Oxygen needs life!➢ O2/O3 in exoplanet

atmosphere will indicate life!

Detecting Exoplanet Atmospheres I

➢ First direct detection: David Charbonneau (Caltech/CFA; 2002) et al. using HST for HD 209458 b

Detecting Exoplanet Atmospheres II➢ Identify the planet

contribution to the spectrum from different (changing) Doppler shift

➢ Done 2012 by Matteo Brogi (Leiden) et al. for τ Boötis b using VLT in Chile

➢ Planet's light is only 0.01% the whole

Detecting Exoplanet Atmospheres III

To date, more than 50 atmospheres have been studied

We are just beginning to probe structure, as for planets in our Solar System: day/night temperature differences & vertical structure

We are beginning to probe composition: Sodium, Water, Carbon Monoxide, Methane

Extreme Example

HD 189733 b

In 2013 HST observations found deep blue color due to reflective clouds containing silicates

In 2007 Spitzer Space Telescope mapped temperature profile: T ~ 1000 K

Wind speeds ~ 2.7 km/s (~ 6000 mph)

⇒ A horizontal rain of semi-molten glass

Habitable Zones:Stellar & Galactic

Habitable Zone: Stars

Simple definition (liquid water) is naïve but practical

Application To Solar System

Venus: runaway greenhouse effect

Earth: warmer than without atmosphere, but… snowball Earth phase?

Mars: runaway glaciation (permafrost) without atmosphere, stripped by solar wind after loss of magnetic field?

Venus/Venera 9

Snowball Earth/650 My ago

Mars/Chaotic terrain/flood

Habitable Zone: Stars

Other factors: The HZ evolves

Close-in planets might tidally lock

Solar flares/CMEs more problematic in low mass stars

The planetary environment is modified……

The Ups & Downs Of Life➢ The fossil record shows that life has not followed

a simple, smooth progression of evolution; there have been major periods of mass extinctions

Extinctions➢ These may be due to numerous events, each of

which is not unique to Earth

➢ 1. impacts

➢ 2. loss of ozone

➢ 3. loss of magnetic field

➢ 4. Increase in cosmic ray flux?

➢ 5. climate change – anthropogenic & Milankovitch cycles

1. Impacts

➢ Falling debris starts world-wide fires

➢ Tsunamis propagate even 1000 km inland

➢ Dust remains in atmosphere for months –temporary global cooling – cessation of photosynthesis

➢ CO2 release causes a following global warming

➢ Noxious chemicals pollute atmosphere & oceans

general disruption of food-chain

Impacts – The Evidence➢ K-T event:

➢ world-wide layer:➢ Iridium etc.

➢ shocked quartz

➢ glass drops, soot

Impacts – Can We Survive?

Even modest events are dramatic

Chelyabinsk event

15 February 2013

Near Earth Asteroid

19 km/s (60 x sound), 20m, 12 kT

Explosion released 500 kT TNT equiv.

1500 people injured

Aside: King Tutankhamun’s Dagger

Made of iron in c. 1330 BCE, before Iron Age smelting!

Same proportions of Fe, Ni, Co as meteorite in Kharga Oasis

Ancients knew of “stones from the sky”

Ann HodgesAlabama1954

Aside: Death & Injury?

Probability Of Destruction

2. Ozone / 3. Magnetic Fields

➢ Ozone protects against damaging UV radiation; it's loss will increase mutation rate➢ Volcanism – indirect (catalytic aerosols)

➢ Magnetic field loss/supernova can provide p/e that promote reactions causing loss

➢ Magnetic field protects against ionizing particles; its loss will also increase mutation rate➢ Earth's field flips every one-tenth- to a few- million

years, with potentially long “drop outs” between polarities

Earth’s Dynamo

4. Increase In Cosmic Ray Flux

Habitable Zone: Galactic

Heavy elements: Need refractory elements (Iron, Nickel…) to form

terrestrial planets

Need radionuclides (40K, 235U…) to heat interior for tectonics/volcanism for atmosphere & dynamo generated

magnetic field to act as shield

Need elements for organic chemistry (CH4, H20…)

