Worlds Unnumbered Worlds Unnumbered

Lecture Twenty-Nine, Apr. 14, 2003Lecture Twenty-Nine, Apr. 14, 2003

Projects due April 25Projects due April 25

• Background papers have been Background papers have been graded (out of 15 points).graded (out of 15 points).

• Happy to offer suggestions or answer Happy to offer suggestions or answer questions for your project.questions for your project.

The search for planets The search for planets around other starsaround other stars

Buy Land!Buy Land!

• Because they are not making a whole lot Because they are not making a whole lot more of it.more of it.

• What if they were making new land by the What if they were making new land by the planet full!planet full!

• It’s time we start searching for these It’s time we start searching for these “worlds unnumbered”. Unnumbered “worlds unnumbered”. Unnumbered because there are so many of them in the because there are so many of them in the galaxy. Unnumbered because we have galaxy. Unnumbered because we have not yet discovered most of them. not yet discovered most of them.

Search for Extrasolar Search for Extrasolar PlanetsPlanets• To find extraterrestrial life!To find extraterrestrial life!• To find good places to colonize?To find good places to colonize?• To understand the possible places for To understand the possible places for

life in the universe.life in the universe.• To see how unique/ rare is earth. To To see how unique/ rare is earth. To

understand better planet formation.understand better planet formation.• It will be very exciting to find It will be very exciting to find

“another earth”.“another earth”.

StarshipsStarships

• Are very very hard to build Are very very hard to build because the distances are because the distances are so great.so great.

• It might cost 100 billion $ It might cost 100 billion $ to send people to Mars. to send people to Mars. This is comparable to the This is comparable to the space station or the Apollo space station or the Apollo moon landings.moon landings.

• A starship might cost our A starship might cost our full gross national full gross national product???product???

• Only for a far future time Only for a far future time when worlds economy is when worlds economy is larger. larger.

Justifying the CostJustifying the Cost

• Maybe starships are too expensive Maybe starships are too expensive just to explore other stars.just to explore other stars.

• Colonization of new worlds could Colonization of new worlds could justify great cost. Or avoiding justify great cost. Or avoiding disaster on earth.disaster on earth.

• For now work hard to explore other For now work hard to explore other stars with sensitive instruments from stars with sensitive instruments from within our solar system.within our solar system.

Extrasolar PlanetsExtrasolar Planets

• Golden age of exploration starts with Golden age of exploration starts with Columbus discovering the new world in 1492.Columbus discovering the new world in 1492.

• New worlds in our solar system? Mars is New worlds in our solar system? Mars is most Earth like but it is still very hostile. Can most Earth like but it is still very hostile. Can we do better in another solar system?we do better in another solar system?

• Now we are looking to discover lots more Now we are looking to discover lots more new worlds around other stars.new worlds around other stars.

• Can we find another Earth? For now we are Can we find another Earth? For now we are only sensitive to more massive planets.only sensitive to more massive planets.

Where should we look in Where should we look in another star system?another star system?

Habitable ZoneHabitable Zone

• Region correct distance from a star for Region correct distance from a star for temp. for liquid water. temp. for liquid water.

• Where in a planetary system to look for Where in a planetary system to look for life.life.

• For Sun habitable zone from beyond Venus For Sun habitable zone from beyond Venus to inside Mars: about 0.75 to 1.5 to inside Mars: about 0.75 to 1.5 Astronomical units. Astronomical units. – Depends somewhat on planets atmosphere.Depends somewhat on planets atmosphere.– For brighter star habitable zone farther out, for For brighter star habitable zone farther out, for

dimmer star, zone closer in.dimmer star, zone closer in.• Assumes heat from star.Assumes heat from star.

Limitations on habitable Limitations on habitable zoneszones• If star is too large.If star is too large.

– It is bright with a large habitable zone far from star. But:It is bright with a large habitable zone far from star. But:– It burns its fuel quickly and will only last millions instead It burns its fuel quickly and will only last millions instead

of billions of years. of billions of years. – There may not be enough time for complex life to evolve.There may not be enough time for complex life to evolve.

