The Search for Extrasolar Planets Since 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 far 2) Astrometric wobble of the star’s orbit Semi-direct Methods: 1) Transits (light blocked by the planet) might also see phases 2) 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

Astrometry This 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 is 1 part in 100 million

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 found Another 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?)

Transits We 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 Project Transits 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 Thermal Radiator Electronics Sunshade 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 characteristics Launched 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 (10 6 ) 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) Digestion C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + E Glucose + Oxygen YIELDS Carbon Dioxide + Water + Energy Photosynthesis 6CO 2 + 6H 2 O + E  C 6 H 12 O 6 + 6O 2 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 GyrFormation and intense bombardment –surface is uninhabitable GyrSurface stabilizes, simple life starts –RNA, DNA; thermophilic progenitor (chemical energy) GyrAnerobic prokaryotes, stromatolite beds –single-celled, no nuclei; oldest fossils formed GyrPhotosynthesis invented, free oxygen –surface life; use of sunlight; oxygen crisis GyrAerobic bacteria, eukaryotes –exploit available oxygen (more energy), cell nucleus Gyrbacteria diversify –Keep changing the mix, experiment GyrSexual reproduction invented –Evolve, baby! Gyrcomplex 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 Earth The 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 Chaos 70 Vir system

The Drake Equation How Likely is Radio Contact With Extraterrestrial Intelligences? N IC = R IC x L IC = R star x P planets x P habitability x P simple life x P complex life x P radio signals x L radio era R IC xL IC rate at which civilizations appear x their lifetime Astronomy R star rate at which stars are formed in the Galaxy P planets probability a star will have planets P habitability probability a planet will be suitable for life Biology P simple life probability bacteria will arise on a suitable planet P complex life probability bacteria will evolve into complex life Sociology P radio signals probability complex life will send out radio signals L radio era total duration during which radio is sent

Evaluating the Odds Optimistically N IC = R IC x L IC = R star x P planets x P habitability x P simple life x P complex life x P radio signals x L radio era Optimistic Estimates R star observed rate: 10 per year P planets observed discoveries: 0.5 P habitability extreme life:0.5 P simple life rapidity of life on Earth1.0 P complex life long time on Earth0.2 P radio signals who knows?0.02 N IC = L radio era /100pick your favorite duration… So if L re is greater than a few hundred years, there’s probably somebody out there. L re needs to be a million years for them to be neighbors (meaning within 1000 ly). The Galaxy’s a big place, and its been around a long time!

…and so, we are listening! Allen Array Rapid Prototype Array Arecibo (Puerto Rico) You can help too! Download (right here in Berkeley). 2005?

Evaluating the Odds Pessimistically N IC = R IC x L IC = R star x P planets x P habitability x P simple life x P complex life x P radio signals x L radio era Pessimistic Estimate R star observed rate: 10 per year P planets observed discoveries: 0.1 (no terrestrials known) P habitability extreme life:0.01 (surface liquid water) P simple life rapidity of Earth life0.1 (we got lucky) P complex life long time on Earth0.01 (looks tough) P radio signals who knows?0.001 (what good are radios?) N IC = L radio 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!