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

Projects due April 25 Background papers have been graded (out of 15 points). Background papers have been graded (out of 15 points). Happy to offer suggestions or answer questions for your project. Happy to offer suggestions or answer questions for your project.

The search for planets around other stars

Buy Land! Because they are not making a whole lot more of it. Because they are not making a whole lot more of it. What if they were making new land by the planet full! What if they were making new land by the planet full! It’s time we start searching for these “worlds unnumbered”. Unnumbered because there are so many of them in the galaxy. Unnumbered because we have not yet discovered most of them. It’s time we start searching for these “worlds unnumbered”. Unnumbered because there are so many of them in the galaxy. Unnumbered because we have not yet discovered most of them.

Search for Extrasolar Planets 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 life in the universe. To understand the possible places for life in the universe. To see how unique/ rare is earth. To understand better planet formation. To see how unique/ rare is earth. To understand better planet formation. It will be very exciting to find “another earth”. It will be very exciting to find “another earth”.

Starships Are very very hard to build because the distances are so great. Are very very hard to build because the distances are so great. It might cost 100 billion $ to send people to Mars. This is comparable to the space station or the Apollo moon landings. It might cost 100 billion $ to send people to Mars. This is comparable to the space station or the Apollo moon landings. A starship might cost our full gross national product??? A starship might cost our full gross national product??? Only for a far future time when worlds economy is larger. Only for a far future time when worlds economy is larger.

Justifying the Cost Maybe starships are too expensive just to explore other stars. Maybe starships are too expensive just to explore other stars. Colonization of new worlds could justify great cost. Or avoiding disaster on earth. Colonization of new worlds could justify great cost. Or avoiding disaster on earth. For now work hard to explore other stars with sensitive instruments from within our solar system. For now work hard to explore other stars with sensitive instruments from within our solar system.

Extrasolar Planets Golden age of exploration starts with Columbus discovering the new world in Golden age of exploration starts with Columbus discovering the new world in New worlds in our solar system? Mars is most Earth like but it is still very hostile. Can we do better in another solar system? New worlds in our solar system? Mars is most Earth like but it is still very hostile. Can we do better in another solar system? Now we are looking to discover lots more new worlds around other stars. Now we are looking to discover lots more new worlds around other stars. Can we find another Earth? For now we are only sensitive to more massive planets. Can we find another Earth? For now we are only sensitive to more massive planets.

Where should we look in another star system?

Habitable Zone Region correct distance from a star for temp. for liquid water. Region correct distance from a star for temp. for liquid water. Where in a planetary system to look for life. Where in a planetary system to look for life. For Sun habitable zone from beyond Venus to inside Mars: about 0.75 to 1.5 Astronomical units. For Sun habitable zone from beyond Venus to inside Mars: about 0.75 to 1.5 Astronomical units. –Depends somewhat on planets atmosphere. –For brighter star habitable zone farther out, for dimmer star, zone closer in. Assumes heat from star. Assumes heat from star.

Limitations on habitable zones If star is too large. If star is too large. –It is bright with a large habitable zone far from star. But: –It burns its fuel quickly and will only last millions instead of billions of years. –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. –For a planet this close in tides will lock its rotation to always keep one side facing the star. Example, near side of Moon always faces Earth. –This lack of day and night is probably not good for life. Therefore look for stars about the same mass as the Sun and look for planets orbiting at about 1 AU. Therefore look for stars about the same mass as the Sun and look for planets orbiting at about 1 AU.

Double Stars Many stars are in double or multiple star systems. Many stars are in double or multiple star systems. Gravity from additional stars can hurl planets from system. Gravity from additional stars can hurl planets from system. Most planetary orbits in multiples star systems are unstable over billions of years. Most planetary orbits in multiples star systems are unstable over billions of years. Multiple star systems are probably not good places to look for planets and life. Multiple star systems are probably not good places to look for planets and life. Look near single stars. Look near single stars.

Worlds Unnumbered To find inhabited worlds we must first find worlds. To find inhabited worlds we must first find worlds. Milkyway galaxy contains stars and we think a significant fraction have planets Milkyway galaxy contains stars and we think a significant fraction have planets Very many possible places for life! Very many possible places for life! We are just starting to be able to detect planets around other stars. We are just starting to be able to detect planets around other stars.

Objects in our search Planets: “low mass” objects including rocky planets like Earth and gas giants such as Jupiter. Question when is a “giant” gas giant big enough to be a “star”? Planets: “low mass” objects including rocky planets like Earth and gas giants such as Jupiter. Question when is a “giant” gas giant big enough to be a “star”? Brown dwarfs: in between objects neither fish nor fowl. More massive then planets but less massive then stars. Brown dwarfs: in between objects neither fish nor fowl. More massive then planets but less massive then stars. Stars: objects more massive then 0.08 the mass of the Sun. Cores get hot enough for nuclear reactions. Stars: objects more massive then 0.08 the mass of the Sun. Cores get hot enough for nuclear reactions.

