Transcript Evolution and the Big Bang, ET Life Lec. 6, Jan 18, 2002

Planet Transits
Lecture Thirty, Apr. 16, 2003
Projects due April 25
• Background papers have been graded (out
of 15 points).
• Happy to offer suggestions or answer
questions for your project.
Extrasolar Planets
• We have now found about 100 planets around
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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.
Doppler method most sensitive to close in
massive planets.
New methods and instruments will allow the
discovery of many more planets.
Artist guess for planet close to 51 Peg
Who lives on these planets?
From 1950s SF movie “The Day The Earth Stood Still”
New Methods to find Planets
One can observe transits of extrasolar
planets with quit small telescopes
Transit of Venus in 1769
The Size of the Solar System
• Johannes Kepler and Edmund Halley in 1716
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predicted that Venus would traverse across the
sun in 1761 and 1769.
Timing these transits from different places on
earth could allow measurement of the
astronomical unit (earth sun distance).
Note transits occur in pairs about every century.
The next transits will occur June 8, 2004 and
June 5, 2012.
Bad weather spoiled observations of the 1761
transit.
Measuring solar system
Angle
Tahiti
0.722 AU
Sun
Venus
London
Deduce parallax angle from time difference of transit
viewed at Tahiti and viewed at London. Allows one to
calculate AU in terms of diameter of earth
The Results
• Observations allowed the AU to be determined
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to within 3% of its modern value.
Secret orders had ship going on after transit and
exploring mysterious southern continent
(Australia and New Zealand).
AU was measured more accurately in modern
times from parallax measurement during a close
approach of asteroid Eros to earth.
AU is now known very accurately from radar
measurements of distances to Mars and Venus.
Extrasolar Transits
• If plane of extrasolar planet’s orbit happens to
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be exactly aligned with us. IE by chance we
happen to observe the system edge on. Then
we can observe a transit.
Star’s light will drop slightly because a small
amount of it is blocked by planet.
Need to very carefully observe brightness of star
as a function of time looking for a small dip.
The Hubble measurements are so accurate they set strong limits on moons
Want telescopes all around world to monitor a star 24 hours a day.
Results since 1995
• We have found about 100 massive planets
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around sun like stars. Some of them orbit close
in. Some orbit at larger distances up to the 5AU
of our Jupiter.
Doppler shift method sensitive to massive
planets relatively close to star.
About 10% of stars have a massive planet within
about 5 AU.
We are not yet sensitive to earth mass planets.
The induced velocity of the star is to small to
measure.
“Jupiters” can be good and bad
• Our Jupiter out at 5 AU helps shield earth from
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impacts (for example Shoemaker Levy 9 was
removed as a threat to earth).
A “Jupiter” near a small planet’s orbit or a
Jupiter in an elliptical orbit can eject “earth’s”
after a while.
The massive planets we have discovered prove
that planets are common in the galaxy!
To find other Jupiters is
interesting. To find other earths
would be exciting!
Talk about new technology for
finding low mass planets next
time.