Transcript Lecture 34. Extrasolar Planets.

Lecture 34. Extrasolar Planets.
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reading: Chapter 9
Extrasolar Planets
Are planets orbiting another star other than the Earth (aka exoplanets)
Star GQ Lupi (A) orbited by a brown dwarf or gas giant (b).
Distance between A and b is 20RJup (Jupiter radii - or 5.2 AU)
Star: 70% solar mass
Planet: 1-42 x MJup (1 Jupiter mass = 318 Earths)
First discovered in the 1995 - over 163 are known.
How many exoplanets per
solar system?
What are the masses of the
exoplanets?
Where do they orbit?
Exoplanets Found So Far
Almost all are gas giants.
Smallest:5-6 Earth Masses
Largest: 25.63 MJup
Gradation between gas giants and brown dwarfs.
Brown dwarfs are sub-stellar objects, not enough mass to ignite.
Lowest mass stars are 75-90 Mjup
Very low brown dwarfs: ~10 Mjup
Many exoplanets are close into the parent star.
Most are within the orbit of Jupiter.
Some have more than one giant exoplanet.
Exoplanets and Metallicity
1. Transit Method
Measures the periodic dimming of
the star.
Planet must orbit between the Sun
and us (the viewers).
Planet must be large enough to be
able to see dimming.
Must rotate soon enough so we
can see cycles of dimming.
Animation copyrighted. From Nick Strobel’s
Astronomy notes, www.astronomynotes.com
2. Doppler Shift
Objects approaching us
sound or light waves are compressed, light waves have a
shorter wavelength, so are blue-shifted.
Objects moving away from us
sound or light waves are stretched, light waves have a
longer wavelength, so are red-shifted.
Stellar Spectroscopy
when you use a prism to separate out the colors of star
light you see two things:
rainbow of color (light emission - continuous wavelengths
make continuous rainbow of color)
absorption lines (dark lines - discrete lines - discrete
wavelengths are absorbed by atoms in the star fingerprint of the chemistry of the star)
2. Doppler Shift, cont.
If a star is approaching us
Absorption lines are compressed, are blue-shifted
If a star is moving away from us
Absorption lines are stretched, are red-shifted.
When a planet orbits a star, the star is
not stationary!
The planet and star orbit around a common
center of mass (COM).
The heavier the planet is, the COM is
further away from the star.
Our own Sun: COM is 47,000 km above
the Sun’s surface.
One complete wobble every 12 years.
2. Doppler Shift, cont.
Observe the Doppler Shift as the star is orbiting around its COM.
Absorption lines are first blue-shifted, then red-shifted.
The bigger the exoplanet, the bigger the Doppler Shift.
Animation copyrighted. From Nick Strobel’s
Astronomy notes, www.astronomynotes.com
2. Doppler Shift, cont.
Our own Sun: red-shifted for 6 years then blue-shifted for 6 years.
What types of planets are easy to find with this method?
Hard to find?
Animation copyrighted. From Nick Strobel’s
Astronomy notes, www.astronomynotes.com
3. Astrometry
Observes the wobble in nearby stars against a backdrop of
distant stars.
Wobble caused by orbiting around the COM.
The wobble is extremely small - very difficult to detect.
Requires very sensitive instruments and way to diminish
atmospheric effects.
Movement of the Sun due to
Jupiter if we were to observe
it 33 light-years away.
No exoplanets have been
discovered yet with this method.
4. Direct Observation
Last March, the Spitzer Space
Telescope detected infrared
radiation emitted from 2
exoplanets
Have hot atmospheres:
~900˚C
so are emitted infrared/heat
radiation
Very Large Telescope Array,
Chile.
Both are several times the mass
of Jupiter and orbits are > 50AU
Four Hot Jupiters Have Been Found
HD149026b orbits the star HD149026.
Its core Is much larger than Jupiter’s.
Was detected photometrically - only a 0.003 magnitude drop in intensity.
Can calculate the size of the planet
Measure Doppler Shift and period
(gives you velocity and mass)
Measure dimming (gives size).
Density is 1.4 g/cm3 (J is 1.33)
Four Hot Jupiters, cont.
Orbital period of HD149026b: 2.88 days!
Orbits at 0.042 AU (Mercury: 0.38 AU)
Surface Temperature 1300˚C!
If gas temperature heats up it can expand to make the planet larger.
Lowest Mass Exoplanet
7.5 Earth Masses
Orbiting Gliese 876
Also has two Jupiters (2.5 Mjup and 0.8 Mjup)
Low mass exoplanet orbits close to the star at 0.04 AU
animation
Discovered using
the Doppler Shift
method.
Most Earth-Like Exoplanet
OGLE-2005-BLG-390Lb
21500 light years away
Orbits at 2-4.1 AU around a cool red dwarf
5.5 Mearth
T is ~50K (~ -220˚C)
May be a terrestrial planet.
Does it have a thick atmosphere?
Image of Doppler Shift in the Starlight Spectrum
Two Ways to Measure Atmospheric Composition
1. Measure what wavelengths of the starlight are absorbed as it
passes through the atmosphere (similar to what method?)
Using this method, sodium was discovered in the atmosphere of
the planet orbiting HD209458. This star is 150 light years away,
transits every 3.5 days. T of the atmosphere is ~1100˚C.
Should be able to detect methane, water vapor, K, O2, O3, CO2….
2. Measure what wavelengths are reflected from the planet’s
atmosphere and reach our telescopes.
Color and reflectivity should give a measure of how many clouds
there are.
Which method is more difficult??
Guinness Book of Planetary Records
Oldest Planet
Largest Planet
Most Distant Planet
Closest Planet
Most Dense
Least Dense
Longest Period
Shortest Period
12.7 Ga
1.32 Rjup
21,500 light years
10.4 light years
1.4 g/cm3
0.33 g/cm3
>2450 years, >55 AU
1.2 days, 0.0225 AU
Lecture 35. Habitable Zones.
reading: Chapters 9, 10