Finding Extrasolar Planets is Hard Stars are far away (light years). Stars are far away (light years). Planets at these distances are very faint and appear very close to the star. Planets at these distances are very 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 planet light. Atmospheric blurring mixes star and planet light. Space dust and Earth’s atmosphere can scatter bright star light obscuring planet. Space dust and Earth’s atmosphere can scatter bright star light obscuring planet.

A Light Year Light travels 186,000 miles per second. Light travels 186,000 miles per second. In one year light travels 6 £ miles. In one year light travels 6 £ 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 trillions of miles! Interstellar journeys need to cover many trillions of miles! Need sensitive means to detect planets 10s of trillions of miles away! Need sensitive means to detect planets 10s of trillions of miles away!

Direct Detection of Planets Direct detection: observe light from planet. Direct detection: observe light from planet. Has never been done. Has never been done. Try in infrared. Planets absorb visible light from star and reradiate it in infrared. Planet is not as overpowered by star’s glare in infrared. Try in infrared. Planets absorb visible light from star and reradiate it in infrared. Planet is not as overpowered by star’s glare in infrared. Technology should improve to allow direct detection of some extrasolar planets soon. Technology should improve to allow direct detection of some extrasolar planets soon.

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

Doppler shift of star light Instead of seeing light from planet, observe indirect effect of planet on star. Instead of seeing light from planet, observe indirect effect of planet on star. Planet and star orbit a common center of mass. Planet and star orbit a common center of mass. Star orbits in a small circle, planet a large circle about the center of mass Star orbits in a small circle, planet a large circle about the center of mass Star Center of Mass Massive Planet

Planet and star move around common center of mass

Star Wobble As the star orbits in a small circle it moves first slowly towards and then slowly away from Earth. As the star orbits in a small circle it moves first slowly towards and then slowly away from Earth. This wobble Doppler shifts the star light ever so slightly. This wobble Doppler shifts the star light ever so slightly. We can detect the effect of the planet in this Doppler shift of the star light without ever being able to detect light reflected from the planet. We can detect the effect of the planet in this Doppler shift of the star light without ever being able to detect light reflected from the planet. Because stars are so massive compared to planets they move in very small circles at very slow speeds (of order a few m/s). Because stars are so massive compared to planets they move in very small circles at very slow speeds (of order a few m/s).

Doppler shift of 51 Pegasi

First extrasolar planet 51 Peg Solar system

Indirect detection of planets No direct information on planets composition, atmosphere … No direct information on planets composition, atmosphere … Instead magnitude of star wobble gives planets mass, period of wobble tells length of planets “year”. Instead magnitude of star wobble gives planets mass, period of wobble tells length of planets “year”. From length of “year” we can determine radius of orbit (how far it is from the star). From length of “year” we can determine radius of orbit (how far it is from the star). We don’t know what the planet looks like, all we know are its mass and orbit. We don’t know what the planet looks like, all we know are its mass and orbit.

Hot Jupiters We were surprised to find massive planets orbiting so close to their stars. We were surprised to find massive planets orbiting so close to their stars. This is not like the solar system. Perhaps solar system had a close in “Vulcan” that fell into the Sun at some point. This is not like the solar system. Perhaps solar system had a close in “Vulcan” that fell into the Sun at some point. Note searches based on stellar wobble are most sensitive to massive planets orbiting close in. We don’t yet have the sensitivity to detect Earth mass planets. Note searches based on stellar wobble are most sensitive to massive planets orbiting close in. We don’t yet have the sensitivity to detect Earth mass planets. Therefore results suggest a variety of solar systems. We have not yet ruled out systems like our own. Therefore results suggest a variety of solar systems. We have not yet ruled out systems like our own.

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

Amateur Planet Searches Finding extrasolar planets is extraordinarily hard and has involved large professional observatories and experienced astronomers. For example the Keck 10 meter telescopes (presently the largest optical telescopes) spend considerable time searching for planets. Finding extrasolar planets is extraordinarily hard and has involved large professional observatories and experienced astronomers. For example the Keck 10 meter telescopes (presently the largest optical telescopes) spend 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 Search Tom Kaye Business owner, paintball gun manufacturing company. Age 41 16” telescope sends star light through optical fiber.

Extrasolar Planets We have now found about 100 planets around other sun like stars by observing small Doppler shifts in the star light. We have now found about 100 planets around other sun like stars by observing small Doppler shifts in the star light. Planets not seen directly. Only have information on planets masses and orbits. Planets not seen directly. Only have information on planets masses and orbits. Doppler method most sensitive to close in massive planets. Doppler method most sensitive to close in massive planets. New methods and instruments will allow the discovery of many more planets. New methods and instruments will allow the discovery of many more planets.