Lecture 14: Searching for planets orbiting other stars III: Using Spectra

1. The Spectra of Stars and Planets 2. The Doppler Effect and its uses3. Using spectroscopy to measure orbits and masses

of exoplanets

NASA Messenger space probe enters into orbit around Mercury

NASA Messenger around Mercury: First Image

Using Spectra for Remote Sensing

• Measuring spectral lines in the spectrum

Using Spectra for Remote Sensing

• Forming spectral lines in the spectrum

Venus in Front of the Sun

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The Sun in One Spectral Line

Using Spectra for Remote Sensing

• Forming spectral lines in the spectrum

Direct Detection of Planets• The best case to-date of a planet-like system detected ininfrared light:

Indirect methods of planet detection

• Newton’s law of universal gravitation

The Astrometric method

• Binary stars - Sirius A & B.

The Astrometric method

• Not successful in planet discovery yet.

The Doppler shift method

• Doppler shift is measured from the spectral lines of two stars in abinary system:

Using Spectra for Remote Sensing

• Measuring spectral lines in the spectrum

The Doppler shift

• What is a Doppler shift ? - true for all waves.

The Doppler shift

• Doppler shift is measured from the spectral lines:

Mass:

Radial velocities seenin star HD 209458 -the variation is dueto a planet that is lessmassive than Jupiter.

(Mazeh et al. 1999;Marcy et al. 2000)

Mass:

• For HD 209458b:

Mp sin(i) = Ms vs P / 2 ap

= const. x (Ms /1.1MSun) Mjup

+ 0.018 + 0.1• Transit light curve helps derive the orbit

inclination: i = 86o.7 + 0.2

• Both Mp and Rp determined to better than 5%!

Mass:

Radial velocities seenin star HD 168443 -they are due to 2 planets,each larger than ourJupiter.

(Marcy & Butler 2002;Mayor et al. 2002)

The Spectra of Planets

• Mars

Main points to take home:

1) Visible light: form of electromagnetic energy (radiation) to which our eyes are sensitive.

2) Spectrum: the amount of light of any given wavelength, emitted or reflected by an object.

3) Thermal spectrum: a simple spectrum that depends only on the object’s temperature.

4) Spectral lines: in emission or absorption; every atom and molecule has a specific set.