• If star is much smaller then Sun.If star is much smaller then Sun.– Habitable zone is close in.Habitable zone is close in.– For a planet this close in tides will lock its rotation to For a planet this close in tides will lock its rotation to

always keep one side facing the star. Example, near side always keep one side facing the star. Example, near side of Moon always faces Earth.of Moon always faces Earth.

– This lack of day and night is probably not good for life.This lack of day and night is probably not good for life.• Therefore look for stars about the same mass as Therefore look for stars about the same mass as

the Sunthe Sun and look for planets orbiting at about 1 and look for planets orbiting at about 1 AU. AU.

Double StarsDouble Stars

• Many stars are in double or multiple star Many stars are in double or multiple star systems.systems.

• Gravity from additional stars can hurl Gravity from additional stars can hurl planets from system.planets from system.

• Most planetary orbits in multiples star Most planetary orbits in multiples star systems are unstable over billions of systems are unstable over billions of years.years.

• Multiple star systems are probably not Multiple star systems are probably not good places to look for planets and life.good places to look for planets and life.

• Look near single stars.Look near single stars.

Worlds UnnumberedWorlds Unnumbered

• To find inhabited worlds we must first To find inhabited worlds we must first find worlds.find worlds.

• Milkyway galaxy contains 10Milkyway galaxy contains 101111 stars stars and we think a significant fraction and we think a significant fraction have planetshave planets

• Very many possible places for life!Very many possible places for life!

• We are just starting to be able to We are just starting to be able to detect planets around other stars.detect planets around other stars.

Objects in our searchObjects in our search

• Planets:Planets: “low mass” objects including “low mass” objects including rocky planets like Earth and gas giants rocky planets like Earth and gas giants such as Jupiter. Question when is a “giant” such as Jupiter. Question when is a “giant” gas giant big enough to be a “star”?gas giant big enough to be a “star”?

• Brown dwarfs:Brown dwarfs: in between objects neither in between objects neither fish nor fowl. More massive then planets fish nor fowl. More massive then planets but less massive then stars.but less massive then stars.

• Stars:Stars: objects more massive then 0.08 the objects more massive then 0.08 the mass of the Sun. Cores get hot enough for mass of the Sun. Cores get hot enough for nuclear reactions. nuclear reactions.

Finding Extrasolar Planets is Finding Extrasolar Planets is Hard Hard

• Stars are far away (light years).Stars are far away (light years).• Planets at these distances are very Planets at these distances are very

faint and appear very close to the star.faint and appear very close to the star.• Faint planet lost in glare of star.Faint planet lost in glare of star.• Atmospheric blurring mixes star and Atmospheric blurring mixes star and

planet light.planet light.• Space dust and Earth’s atmosphere can Space dust and Earth’s atmosphere can

scatter bright star light obscuring scatter bright star light obscuring planet.planet.

A Light YearA Light Year

• Light travels 186,000 miles per second.Light travels 186,000 miles per second.• In one year light travels 6 In one year light travels 6 ££ 10 101212 miles. miles.• Thus a light year is 6 trillion miles!Thus a light year is 6 trillion miles!• The nearest star is 4.3 light years away.The nearest star is 4.3 light years away.• Interstellar journeys need to cover many Interstellar journeys need to cover many

trillions of miles!trillions of miles!• Need sensitive means to detect planets Need sensitive means to detect planets

10s of trillions of miles away! 10s of trillions of miles away!

Direct Detection of PlanetsDirect Detection of Planets

• Direct detection: observe light from Direct detection: observe light from planet.planet.

• Has never been done.Has never been done.

• Try in infrared. Planets absorb visible light Try in infrared. Planets absorb visible light from star and reradiate it in infrared. from star and reradiate it in infrared. Planet is not as overpowered by star’s Planet is not as overpowered by star’s glare in infrared.glare in infrared.

• Technology should improve to allow direct Technology should improve to allow direct detection of some extrasolar planets soon.detection of some extrasolar planets soon.

The sun is 10The sun is 101010 times as bright as the earth in visible times as bright as the earth in visible light!light!

Doppler shift of star lightDoppler shift of star light

• Instead of seeing light from planet, observe Instead of seeing light from planet, observe indirect effect of planet on star.indirect effect of planet on star.

