Planet TransitsPlanet Transits

Lecture Thirty, Apr. 16, 2003Lecture Thirty, Apr. 16, 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.

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.

Artist guess for planet close to 51 PegArtist guess for planet close to 51 Peg

Who lives on these Who lives on these planets?planets?

From 1950s SF movie “The Day The Earth Stood Still”From 1950s SF movie “The Day The Earth Stood Still”

New Methods to find New Methods to find PlanetsPlanets

One can observe transits of One can observe transits of extrasolar planets with quit small extrasolar planets with quit small

telescopestelescopes

Transit of Venus in 1769Transit of Venus in 1769

The Size of the Solar The Size of the Solar SystemSystem• Johannes Kepler and Edmund Halley in Johannes Kepler and Edmund Halley in

1716 predicted that Venus would traverse 1716 predicted that Venus would traverse across the sun in 1761 and 1769.across the sun in 1761 and 1769.

• Timing these transits from different places Timing these transits from different places on earth could allow measurement of the on earth could allow measurement of the astronomical unit (earth sun distance).astronomical unit (earth sun distance).

• Note transits occur in pairs about every Note transits occur in pairs about every century. The next transits will occur June century. The next transits will occur June 8, 2004 and June 5, 2012.8, 2004 and June 5, 2012.

• Bad weather spoiled observations of the Bad weather spoiled observations of the 1761 transit. 1761 transit.

Measuring solar systemMeasuring solar system

SunSun LondonLondon

TahitiTahiti

VenusVenus

Deduce parallax angle from time difference of Deduce parallax angle from time difference of transit viewed at Tahiti and viewed at London. transit viewed at Tahiti and viewed at London. Allows one to calculate AU in terms of diameter of Allows one to calculate AU in terms of diameter of earthearth

AngleAngle

0.722 AU0.722 AU

The ResultsThe Results

• Observations allowed the AU to be Observations allowed the AU to be determined to within 3% of its modern value.determined to within 3% of its modern value.

• Secret orders had ship going on after transit Secret orders had ship going on after transit and exploring mysterious southern continent and exploring mysterious southern continent (Australia and New Zealand).(Australia and New Zealand).

• AU was measured more accurately in modern AU was measured more accurately in modern times from parallax measurement during a times from parallax measurement during a close approach of asteroid Eros to earth.close approach of asteroid Eros to earth.

• AU is now known very accurately from radar AU is now known very accurately from radar measurements of distances to Mars and measurements of distances to Mars and Venus.Venus.

Extrasolar TransitsExtrasolar Transits

• If plane of extrasolar planet’s orbit happens If plane of extrasolar planet’s orbit happens to be exactly aligned with us. IE by chance to be exactly aligned with us. IE by chance we happen to observe the system edge on. we happen to observe the system edge on. Then we can observe a transit. Then we can observe a transit.

• Star’s light will drop slightly because a Star’s light will drop slightly because a small amount of it is blocked by planet.small amount of it is blocked by planet.

• Need to very carefully observe brightness Need to very carefully observe brightness of star as a function of time looking for a of star as a function of time looking for a small dip. small dip.

The Hubble measurements are so accurate they set strong limits The Hubble measurements are so accurate they set strong limits on moonson moons

Want telescopes all around world to monitor a star 24 hours a Want telescopes all around world to monitor a star 24 hours a day.day.

Results since 1995Results since 1995

• We have found about 100 massive planets We have found about 100 massive planets around sun like stars. Some of them orbit around sun like stars. Some of them orbit close in. Some orbit at larger distances up close in. Some orbit at larger distances up to the 5AU of our Jupiter.to the 5AU of our Jupiter.

• Doppler shift method sensitive to massive Doppler shift method sensitive to massive planets relatively close to star.planets relatively close to star.

• About 10% of stars have a massive planet About 10% of stars have a massive planet within about 5 AU.within about 5 AU.

• We are not yet sensitive to earth mass We are not yet sensitive to earth mass planets. The induced velocity of the star is planets. The induced velocity of the star is to small to measure. to small to measure.

““Jupiters” can be good and Jupiters” can be good and badbad• Our Jupiter out at 5 AU helps shield earth Our Jupiter out at 5 AU helps shield earth

from impacts (for example Shoemaker from impacts (for example Shoemaker Levy 9 was removed as a threat to earth).Levy 9 was removed as a threat to earth).

• A “Jupiter” near a small planet’s orbit or a A “Jupiter” near a small planet’s orbit or a Jupiter in an elliptical orbit can eject Jupiter in an elliptical orbit can eject “earth’s” after a while.“earth’s” after a while.

• The massive planets we have discovered The massive planets we have discovered prove that planets are common in the prove that planets are common in the galaxy! galaxy!

To find other Jupiters is To find other Jupiters is interesting. To find other interesting. To find other earths would be exciting! earths would be exciting!

Talk about new technology for Talk about new technology for finding low mass planets next finding low mass planets next

time.time.