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 lines 3.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 technical limits of telescopic observations

Telescopes: 2 basic designs Telescopes with a lens for an objective are refractors:

Telescopes: 2 basic designs All large telescopes are reflectors: with a mirror, instead of a lens.

Telescopes Harvard is a partner in the construction of the largest new telescope: The Giant Magellan Telescope (D ~ 25 m)

Light and Telescopes - Optics Resolution - the ultimate limitation comes from the wave properties of light: diffraction

Light and Telescopes - Optics Resolution - the ultimate limitation comes from the wave properties of light: diffraction

Light and Telescopes - Optics Resolution - the ultimate limitation comes from the wave properties of light: diffraction

Telescopes: 2 basic designs All large telescopes are reflectors: with a mirror, instead of 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

Plots 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 2000 Detection: 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.