Download Other Worlds - UC Berkeley Astronomy w

Reminders!
Website: http://starsarestellar.blogspot.com/
Lectures 1-9 are available for download as study
aids.
Reading: You should have Chapters 1-8 done, so
that you have all the necessary reading done for the
midterm.
Homework: Homework #2 is due TODAY.
Homework #3 is due next Wednesday, June 17th.
Homework #3 is short--might be a good time to
concentrate on your projects!!!
I should have Homework #2 solutions up soon to
help you study for the midterm.
Other Worlds
Today’s Lecture:
Chapter 9, pages 184-209
• Detecting extra-solar planets
Doppler Shifts
Transits
Microlensing
• Properties of extra-solar planets
Hot Jupiters
Extra-solar planets
• It’s a huge technical challenge to find extrasolar
planets! The parent star completely outshines the
planet, making direction detection next to impossible.
• Doppler effect: Look for periodic “wobble” of a star
(sinusoidal change in its radial velocity) due to the
motion of a planet around it. We can estimate the mass
of the planet using Kepler’s third law.
(Important note: You only get the minimum mass of the
planet this way, since you don’t know the inclination of
the orbit. In other words, the measured radial velocity of
the star may be only part of its total orbital velocity.)
Extra-solar planets (cont.)
• October 1995: Breakthrough in optical search
for extra-solar planets -- a planet found by a
Swiss team around star 51 Pegasi!
(called 51 Pegasi b)
• 51 Pegasi b is weird! Half the mass of
Jupiter and a period of only 4.2 days!!!
Confirmed by Geoff Marcy (UC Berkeley) and
Paul Butler at Lick Observatory. They
subsequently found > 100 more planets (mostly
at Lick and Keck)
Over 350 extra-solar planets are now known!
Lots of selection biases
• Most of these stars are (barely) visible to the
naked eye. Distance ~ 50 ly
• Technology at optical wavelengths is not yet good
enough (usually) to detect Earth-sized planets
• We preferentially find very small orbits, high
eccentricities, and massive planets because
these produce the greatest Doppler shift.
• Most of these planets are unlike anything that we
see in our Solar System, forcing us to rethink our
understanding of how the Solar System formed.
Other methods for discovery
• Transits: The planet passes in front of its parent star,
blocking the star’s light a little (making it dimmer).
Advantage: We can measure the planet’s relative radius
and probe its atmosphere.
• Microlensing: As the star and its planet pass in front
of a background star, they magnify the more distant star
(light bending from General Relativity - Einstein). Extra
magnification indicates the presence of a the star.
Advantage: the planet can be further away from the
star!
Planetary transit: light curve
(star brightness vs time)
The first observed transit
HD 209458 with Hubble Space Telescope
1.5%
Dip!
Measure a planet’s radius
Consider a star with radius Rstar and planet Rplanet.
Before the planet passes in front of the star, the area of the
star seen is
Area of star = π(Rstar)2
When the planet passes in front, we instead see an area
Area seen = π(Rstar)2 - π(Rplanet)2
The fractional change in area seen is therefore
(Area of star - Area seen)/(Area of star) = (Rplanet / Rstar)2
Example: If the luminosity changes by 2% or 0.02, then
the planet must have a radius that is (0.02)1/2 = 0.14 of the
star’s radius.
Notable Extra-solar Planets!
• OGLE-2005-BLG-390Lb: First planet found using
microlensing. Mass of 5.5MEarth and distance of 2.6 AU.
• HD 209458 b and HD 189733 b: First spectra of extrasolar
planets. Found water vapor and other mysterious absorption
lines. Also first transiting planet.
• TrES-4: Lowest density planet ever at 0.2 g/cm3! The same
density of balsa wood! What makes the planet so bloated?
• HD 189733 b: The first organic molecule found in a planet’s
atmosphere (methane). What is making methane?
• Fomalhaut b: The first direct image of an extrasolar planet.
Mass of 3MJupiter. Found due to effects on a debris disk.
• Gliese 581 e: The smallest mass at 1.9MEarth and distance of
0.03 AU.