The Search for Extrasolar PlanetsSince it appears the conditions for planet formation are common, we’d like to know

how many solar systems there are, and what they look like.

Indirect Methods:1) Doppler shift of the star’s orbit this is the main one so far2) Astrometric wobble of the star’s orbit

Semi-direct Methods:1) Transits (light blocked by the planet)

might also see phases2) Microlensing (planet’s gravity)

Direct Methods:1) Planet imaged directly (perhaps with coronograph)

reflected or emitted (IR or radio) light 2) Planet imaged by interferometer

AstrometryThis works best for large orbits (which take a long time) and stars that are nearby. Interferometry would allow very small motions to be measured.

Precision Radial Velocity Searches

Shift is1 part in100million

Discovery of Extrasolar planets

We get the orbital period, semimajor axis, and a lower limit on the mass of the planet. This can only do giant planets relatively close in (but could see Jupiter).

A Big Surprise : Close-in Jupiters

It is easiest to find a massive planet that is close to the star (it repeats quickly and has a large velocity amplitude). The first discovery, 51 Peg, had a 4 day orbit (0.05 AU!) and the mass of Jupiter. Many are now known, but that doesn’t mean they are most common, just easiest to find (and present in some numbers).

Properties of the systems

foundAnother surprise was that many of the orbits are eccentric (like binary stars). In a few cases, there are several planets.

How did the close Jupiters get there?1) They could have been dragged there by the accretion disk.

Corollary : some planets fall into the star!2) They could have gotten there by interacting with another planet.3) They could have formed there (direct collapse mechanism?)

TransitsWe can watch for the dimming of the star if the planet crosses in front of it. This is by the ratio of their areas: 1% for Jupiter and 0.008% for the Earth. This has been seen for one case (confirming the radial velocity detections).HST measurement of HD209458

The Kepler ProjectTransits provide the only way right now that we can reliably study the occurrence of extrasolar terrestrial planets (none known now).

Primary Mirror

CCD’s

Schmidt Corrector

ThermalRadiator

ElectronicsSunshade

A wide-view telescope monitors 100,000 stars in a single field for >4 years to detect Earth-size planets

•Finds hundreds of terrestrial planets within 2 AU of stars•For Earth-size and larger planets, determines:

–Frequency –Size distribution –Orbital distribution –Association with stellar characteristicsLaunched 2007?

“Microlensing” : Gravitational lenses

In principle, this method could even see Earth-mass planets. You have to have a huge and long-time monitoring program with high time resolution and good photometric precision.

The downside is that you will only detect the planet once, and can’t learn anything more about it. One tentative detection has been claimed (but how to confirm it?).

The Problem with Direct Imaging

1) The host star is FAR brighter (106) than any planet (except very young Jupiters in the infrared).

2) The planet is VERY close in angle (micro-arcsecs) to the star, so any stray light from the star can overwhelm the light from the planet.

Reflected light

Thermal emission

Nulling Interferometry

You can try to keep the star at a destructive null fringe, while the planet will be slightly off the fringe and so still visible. Might be able to reduce the star’s brightness by a million times?

Interferometric Missions

Darwin

Terrestrial Planet Finder

Perhaps a decade from now we will be able to directly image older extrasolar giant planets.

Search Methods : what

they can find

Eventually, imaging terrestrial planets?

Even if we can just get a spectrum, we might be able to detect life.

The Elements of Life• Organic Chemistry

– By definition, involves H,C,N,O• Most common elements (produced by most stars)• Well dispersed and available

– Occurs even in interstellar space• Many organic compounds found in ISM, comets, meteors

(despite extremely harsh conditions)– Easily delivered to early Earth, or produced locally

• Biochemistry– Requires liquid water?– Arises naturally when basic conditions met?

• What is “life”?– System out of chemical equilibrium which extracts energy from its

environment to maintain itself– Energy source could be heat, light, chemical, other? – Reliably reproduces, with opportunity for evolution– Able to store and decode information for this

Basic Chemistry of Life (here)

DigestionC6H12O6 + 6O2 6CO2 + 6H2O + E

Glucose + Oxygen YIELDS Carbon Dioxide + Water + Energy

Photosynthesis6CO2 + 6H2O + E C6H12O6 + 6O2

Carbon Dioxide + Water + Energy YIELDS Glucose + Oxygen

From H,C,N,O (plus some trace amounts of heavier elements like P and Fe) are built nucleic acids, proteins, carbohydrates, and lipids, which can do the chemistry needed for both metabolism and evolution.