Habitable Zone: Galactic

Heavy elements: More common in inner galaxy

But excess of heavier elements might enhance seeding of gas giants, whose migration disrupts orbits of inner planets

Habitable Zone: Galactic

Catastrophic events: Supernovae produce damaging radiation; they are more

frequent in the inner Galaxy where there’s a high density of stars

Regions of higher Galactic tide might lead to higher rates of cometary impacts

Spiral arms contain molecular clouds, encounters with which might lead to higher rates of cometary impacts

Habitable Zone: Galactic

Search For Life Beyond Earth

Exoplanet- related Astronomical Techniques

(See also Solar System exploration, CETI, SETI & space travel)

Kepler Shadowgrams

The Kepler satellite is monitoring stars for the telltale periodic dimming of starlight as a planet transits

Suppose an alien civilization has constructed a light weight “gossamer” billboard orbiting their star; it's shape would be evident to us in the shadowgram:

(rotating triangle)

Shadowgrams contd. This could be used passively – for generations – or

Actively, like semaphore, sending information as binary digits

(louvered strip)

(multiple transits by groups)

What Is This?

Civilizations Need Energy

A data center can use as much as medium sized town

Globally, “data warehouses” use 30 billion watts –30 nuclear power plants; the USA accounts for about 1/4-1/3 of that

Up to 70% of the power is used for cooling/air handling

This is just one example of how an advancing civilization's energy use rises dramatically as technology develops

We Borrow Energy

Energy is not created or destroyed, it just changes form

eg, Potential ⇒ Kinetic ⇒ Heat (falling object)

eg, Electrical ⇒ Heat (electronics)

An advanced civilization with vast energy needs will generate a vast amount of waste energy

Use 'degrades' energy; we can expect the waste energy to show up as heat – radiation in infra-red (IR)

Kardashev Scale

Nikolai Kardashev, Russian astrophysicist, b. 1932

Type I civilization: utilizes as much energy as we do today (about 16 TW) Redefine? Use total insolation? At less than 1/10,000

that, we might be called Type 0

Type II civilization: capable of utilizing the entire energy output of its parent star

Type III civilization: capable of utilizing the entire energy output of its galaxy

Kardashev Scale contd.

Type I civilization: planet's energy supply; this might be achieved within centuries of the start of industry/technology

Type II civilization: energy output of its parent star; within millenia [Michio Kaku]

Type III civilization: energy output of its galaxy; within 100,000 to 1 million years

How can you tap into a star/galaxy's energy?

Dyson Sphere

Freeman Dyson, British/American physicist, b. 1923

Dyson Sphere

Dyson Shell: practical? drift w.r.t. star

no force holding interior biosphere

no known material is strong enough to withstand stress

not enough material in solar system for more than thin shell

Dyson Swarm (or Bubble or Net) independent energy traps/habitats; incremental

construction: won't get 100% star's light but maybe viable

Is The Time Frame Realistic?

Once a technology exists, it spreads on a time scale short compared with that of its context [see examples on next slide]

The Universe is 13.8 billion years old; the Earth formed 4.5 billion years ago, 9 billion years after the Universe; it's easy to imagine another star/planet/civilization that arose 8 billion years after the Big Bang, giving it a 1billion year 'head start' on us

The million year time frame suggested for utilizing a galaxy's energy is only 1/1000 this

Internet Access

Mobile Phones

A Search For Waste Energy

Jason Wright at Penn State PI

Funded by the John Templeton Foundation

Using WISE (Wide-field Infrared Survey Explorer): radiation from 1AU sized objects, 200-300K, in far IR

Search for 'astronomically anomalous' IR emission from the vicinity of (unseen?) stars (Type II civilization) and ditto from whole galaxies (Type III civilization) – a web of stars enshrouded in 'industrial' megastructures

Essential Points:

Exoplanets are common

Today we could detect Oxygen in exoplanet atmospheres, an almost certain

indicator of (maybe only primitive) life

Signals from “billboards” orbiting stars – an indicator of life a little more advanced than us

Evidence of vastly more advanced civilization via their waste energy

We don’t have to wait for ET to visit!