• Planet and star orbit a common center of Planet and star orbit a common center of mass.mass.

• Star orbits in a small circle, planet a large Star orbits in a small circle, planet a large circle about the center of masscircle about the center of mass

StarStar

Center of MassCenter of MassMassive Massive PlanetPlanet

Planet and star move around Planet and star move around common center of masscommon center of mass

Star WobbleStar Wobble

• As the star orbits in a small circle it moves As the star orbits in a small circle it moves first slowly towards and then slowly away first slowly towards and then slowly away from Earth.from Earth.

• This wobble Doppler shifts the star light ever This wobble Doppler shifts the star light ever so slightly.so slightly.

• We can detect the effect of the planet in this We can detect the effect of the planet in this Doppler shift of the star light without ever Doppler shift of the star light without ever being able to detect light reflected from the being able to detect light reflected from the planet.planet.

• Because stars are so massive compared to Because stars are so massive compared to planets they move in very small circles at planets they move in very small circles at very slow speeds (of order a few m/s). very slow speeds (of order a few m/s).

Doppler shift of 51 PegasiDoppler shift of 51 Pegasi

First extrasolar planetFirst extrasolar planet

51 Peg51 Peg

Solar systemSolar system

Indirect detection of planetsIndirect detection of planets

• No direct information on planets No direct information on planets composition, atmosphere …composition, atmosphere …

• Instead magnitude of star wobble gives Instead magnitude of star wobble gives planets mass, period of wobble tells length planets mass, period of wobble tells length of planets “year”. of planets “year”.

• From length of “year” we can determine From length of “year” we can determine radius of orbit (how far it is from the star).radius of orbit (how far it is from the star).

• We don’t know what the planet looks like, We don’t know what the planet looks like, all we know are its mass and orbit.all we know are its mass and orbit.

Hot JupitersHot Jupiters

• We were surprised to find massive planets We were surprised to find massive planets orbiting so close to their stars.orbiting so close to their stars.

• This is not like the solar system. Perhaps solar This is not like the solar system. Perhaps solar system had a close in “Vulcan” that fell into the system had a close in “Vulcan” that fell into the Sun at some point.Sun at some point.

• Note searches based on stellar wobble are most Note searches based on stellar wobble are most sensitive to massive planets orbiting close in. We sensitive to massive planets orbiting close in. We don’t yet have the sensitivity to detect Earth don’t yet have the sensitivity to detect Earth mass planets.mass planets.

• Therefore results suggest a variety of solar Therefore results suggest a variety of solar systems. We have not yet ruled out systems like systems. We have not yet ruled out systems like our own.our own.

Greatly magnified motion of center of sun from Jupiter and other Greatly magnified motion of center of sun from Jupiter and other planetsplanets

Amateur Planet SearchesAmateur Planet Searches

• Finding extrasolar planets is Finding extrasolar planets is extraordinarily hard and has involved extraordinarily hard and has involved large professional observatories and large professional observatories and experienced astronomers. For example experienced astronomers. For example the Keck 10 meter telescopes (presently the Keck 10 meter telescopes (presently the largest optical telescopes) spend the largest optical telescopes) spend considerable time searching for planets.considerable time searching for planets.

• However finding new worlds is exciting.However finding new worlds is exciting.

• Can amateurs get involved?Can amateurs get involved?

Amateur Planet SearchAmateur Planet Search

Tom KayeTom KayeBusiness owner, paintball gun Business owner, paintball gun manufacturing company. Age 41manufacturing company. Age 41

16” telescope sends 16” telescope sends star light through star light through optical fiber.optical fiber.

Extrasolar PlanetsExtrasolar Planets

• We have now found about 100 planets We have now found about 100 planets around other sun like stars by observing around other sun like stars by observing small Doppler shifts in the star light.small Doppler shifts in the star light.

• Planets not seen directly. Only have Planets not seen directly. Only have information on planets masses and orbits.information on planets masses and orbits.

• Doppler method most sensitive to close in Doppler method most sensitive to close in massive planets.massive planets.

• New methods and instruments will allow New methods and instruments will allow the discovery of many more planets.the discovery of many more planets.