Lecture 34. Extrasolar Planets.

reading: Chapter 9

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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 massPlanet: 1-42 x MJup (1 Jupiter mass = 318 Earths)

First discovered in the 1995 - over 163 are known.

How many exoplanets persolar system?

What are the masses of theexoplanets?

Where do they orbit?

Exoplanets Found So Far

Almost all are gas giants.Smallest:5-6 Earth MassesLargest: 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

Animation copyrighted. From Nick Strobel’sAstronomy notes, www.astronomynotes.com

Measures the periodic dimming ofthe star.

Planet must orbit between the Sunand us (the viewers).

Planet must be large enough to beable to see dimming.

Must rotate soon enough so wecan see cycles of dimming.

2. Doppler Shift

Objects approaching ussound or light waves are compressed, light waves have ashorter wavelength, so are blue-shifted.

Objects moving away from ussound or light waves are stretched, light waves have alonger wavelength, so are red-shifted.

Stellar Spectroscopywhen you use a prism to separate out the colors of starlight you see two things:

rainbow of color (light emission - continuous wavelengthsmake continuous rainbow of color)

absorption lines (dark lines - discrete lines - discretewavelengths are absorbed by atoms in the star -fingerprint of the chemistry of the star)

2. Doppler Shift, cont.

If a star is approaching usAbsorption lines are compressed, are blue-shifted

If a star is moving away from usAbsorption lines are stretched, are red-shifted.

When a planet orbits a star, the star isnot stationary!

The planet and star orbit around a commoncenter of mass (COM).

The heavier the planet is, the COM isfurther away from the star.

Our own Sun: COM is 47,000 km abovethe 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’sAstronomy notes, www.astronomynotes.com

Animation copyrighted. From Nick Strobel’sAstronomy 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?

3. Astrometry

Movement of the Sun due toJupiter if we were to observeit 33 light-years away.

No exoplanets have beendiscovered 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 SpaceTelescope detected infraredradiation emitted from 2exoplanets

Have hot atmospheres:~900˚Cso are emitted infrared/heatradiation

Very Large Telescope Array,Chile.

Both are several times the massof 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 planetMeasure 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 MassesOrbiting Gliese 876Also has two Jupiters (2.5 Mjup and 0.8 Mjup)Low mass exoplanet orbits close to the star at 0.04 AU

animation

Discovered usingthe Doppler Shiftmethod.

Most Earth-Like Exoplanet

OGLE-2005-BLG-390Lb21500 light years awayOrbits at 2-4.1 AU around a cool red dwarf5.5 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 itpasses through the atmosphere (similar to what method?)

Using this method, sodium was discovered in the atmosphere ofthe 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’satmosphere and reach our telescopes.

Color and reflectivity should give a measure of how many cloudsthere are.

Which method is more difficult??

Guinness Book of Planetary Records

Oldest Planet 12.7 GaLargest Planet 1.32 Rjup

Most Distant Planet 21,500 light yearsClosest Planet 10.4 light yearsMost Dense 1.4 g/cm3

Least Dense 0.33 g/cm3

Longest Period >2450 years, >55 AUShortest Period 1.2 days, 0.0225 AU

Lecture 35. Habitable Zones.

reading: Chapters 9, 10