Emergence of Life on the Earth

• 0.0-0.5 Gyr Formation and intense bombardment– surface is uninhabitable

• 0.5-1.0 Gyr Surface stabilizes, simple life starts– RNA, DNA; thermophilic progenitor (chemical energy)

• 1.0-2.0 Gyr Anerobic prokaryotes, stromatolite beds– single-celled, no nuclei; oldest fossils formed

• 2.0-2.5 Gyr Photosynthesis invented, free oxygen– surface life; use of sunlight; oxygen crisis

• 2.5-3.0 Gyr Aerobic bacteria, eukaryotes– exploit available oxygen (more energy), cell nucleus

• 3.0-3.5 Gyr bacteria diversify– Keep changing the mix, experiment

• 3.5-4.0 Gyr Sexual reproduction invented– Evolve, baby!

• 4.0-4.5 Gyr complex organisms appear– Let’s get together! Let’s get it together!

The “Tree of Life”Genetic analysis gives us a window into the distant past, and clues on how life developed. Most of the biomass on the Earth is still bacterial, and they are best at filling ecological niches. Extreme life is found in amazing places.

Climate on the EarthThe Sun is getting brighter, and was 30% fainter in the beginning. We’d be frozen now without greenhouse gases (and really frozen then). Somehow the greenhouse effect has been regulated to keep liquid water on the surface. In less than a billion years, it will be hard to stop a runaway greenhouse on Earth (like Venus).

Habitable Zones (liquid surface water)

Many other conditions may be

“habitable”Life here could have started at the bottom of the ocean at volcanic vents.

Life on Earth could be Martian

Mars may have been ready for life first, and seeded the Earth. We know rocks travel safely between them. We should go and see!

SETI : the search for extraterrestrial intelligence• Our only real hope of detecting ET (unless they come to us) is by listening to the

radio– Radio travels at the speed of light, over the whole Galaxy– Radio is a low energy way to send a message– We already have the ability

to send and receive across the Galaxy

• Where should we listen?– Not the currently known extrasolar systems! – Solar-type stars? Milky Way?

• How should we listen?– Frequencies that are relatively quiet. – How narrow-band?The “water hole”?

• What should we listen for?– A regular carrier pattern. Complexity.

• What are the odds we will hear something?

– The Drake equation

Orbital Chaos70 Vir system

The Drake Equation How Likely is Radio Contact With Extraterrestrial Intelligences? NIC = RIC x LIC = Rstar x Pplanets x Phabitability x

Psimple life x Pcomplex life x Pradio signals x Lradio era

 

RICxLIC rate at which civilizations appear x their lifetime 

AstronomyRstar rate at which stars are formed in the GalaxyPplanets probability a star will have planetsPhabitability probability a planet will be suitable for life 

BiologyPsimple life probability bacteria will arise on a suitable planetPcomplex life probability bacteria will evolve into complex life 

SociologyPradio signals probability complex life will send out radio signalsLradio era total duration during which radio is sent

Evaluating the Odds Optimistically

NIC = RIC x LIC = Rstar x Pplanets x Phabitability xPsimple life x Pcomplex life x Pradio signals x Lradio era

 

Optimistic EstimatesRstar observed rate: 10 per yearPplanets observed discoveries: 0.5 Phabitability extreme life: 0.5Psimple life rapidity of life on Earth 1.0Pcomplex life long time on Earth 0.2Pradio signals who knows? 0.02 NIC = Lradio era/100 pick your favorite duration…

So if Lre is greater than a few hundred years, there’s probably somebody out there. Lre needs to be a million years for them to be neighbors (meaning within 1000 ly). The Galaxy’s a big place, andits been around a long time! 

…and so, we are listening!

Allen Array

Rapid Prototype Array Arecibo (Puerto Rico)

You can help too! Downloadseti@home (right here in Berkeley).

2005?

Evaluating the Odds Pessimistically

NIC = RIC x LIC = Rstar x Pplanets x Phabitability xPsimple life x Pcomplex life x Pradio signals x Lradio era

 

Pessimistic EstimateRstar observed rate: 10 per yearPplanets observed discoveries: 0.1 (no terrestrials known)Phabitability extreme life: 0.01 (surface liquid water)Psimple life rapidity of Earth life 0.1 (we got lucky)Pcomplex life long time on Earth 0.01 (looks tough)Pradio signals who knows? 0.001 (what good are radios?) NIC = Lradio era/100 million duration doesn’t much matter…

Pessimistic Conclusion:

There’s nobody home (except for us!). Let’s be careful, live long, and prosper!