Lecture 34. Extrasolar Planets. 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 20R Jup (Jupiter radii - or 5.2 AU) Star: 70% solar mass Planet: 1-42 x M Jup (1 Jupiter mass = 318 Earths) First discovered in the 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: M Jup Gradation between gas giants and brown dwarfs. Brown dwarfs are sub-stellar objects, not enough mass to ignite. Lowest mass stars are M jup Very low brown dwarfs: ~10 M jup 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 Animation copyrighted. From Nick Strobel’s Astronomy notes, 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.

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,

Animation copyrighted. From Nick Strobel’s Astronomy notes, 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?

3. Astrometry 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. 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.

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 HD Its core Is much larger than Jupiter’s. Was detected photometrically - only a 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/cm 3 (J is 1.33)

Four Hot Jupiters, cont. Orbital period of HD149026b: 2.88 days! Orbits at 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 M jup and 0.8 M jup ) 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 light years away Orbits at AU around a cool red dwarf 5.5 M earth 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 HD 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, O 2, O 3, CO 2 …. 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 Planet12.7 Ga Largest Planet1.32 R jup Most Distant Planet21,500 light years Closest Planet10.4 light years Most Dense1.4 g/cm 3 Least Dense0.33 g/cm 3 Longest Period>2450 years, >55 AU Shortest Period1.2 days, AU

Lecture 35. Habitable Zones. reading: Chapters 9, 10