Lecture 13: Searching for planets orbiting other stars I: Properties of Light

1. How could we study distant habitats remotely ? 2. The nature of light - spectrum, spectral lines3. Using spectroscopy to do remote sensing of

exoplanets

NASA Messenger space probe enters into orbit around Mercury

The nature of light

• Light - electromagnetic waves that have:

Wavelength

Frequency

Speed

… and

Energy

Direct Detection of Planets

• Direct detection is challenging because of the technicallimits oftelescopicobservations

Telescopes: 2 basic designs

• Telescopes with a lens for an objective are refractors:

Telescopes: 2 basic designs

• All large telescopesare reflectors:

with a mirror, insteadof a lens.

Telescopes

• Harvard is a partnerin the constructionof the largest newtelescope:

The Giant Magellan Telescope (D ~ 25 m)

Light and Telescopes - Optics

•Resolution - theultimate limitationcomes from thewave properties oflight:

diffraction

Light and Telescopes - Optics

•Resolution - theultimate limitationcomes from thewave properties oflight:

diffraction

Light and Telescopes - Optics

•Resolution - theultimate limitationcomes from thewave properties oflight:

diffraction

Telescopes: 2 basic designs

• All large telescopesare reflectors:

with a mirror, insteadof a lens.

Light and Telescopes - Optics

• Resolution and telescope spider diffraction

Light and Telescopes - Optics

• Resolution and telescope spider diffraction

Telescopes: 2 basic designs

• Telescopes with a lens for an objective are refractors: suffer from chromatic aberration

The nature of visible light

White light is a mixture of the colors;

monochrome light behaves like waves of the same wavelength.

The nature of visible light

• Visible light: a form of electromagnetic energy / radiation that our eyes are sensitive to.

The Spectrum

• Can tell us temperature:

a thermal radiation spectrum is a continuous spectrum of light that depends only on the temperature of the object that emits it.

The Spectrum

• Thermal spectrum:

the spectrum of the Sun is roughly similar to a thermal spectrum.

The Spectra of Stars

• The Sun vs. a smaller, cooler star (M-star),• The wavelength at which a star’s spectrum peaks, reveals the star’s surface temperature:

Using Spectra for Remote Sensing

• Forming spectral lines in the spectrum

Using Spectra for Remote Sensing

• Measuring spectral lines in the spectrum

Electron Orbits in Atoms

Electron Orbits in AtomsPlots of electron density shapes of 1s, 2p and 3d orbitals:

Atoms and Spectral Lines• Spectral lines correspond to the energy of a transition an electron makes between two distinct states.

Model: Seager & Sasselov 2000Detection: Charbonneau et al 2002

Molecules and Spectral Lines• Spectral lines of molecules also correspond to the

energy to transit between distinct states

The Spectra of Stars

• The Sun vs.a smaller, cooler star (M-star)

The Spectra of Planets

• Mars

Using Spectra for Remote Sensing

• Forming spectral lines in the spectrum

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.